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Switch between displaying air/fuel ratios for two different fuels ◀ Accurate air/fuel ratio and lambda measurement and display ◀ Wideband and narrowband O2 sensor compatible outputs ◀ Several display options, including wireless via Bluetooth ◀ Optional exhaust pressure correction for readings ◀ Correct sensor heat-up procedure implemented ◀ Compact size, fitting in a 120 x 70mm case ◀ Factory-calibrated oxygen sensor ◀ Part 3 of John Clarke’s WIDEBAND Fuel Mixture Display Our Wideband Fuel Mixture Display (WFMD) includes Bluetooth support, fits in a compact case and can compensate for higher exhaust gas pressures. This month, we give the complete construction, setting up and installation details. W hile the Wideband Fuel Mixture Display (WFMD) uses multiple surface-mount components, it’s pretty straightforward to assemble. Most parts are mounted on a double-sided, plated-through PCB coded 05104231 that measures 103.5 × 63.5mm (not 160 × 98.5mm as stated in the parts list in the April issue). It is housed within a 120 × 70 × 30mm plastic enclosure. An 8-pin circular multi-pole panel plug connector provides the interface to the external wideband sensor. This sensor is mounted in the exhaust stream (either directly or via an adaptor pipe) and connects to the controller via a 7-way extension cable. The enclosure also hosts cable glands for the power input, pressure sensor and volt/amp panel meter (or external multimeter) leads. The WFMD provides a simulated narrowband sensor output. This enables a vehicle’s existing narrowband sensor to be replaced with the 72 Silicon Chip Bosch LSU4.9 and still provide for normal engine operation by connecting the narrowband signal to the ECU. If your engine already uses a wideband oxygen sensor instead, the simulated narrowband output will not be a suitable replacement signal source. In that case, you can add the Bosch wideband oxygen sensor to the exhaust pipe as a standalone unit driven by the Wideband Fuel Mixture Display to observe the Air/Fuel mixture, leaving the ECU’s oxygen sensor(s) alone. PCB assembly Fig.13 shows the parts layout on the PCB. While there are components on both sides, we recommend fitting all the top-side SMDs before you solder any to the underside. That way, the board will still sit flat until you have mounted all the top-side SMDs. Begin by fitting the ICs. These are not overly difficult to solder, provided Australia's electronics magazine you have magnification of the work area and a fine-tipped soldering iron. Be sure to install the correct IC in each place and, in particular, double-check the orientation of each before soldering. Do not mix up IC2 and IC3. While IC2 is an OPA2171AID, IC3 can be either the OPA2171AID or an LMC6482AIM. Our kits will likely be supplied with two OPA2171AIDs; in that case, IC2 and IC3 will be the same type. To solder each IC, align the pins with the pads on the PCB, ensure pin 1 is in the correct position and then solder a corner pin. Check the IC alignment and, if necessary, remelt the solder and adjust the alignment until the pins are all centred over their pads. Solder the diagonally opposite pin of the IC before soldering the remaining pins; applying a little flux paste before soldering them will make that easier. siliconchip.com.au That takes care of all the SMDs on the top side. Now flip the board over and fit the SMDs on the underside, which include eight resistors, five capacitors, one zener diode, three regular diodes and two transistors: Q2 (BC817, NPN) and Q3 (BC807, PNP). Use the same techniques as before to mount all those components. Through-hole parts Fig.13: the overlay diagram for the Wideband Fuel Mixture Display (WFMD); we recommend fitting the components on the underside last. Any solder bridges that form can be cleared using a bit of extra flux paste and some solder wick. The resistors can be mounted next; all are surface-mount types that will be printed with a coded resistance value. For the 1% resistors, this is usually a four-digit code where the first three digits are the resistance value and the fourth value is the zeros multiplier. A code of 1003 means 100 with three zeros for 100kW. If it’s a three-digit code instead, it will be 104 (10 with four more zeros). For lower resistance values, the label could be just the resistance, eg, a 10W resistor might read 10 or 10R. A 100W resistor may be printed with 1000; the last zero indicates there are no zeros added to the value of 100. If present, R represents a decimal point, so a 0.1W resistor may read R100 or 0R10, although that resistor should be obvious as it is larger than the others. If you are unsure, check the resistor’s value with a multimeter set to siliconchip.com.au read ohms, but be careful not to press so hard on it with the probes that it goes flying off, never to be seen again. Next, install the SMD diodes and zener diodes. Most will have the type number on the top of the diode body, although you might need a magnifier to read the markings. Take care to orientate each with the anode and cathode (the end with a stripe) positioned as shown on the overlay diagram. We also placed a + near the cathode end on the PCB screen printing for clarity. Transistors Q4 and Q5 can go in next. Be sure to use the correct transistor in each place; Q4 is a BC847, while Q5 is a BC817. These are threepin SOT-23 surface-mount types, both NPN transistors. Follow with the surface-mount capacitors. These are unmarked, so you will need to rely on the packaging markings (or, in a pinch, a capacitance meter) to find their value. They are not polarised and can be installed either way on the PCB. Australia's electronics magazine Now we can move on to the throughhole parts, starting with the sole through-hole diode, D1 (1N4004), with its cathode stripe facing as shown in Fig.13. The through-hole capacitors are either MKT polyester or electrolytic types. The electrolytic capacitors need to be oriented with the polarity indicated, with the longer (positive) leads into the pads marked with a + and the negative stripes on the opposite side. All the electros are 100μF except for one 10μF type, so watch out for that. The MKT polyester capacitors can be mounted either way. Similar to the SMD resistors, they may have a coded value in picofarads instead of a direct value. The 470nF capacitor’s marking could be 474; 220nF could be marked as 224, and 100nF could be 104. Once those are soldered in and the leads cut short on the underside of the PCB, REG1, REG2 and Q1 can be installed. These parts are all in TO-220 packages that mount vertically, as far down as the device leads allow. Make sure that each device goes in the correct location and orientation, with the metal tabs toward the edge of the PCB. Once they are in, install the twoway pin headers for JP1, JP2 and JP3. Orientate LED1 as shown in Fig.13, locate its lens about 6mm above the board surface and then solder and trim the leads. The 13 trimpots (VR1-VR13) can now go in. Check that the correct value is installed at each location, and orientate each one with its adjusting screw as shown on the overlay. Using the correct orientation ensures that the voltages (or required resistance) at their wipers increase with clockwise rotation. Once again, these trimpots may be marked with a code other than the actual resistance value in ohms. So the 500W trimpot may be coded 501 (50 plus one zero), the 1kW trimpot may June 2023  73 be coded as 102 (10 plus two zeroes), the 10kW trimpots may be 103, and the 500kW trimpot may be 504. Bluetooth module The HC-05 Bluetooth Module can come with a right-angle or straight 6-way header strip. If you have a right-angle header, a 6-way header can be installed on the PCB so that the HC-05 right-angle header can be soldered to it, as shown in the overlay diagram. If your HC-05 has a straight header, it is easily installed by inserting the 6-way pin header into the holes allocated and soldering it. Switch S1 is also installed at this stage. If you intend to program microcontroller IC1 yourself instead of using the pre-programmed IC from the Silicon Chip Online Shop (the one supplied in kits is also programmed), a 6-way in-circuit serial programming (ICSP) header will need to be installed (CON1). Boxing it up With the PCB finished, it can be installed in the enclosure. The PCB rests inside the case on the integral mounting bushes. Four screws and nuts secure it; however, the screws do not pass through the bushes but off to the side. Drill the holes for these screws by placing the PCB into the case and drilling four 3mm holes through the PCB mounting holes. The ends of the enclosure can then be drilled and filed for the circular connector and cable glands. You only need to drill holes for the glands you are using. The corresponding holes are not required if you are not using the volt/amp meter or pressure sensor. Fig.14 shows the drilling details for two cable gland sizes that will fit within the designated enclosure; use the correct size hole for each gland you are using. Once the holes are drilled and shaped, mount the glands and the connector in position. Then run the A modified volt/amp LED panel meter in a Jiffy box makes for a convenient way to get a live readout of the air/fuel ratio and lambda. wiring as shown in Fig.15. Use minimum 7.5A-rated wire for the 12V supply, ground and heater wires. For the 8-pin circular panel connector, first connect the sensor leads to the PCB, with the heater and ground leads at the other end. Then cover each soldered pin on the connector with heatshrink tubing to avoid shorts and prevent the leads from breaking. That means you have to slide a length of heatshrink over each lead before soldering it to the connector. After soldering, push the heatshrink over the connection and shrink it using a hot-air gun. The power supply leads must be fed through the cable gland before connecting them to the PCB. The negative lead connects to the vehicle chassis near the battery negative wire, while the +12V lead goes to the vehicle’s switched ignition circuit via an inline fuse holder. Alternatively, for temporary use, the cigarette lighter or 12V DC socket can be used via a plug connector. Finally, secure the board using four M3 × 15mm screws and nuts. Tighten up the cable glands and circular connector to the sides of the enclosure. Sensor extension cable The sensor extension cable is made using a 6-way sheathed and shielded lead from TechEdge (see the parts list). It’s wired as shown in Fig.16. Ensure the wiring is done correctly and use heavy-duty (7.5A minimum) leads in the cable for the H+ and H− leads. The wiring is shown from each connector’s back (wiring side). The 6-pin connector includes rubber sealing glands to be placed over each lead before it is attached to the 2.8mm female crimp spade terminals. Before Fig.14: the drilling diagram for both sides of the bulkhead case. 74 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.15: the overall wiring diagram for the WFMD. Note the use of 7.5A-rated wire where required. attaching the oxygen sensor plug, you must remove the purple locking clip from the socket. Setting it up Power must not be applied at this stage since the 5V supply is not set. Also, leave the oxygen sensor unplugged from the WFMD and ensure there are no jumpers on JP1, JP2 or JP3. It’s then simply a matter of following this step-by-step procedure. With the sensor unplugged and no power connected: 1. Connect a multimeter between TP10 and Rcal, set the meter to read ohms and adjust trimpot VR3 for a reading of 311W. 2. Measure the resistance between TP1 and GND and adjust VR1 for a reading of less than 341W. This ensures a maximum of 5V at TP1 when power is switched on. Apply power (12V) to the circuit, monitor the voltage between TP1 and TP GND and adjust VR1 for a reading of 5.00V. 3. Connect the multimeter between TP GND and TP17 and adjust VR13 for 4V. This initially sets the enginestart battery voltage threshold to 12V. 4. Monitor the voltage between TP6 and TP GND and adjust VR2 for a reading of 3.3V. 5. Monitor the voltage between TP15 and TP GND and adjust VR4 for a reading of 3.92V. 6. Check that TP2 is at about 12V (it will be slightly lower than 12V if the supply is only 12V). 7. Check that the voltage at TP3 is close to -3V, although it could be as low as -2.5V. If this voltage is positive, check the orientation of diodes D2-D4, the placement of Q2 & Q3 and the orientation of the 100μF capacitors. 8. Check that the voltage at TP4 is near +33V. If incorrect, check the orientation of diodes D5-D9 and ZD2. Also check that Q4 is the correct type. 9. With the sensor still unplugged, check that the status LED is initially at low brightness when power is applied. It should then flash at 1Hz, indicating an error with the sensor connection. Fig.16: the wiring diagram for the extension cable, which connects to the Bosch LSU4.9 wideband sensor. siliconchip.com.au Australia's electronics magazine June 2023  75 For more information on making labels, see: siliconchip.au/Help/FrontPanels Calibration Fig.17: the drilling diagram for the external panel meter, which fits inside a UB5 enclosure. The wiring is shown in Fig.15 and the photo opposite. 10. If using the pressure sensor, connect it now and measure its output voltage at the connection to the PCB with both air inputs open to the atmosphere. Adjust VR11 until TP11 is at half the sensor output voltage. This sets pressure calibration to 25mV/kPa. If using a different sensor, you should be able to adjust VR11 so that the calibration is the same. 11. Adjust VR12 until there is no voltage between TP11 and TP12. If adjusting at an altitude above sea level, reduce the value at TP12 by 27.5mV for each 100m above sea level. This is valid up to about 900m. Above that altitude, the adjustment becomes non-linear and will need to be set when at a lower altitude. Leave the adjustment at the 900m level initially, with TP12, 247mV below TP11, until you can redo this at an altitude below 900m. 12. Once step 11 is fully completed, plug the smaller pressure sensor port with silicone sealant to prevent pressure changes at this port. Air/fuel ratio and lambda metering The three methods of displaying the air/fuel ratio and/or lambda include using a multimeter, a volt/amp panel meter or via Bluetooth to a computer or Android-based phone or tablet. When using a multimeter, connect it between MV+ and GND and set it to measure volts to monitor the air/fuel ratio, or between MM and GND for the lambda value. If using a panel meter, connect it as shown in Fig.15 and the photo opposite. It would be a good idea to use a long cable between the WFMD unit and the meter. The wire colours shown 76 Silicon Chip in Fig.15 match the meter wires supplied with the specified meter – yellow and red for current, red and black for voltage and thinner red/black wires for power. The meter needs to be modified by removing its onboard current shunt. A 1W resistor on the WFMD PCB replaces this. To do this, remove the meter’s internal PCB from its surround by levering the side clips and prising it out. The meter shunt is a U-shaped piece of stiff wire between the current measuring wires. It can be desoldered one end at a time and levered out, or simply cut a section out of it. The meter can also be installed in a small UB5 enclosure with the wiring via a cable gland on one side. We made the cutout in the base rather than the lid, as shown in Fig.17. Drill a series of holes around the inside perimeter of the cutout, knock out the inside piece and file it to the correct shape. The meter surround must be installed first before inserting the meter PCB into it. The existing V (Volt) and A (Amps) labels on the meter display can be covered over with lambda and air/ fuel labels, as seen in our photos. These labels are included in the front panel artwork download on the Silicon Chip website (Fig.18). Print them onto suitable sticky labels and attach them to the meter front screen. The front panel label can also be printed out and attached similarly. The air/fuel ratio can be shown for two different fuels, such as petrol and LPG, or E10 and standard 91 octane petrol, designated AF1 and AF2. For example, you could set AF1 for petrol (14.7:1 stoichiometric) and AF2 for LPG (15.5:1). The two readings are selected using jumper shunt JP3. When a jumper is in, the selection is AF1; when the jumper is out, it is AF2. The JP3 contacts can be wired to a toggle switch or other latching type to easily switch between the two options. VR5 and VR6 set the stoichometric air/fuel ratios for AF1 and AF2 for the meter display, respectively, while VR7 and VR8 set the equivalent values for the remote Bluetooth display. The adjustments can be made by inserting a jumper shunt on JP2. This sets the WFMD to produce a lambda 1 output. For the multimeter (MM) output, adjust VR9 for a 1.00V reading in this condition. The narrowband output does not require calibration and should already be at 450mV ±5mV. If using the panel meter, adjust VR10 to show 1.00 on the current (A) display. For the voltage display or to calibrate the MV+ output, adjust VR5 for the desired stoichiometric AFR reading with JP3 in and similarly adjust VR6 with JP3 out. The maximum AFR that can be set for lambda = 1 is 17.9. This results in an output of 33V (AFR 33:1) for a lambda of 1.84. To calibrate the Bluetooth display, switch off the power to the WFMD unit and then switch it on with a jumper shunt at JP2. Then open the GUI and connect it to the WFMD (details on doing that are in the panels). For AF1, adjust VR7 for a reading at TP7 that is one-tenth the desired stoichiometric AFR (eg, 1.47V for 14.7:1). Make the same adjustment at TP8 using VR8 for AF2. The trimpots may require a slight re-adjustment when viewed on the Fig.18: the labels for the WFMD and its panel meter. The main label would look best printed on a transparent label (you can download a PDF from our website). Australia's electronics magazine siliconchip.com.au GUI display. The JP3 setting can be changed to switch between calibrating AF1 and AF2. Note that removing JP2 will not stop the Bluetooth display from showing the lambda of 1 immediately. You will need to switch the power off and then on again with JP2 out before the Bluetooth display will show other values. Testing with the O2 sensor The next step is to check the controller’s operation with the oxygen sensor connected. Switch off power to the WFMD and connect the sensor lead to the controller. Now check that there is resistance between the sensor’s H+ and H− heater terminals, measured at the PCB H+ and H− terminals. You should get a reading of about 3.2W at 20°C. When power is applied, the sensor will become hot, so first remove the plastic protective cap. Place the sensor on a surface that can withstand rapid temperature changes and temperatures up to 200°C. Glass cookware (eg, Pyrex) is ideal, but do not hit the sensor against the glass or its ceramic element could crack. You could also use a clean brick, flat stone, or ceramic tile. Remember that the sensor tip can become hot enough to burn skin when power is applied. You will need a 12V supply that can deliver about 2A. Apply power and check that LED1 lights dimly for around 10s before flashing rapidly. Any display connected should show near full lean readings, such as a lambda of 1.84. If nothing happens, check that the VR13 adjustment gives a voltage reading of 4V at TP17. If your supply is just under 12V, you will need to readjust VR13 so that TP17 has a lower supply voltage threshold setting for the WFMD to start. The panel meter can be wired up in a UB5 case as shown. If the controller still doesn’t appear to be operating correctly, check for assembly or wiring errors. You can also test the sensor temperature control by installing JP1. The voltage across the sensor cell is then echoed at the narrowband output. Typically, this should be around 684mV ±10mV. Having completed the above tests, adjust VR13 so that TP17 is at 4.33V. This sets the controller to wait until the supply voltage reaches 13V (4.33V × 3), meaning the engine must start before it begins heating the sensor. Additional tests can also be carried out after the oxygen sensor is fitted to a vehicle. The Bosch LSU4.9 wideband sensor can be installed in the exhaust pipe by screwing it into the existing threaded boss of the original narrowband sensor or by adding a suitable threaded boss. This should be as close to the engine as possible. However, the exhaust gas temperature at the sensor must be under 780°C under all engine operating conditions, Fig.19: the Bosch sensor must be mounted perpendicular to the exhaust stream, and it must always be inclined 10° or more horizontally. siliconchip.com.au Australia's electronics magazine or the sensor might overheat. In general, installing the wideband sensor in the same position or near the existing narrowband sensor will be OK. You can check for sensor overheating by monitoring the heater impedance with jumper JP1 shorted. In this case, the narrowband output shows the sensor cell impedance. A reading much lower than 680mV DC indicates overheating. In that case, relocate the sensor to a cooler section of the exhaust manifold, further from the engine. The following points should also be taken into consideration: 1. If the sensor is to be used in a turbocharged engine, it must be installed after the turbocharger. 2. The exhaust pipe section before the sensor should not contain any pockets, projections, protrusions, edges or flex-tubes etc, to avoid the accumulation of condensation water. It is recommended to locate the sensor on a downward-sloping section of the pipe. 3. The sensor must be mounted perpendicular to the exhaust stream so it can constantly monitor fresh exhaust gas. It must always be inclined at least 10° from horizontal – see Fig.19. This inclination limit must account for the vehicle being on sloping ground. This is necessary to prevent condensation from collecting between the sensor housing and the element. 4. The recommended material for the threaded boss in the exhaust pipe is temperature-resistant stainless steel to the following standards: DIN 174401.4301 or 1.4303, SAE 30304 or 30305 (USA). Fig.20 shows the threaded boss dimensions. The sensor June 2023  77 The O2 sensor is shown above, with it attached to the extension cable at right. thread must be covered completely when the sensor is installed. 5. Applying high-temperature grease on the boss screw threads is recommended. The tightening torque is 40-60Nm (30-45ft-lbs). 6. The sensor must be protected if an under-sealant such as wax, tar or spray oil is applied to the vehicle. 7. The sensor must not be exposed to strong mechanical shocks (eg, installation or removal using an impact driver). If it is, the sensor element could crack and destroy the sensor without visible damage to the housing. 8. The sensor and its connecting cable should be positioned to avoid damage due to stones or other debris thrown up by the wheels. 9. Do not expose the sensor to water drips from the air conditioner or sources such as windscreen run-off during rain or when using the windscreen washer. The resulting thermal stress could damage the sensor. 10. The sensor heater must remain off until the engine starts. This means that VR13 must be correctly adjusted to ensure heating does not begin until after the engine has started and the battery voltage rises. Using the S-curve (narrowband) output As mentioned earlier, the S-curve narrowband output from the WFMD can replace the signal from a narrowband sensor. That is only possible if the vehicle originally uses a zirconia-­ type narrowband oxygen sensor. If the vehicle already has a wideband sensor, its output should not be replaced with the S-curve signal from the WFMD. A less common type of narrowband lambda sensor has a ceramic element made of titanium dioxide. This type does not generate a voltage but instead changes its resistance according to the oxygen concentration. This type of sensor cannot be simulated using the S-curve signal from the WFMD. Identifying the sensor leads To replace the existing sensor with the S-curve output, you must first identify the leads running from the sensor to the ECU. If you have a vehicle wiring diagram, that will make it much easier. Typically, there are four narrowband sensor variations: 1. If the sensor has one lead, this will be the signal wire, and the sensor body will be ground. 2. If the sensor has two leads, one will be the signal lead, and the other will either be the signal common or, in the case of a heated sensor, a +12V heater lead. For a heated sensor, the body forms a common ground for both the signal and heater circuits. 3. A three-wire sensor usually has Heater+ (H+), Heater− (H−) and a sensor signal lead, with the body as the signal ground. Alternatively, it could have a sensor signal lead, a sensor ground lead and a heater H+ lead, with the sensor body as heater H−. 4. A four-wire sensor is similar to a three-wire sensor but with ground leads for both the signal ground and H−. Screen 1 (left): the export settings for the Windows version of the GUI application. Screen 2 (below): to run the Windows application, you need to run the “air_display_3_pde” executable file by double-clicking it or similar. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au Making a Bluetooth connection with a PC Fig.20: the dimensions required for the threaded boss which goes into the exhaust pipe. The thread on the sensor must be completely covered when installed. Having more than four leads suggests that the sensor is probably a wideband type. In each case, the leads are quite easy to identify, but first, a word of warning. Do not measure the narrowband sensor impedance with a multimeter set to read ohms. The current produced by the meter when measuring resistance could damage the sensor. Note also that the maximum loading for the sensor is 1μA. This means that to measure the voltage produced by a narrowband sensor, the meter must have an input impedance higher than 1MW. Digital multimeters (DMMs) generally have an input impedance much higher than 1MW, but an analog meter may not have the required high impedance. The first step in identifying the leads is to set your DMM to read DC volts, then connect the negative lead of the DMM to the chassis. Next, start the engine and probe the sensor leads with the DMM’s positive lead. A sewing pin can be used to pierce the wire Several graphical user interface (GUI) applications allow you to view the air/ fuel ratio and lambda values via Bluetooth. For a computer running Windows, we use an application based on ‘Processing’ (https://processing.org/) written by Tim Blythman. The download includes the Processing source code (“air_display_3_pde. pde”) plus a standalone version that will run in Windows. For macOS computers, the Processing file can be loaded into the Processing software for macOS (available from https://processing.org/) and then run or exported to a standalone app using the File → Export Application option. Screen 1 shows the export settings for the Windows version. When using Processing on a Mac, the macOS options for the platform will be available instead of the Windows and Linux options. By ticking the Embed Java box, the program will run without having Java installed on the computer. The Windows standalone application folder contains 268 files totalling 258MB. To run it, double-click the “air_display_3_pde.exe” file (see Screen 2). The GUI allows the COM port for the HC-05 to be selected using the < and > keys on your keyboard. Note that you don’t need to press shift or caps lock; just press the keys with those labels. Once the correct COM port has been selected, press Enter/Return. Help is available by pressing the H key. Pairing Before the display can show values, you must pair the HC-05 Bluetooth module with the computer. To do this on a Windows machine, click Start → Settings → Bluetooth & Devices, then power up the WFMD unit with a jumper shunt in JP2. This is so the WFMD will show lambda=1 values. The HC-05 Bluetooth module will be powered, and its LED should blink at 4Hz. Click “Add Device” on the computer to find the HC-05 Bluetooth Module. When found, enter the password (1234 or 0000). If the Bluetooth connection does not occur, try pressing and holding button S1 next to the Bluetooth module when power is applied to the WFMD. Hold it until pairing occurs. The computer will automatically pair with the HC-05 module when both are subsequently powered up and the computer’s Bluetooth is on. You will then need to know the COM port it has been allocated. To do this, under Bluetooth and Devices, select Devices, then scroll down to More Bluetooth Settings. Open Settings, select the COM Ports button, and the connected COM ports will be shown, similar to Screen 3. Make a note of the COM port that the HC-05 connects to. When you select the correct COM port on the GUI and press Enter/Return, the HC-05 module should change its onboard LED flash rate. It should give two flashes per 1.5 seconds, indicating that communication has been established. The display should then show a lambda value of 1.00 and the stoichiometric AFR set by JP3 and trimpot VR7 (JP3 closed) or VR8 (JP3 open) – see Screen 4. You can now make the final adjustments to VR7 and VR8 for the required air/fuel ratio readings. Remove the shunt from JP2 after switching the power off, and the WFMD is ready for use. Screen 3: you need to make a note of which COM port the HC-05 module is connected to. In this example, it’s connected to COM4. Screen 4: when the application is up and running it should initially show you the stoichiometric air/fuel ratio (AFR) and lambda value. siliconchip.com.au Australia's electronics magazine June 2023  79 Setting up the Android app There are two ways to install the Android app: via the Google Play store or a downloaded APK file. For the Play Store, open Google Play and search for “Silicon Chip WFMD”. You should find the “Silicon Chip WFMD BT interface” app. Clicking the Install button should be all you need to do. Otherwise, you can go directly to the page via this link: siliconchip.au/link/abl6 To install the APK file, first, you need to enable the “Install apps from external sources” option. Unfortunately, this appears in different places on different devices. In some cases, it will be under Settings → Apps → Special app access or Settings → Apps → Advanced → Special app access. We have also seen it under Settings → Security → More settings → Install apps from external sources. If your Settings has a search option, as many do now, you can try searching for “unknown” (Install unknown apps) or “special” (Special app access). That method can be a lot faster than trawling through the settings. Once enabled, download or copy the APK file (available from the Silicon Chip website) onto your device and launch it. Some devices may prompt for granting the above permission when you do this, if you haven’t already. After installing the APK file, we recommend turning that setting back off to avoid unwanted, malicious apps from being installed. Next, you need to pair the Bluetooth device. Put a shunt on JP2 and prepare the WFMD for being powered up. Remember that it might need a supply voltage above 13V to be enabled, in which case you will have to start the engine. Go to Settings → Bluetooth on your device, then power up the WFMD. A new Bluetooth device should appear shortly after – see Screen 5. Click on it, then enter the password (1234 in most cases, although some modules may use 0000). If the Bluetooth connection does not occur, try pressing and holding button S1 next to the Bluetooth module when power is applied to the WFMD. Hold it until pairing occurs. Once paired, launch the app. There are three buttons on the main screen, visible in Screen 6, and pressing the one marked “Connect Bluetooth Device” should allow you to select from a list of Bluetooth devices, choose the one for the WFMD. The lambda and AFR displays should starting show data, with lambda = 1 (due to JP2 being shorted) and your stoichiometric AFR. You can now fine-tune the value(s) using VR7, VR8 and JP3. Once it’s all working, power the WFMD down, remove the shorting block from JP2, power it back up and check that the values are displayed correctly. If the app complains about Bluetooth Permissions or does not show any devices to connect to, ensure that Bluetooth is turned on and also check that the Nearby Devices permission is allowed under permissions for the Silicon Chip WFMD BT interface app. We found that the app would occasionally say that the permission had been denied, even when it was allowed, but that did not actually prevent it from working. Screen 5: the Bluetooth connection should appear shortly after starting the WFMD. If it doesn’t, press and hold S1 next to the Bluetooth module while power is applied to the WFMD. Keep holding it until pairing occurs. 80 Silicon Chip Australia's electronics magazine insulation, but make sure you seal any holes you make with neutral-cure silicone sealant afterwards, to prevent corrosion. The sensor’s H+ lead will be at +12V, while its signal voltage lead will vary, cycling about an average of 450mV once the sensor has finished heating. Once these two leads have been identified, switch off the engine and unplug the sensor. The H− terminal can now be identified – it’s the one that gives a low resistance reading (typically 5W and usually less than 10W) to the previously identified H+ terminal. If there is no such wire, the H− connection is via the chassis. But ensure you do not connect the meter probe to the previously identified signal terminal when the meter is set to read ohms! The signal ground wire will be the one remaining wire (or the chassis connection, if there are none remaining). Error codes In some cars, the ECU will check that the sensor is connected and produce an error code if it detects anything is amiss. In most cases, the S-curve narrowband signal from the WFMD unit will be accepted as valid, but there can be exceptions. First, the ECU may check the sensor’s impedance to determine if it is sufficiently heated (ie, when its impedance falls below a particular value). The impedance the ECU will measure at the WFMD’s narrowband output will be 100kW, which might be out of range for some sensors. If this happens, you will need to change the value of that 100kW output resistor to stop the ECU from generating an error code. Check the sensor data from the manufacturer to determine the expected impedance. Failing that, experiment with different values. It could be above or below 100kW. Heater fault indications Some ECUs will also indicate a fault if the heater leads to the oxygen sensor are disconnected. In that case, you will have to keep those wires connected to the old sensor and mount it away from parts that could melt, such as rubber and plastics. Ideally, mount it against the metal chassis. If doing this, ensure the heated sensor cannot be accidentally touched as it can run very hot. You could place a metal cover over it for protection. siliconchip.com.au Alternatively, you could make up a resistance box with the same nominal resistance as the sensor’s heater element when it is hot. The hot resistance will be higher than the cold resistance. It can be measured by disconnecting the sensor lead after the engine has reached operating temperature and then measuring the heater resistance using a DMM. The resistors should be installed in a diecast case and must be rated to handle the expected power dissipation, assuming a 14.8V maximum supply and a 50% power derating. For example, if the heater’s hot resistance is 12W, it will dissipate up to 18.25W (14.8V2 ÷ 12W). In practice, given the derating requirement, a 40W resistor would be needed. In this case, the heater could be simulated by connecting four 47W 10W resistors in parallel. Make sure the resistors are secured, and all wiring is prevented from shorting to the enclosure and supported from breakage due to movement. Using the narrowband output If feeding the WFMD’s narrowband output to the ECU, connect the S-curve output to the sensor+ signal input of the ECU. Do not make a direct connection to the sensor’s negative input to GND on the WFMD unit, as that could cause a ground loop. Usually, the ground connection will not be required, but if necessary, add a 10W ¼W resistor in series to minimise the ground current. Check that there is at least 4.33V at TP17 (adjusted using VR13) to ensure the engine is started before the sensor is heated. Ideally, you should use an enginecode reader to check for and clear any resulting fault codes. However, without access to this, fault codes can usually be cleared by disconnecting the vehicle’s battery for a minute or so. This method of clearing faults does have its drawbacks. Disconnecting the battery may affect a security-coded sound system, meaning that the security code will have to be re-entered. Any clocks will be reset, and also it could reset some of the learned parameters stored in the car’s ECU or transmission controller. Learned parameters include engine timing (to prevent pinging), fuel injector trims and transmission shift rates. These are tabled values made by the ECU and/or TCU during normal siliconchip.com.au Tips on removing or replacing an oxygen sensor To remove an existing oxygen sensor, first make sure you remove the correct sensor. The required sensor is the one that’s between the exhaust manifold and the catalytic converter. A second oxygen sensor may be located downstream from the catalytic converter to monitor its operation. Removing the narrowband sensor may be difficult if you do not have the correct tools. The required tool depends on the sensor’s placement. With limited access, you may have to resort to using an open-ended 22mm (or ⅞-inch) spanner. In most cases, though, you should be able to use a special oxygen sensor removal tool. This is a 22mm socket with a slit along one side so that you can slip it over the oxygen sensor wiring. It’s common for the original oxygen sensor to seize in the threaded boss in the exhaust manifold pipe, in which case the hexagonal section will refuse to budge. If using an open-ended spanner, it will tend to spread open under tension and slip, rounding off the hexagonal edges of the sensor nut. Removing a seized oxygen sensor can be tricky, even with the correct tool. We used a thread-penetrating lubricant such as “Loctite Freeze & Release Lubricant” (FAR IDH1024403) to help free it. We have also heard good things about Cre-Oil for this job. Other ‘penetrating oils’ are available from SCA, Chemtools, Protech, Master etc. Due to the risk of rounding, it’s generally a good idea to spray the junction of the O2 sensor and threaded boss with one of these penetrating oils and wait a little while (eg, half an hour or more) before attempting removal. Suppose it proves impossible to remove, and you are not concerned about damaging the original sensor. In that case, you can use a hacksaw or grinder to cut the sensor apart just above the 22mm hexagon nut section. Then you can use a 22mm hexagonal socket and breaker bar for added leverage to remove the remaining section. If you refit the existing sensor, apply high-temperature grease to the screw threads. That will make it easier to remove next time. A new sensor (such as the Bosch LSU4.9 sensor) will probably be supplied with this grease already applied to the thread, or supplied in a small sealed plastic bag along with the sensor. The factory oxygen sensor on a Volkswagen Golf Mk.7. Typically, oxygen sensors are generally installed in a similar position a short distance from the exhaust manifold. Access is not too bad in this case. Note the heat shielding over the exhaust manifold, with a hole for the sensor (and on the firewall behind it). Australia's electronics magazine June 2023  81 SC6721 Kit ($120 + postage) Includes the PCB and all the parts that mount directly on it; the microcontroller comes pre-programmed (the Bluetooth module is also included). You need to separately purchase the oxygen sensor, case, wiring, fuse holder, off-board connectors (including those for the O2 sensor) and optional parts like the pressure sensor and LED display. operation to improve engine running and fuel economy based on oxygen sensor readings and knock sensing, and optimise shift speeds while preventing hard shifts. If they are cleared, the engine and transmission may take a while to restore these parameters. Some automatic transmissions also ‘learn’ and adapt to driving style and can take some time to retrain after a power cut. If, despite everything you do, the engine still runs poorly or the ECU logs a fault code, the original narrowband sensor will need to be reinstalled. In that case, the wideband sensor can still be installed separately. Pressure sensor connections If you wish to use the pressure sensor, which will give more accurate readings, it is necessary to drill a small hole through the exhaust pipe and then braze a short length of metal tubing (steel or brass) to the pipe near the sensor. This should be located downstream from the sensor, so it doesn’t provide a condensation point above the sensor. The tube length should be such that the exhaust pipe heat is dissipated sufficiently for the rubber pressure tubing to attach without softening or burning. If you don’t wish to monitor the pressure, leave the pressure sensor disconnected from the WFMD unit. The WFMD will then operate assuming standard atmosphere pressure within the exhaust at the sensor location. The resulting error will depend on how much back-pressure the exhaust system generates at a given throttle setting. Screen 6: the Android app looks like this when data is being received from the WFMD. Tailpipe mounting If you do not wish to install the wideband oxygen sensor permanently, an alternative is to mount it in a tailpipe extension. This tailpipe extension can then be slid over the end of the tailpipe and clamped in position, as shown in Fig.21. However, readings obtained using this method will be affected by the catalytic converter, so they won’t be as accurate. That’s because the catalytic converter alters the exhaust gas oxygen content. Some catalytic converters also include an air bleed to feed oxygen into the exhaust, allowing full catalytic operation with rich gases and minimising unburnt fuel. This won’t be a problem in older vehicles that don’t have a catalytic converter. Also, consider the effect of exhaust dilution, where air mixes with the exhaust near the tailpipe. This can cause a slightly leaner than actual reading. When the sensor is fitted to a tailpipe extension, TP17 in the Wideband Fuel Mixture Display unit can be set for less than 4.33V. This will allow the sensor heating to start immediately when the WFMD unit is powered, instead of having to wait until the battery voltage rises when the engine is started. This is acceptable, provided the sensor is stored upright in a dry environment, to prevent moisture condensing in the sensor. Follow Fig.21 closely if you intend to mount the sensor in a tailpipe extension. Using the dimensions shown, the sampled exhaust gas is taken sufficiently upstream from the end of the tailpipe to prevent dilution with outside air. The pipe and clamp can be steel or brass, but use a stainless steel SC boss to mount the sensor. Fig.21: the Bosch sensor can also be mounted in the tailpipe. It should be mounted as shown in this diagram to minimise exhaust gas dilution. 82 Silicon Chip Australia's electronics magazine siliconchip.com.au