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Pt.3: By JOHN CLARKE Wideband Oxygen Sensor Controller Mk.2 In Pt.2 last month, we gave the full assembly details for our new Wideband Controller Mk.2 and its companion Display Unit. Our final article this month describes how the oxygen sensor is installed in a car and connected to the Wideband Controller. A S MENTIONED in Pt.1, the Bosch LSU4.9 wideband sensor can be installed in the exhaust pipe by screwing it into a suitable threaded boss. This should be positioned as close to the engine as possible. Note, however, that the exhaust gas temperature under all engine operating conditions at the sensor position must be less than 780°C otherwise the sensor may overheat. In general, installing the wideband sensor in the same position as the existing narrowband sensor should be OK. You can check for sensor overheating by monitoring the heater impedance. This is done with jumper JP1 installed. The wideband output as measured between the tip and sleeve of a 3.5mm jack plug should normally be 684mV DC or about ±2% above and below this. If the Display Unit is connected (and is set up to measure 82  Silicon Chip lambda), it should show 0.85 (0.840.86 range). If the sensor has overheated, the above-listed voltage or value will fall significantly. And if it’s severely overheated, the Wideband Controller indicating LED will revert to its dim indication. An overheating sensor will have to be relocated to a cooler section of the exhaust manifold, ie, further from the engine. The following points should also be taken into consideration: (1) The sensor must not be mounted in the exhaust manifold of a turbocharged engine. Instead, it must be installed after the turbocharger. (2) The exhaust pipe section prior to the sensor should not contain any pockets, projections, protrusions, edges or flex-tubes etc, to avoid the accumulation of condensation water. Locating the sensor on a downward slope of the pipe is recommended. (3) The sensor must be mounted perpendicular to the exhaust stream so that it can constantly monitor fresh exhaust gas. It must also be mounted so that it is inclined at least 10° from horizontal (electrical connection upwards) – see Fig.19. This is necessary to prevent condensation collecting between the sensor housing and the element. (4) The recommended material to use 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 (US). Fig.20 shows the threaded boss dimensions. Note that the sensor thread must be covered completely. (5) The use of high-temperature resistant grease on the screw thread of the boss is recommended. The tightening siliconchip.com.au (VERTICAL PLANE) Mounting The Oxygen Sensor On The Exhaust 25 10.5 +/-0.35 3 > 10° 23 ALL DIMENSIONS IN MILLIMETRES (HORIZONTAL PLANE) Fig.19: the Bosch wideband sensor must be fitted to the exhaust pipe at an angle of at least 10° above horizontal. This is necessary to ensure that any condensation drains out of the unit. torque is from 40-60 Nm. (6) The sensor must be protected if an under-sealant such as wax or tar or spray oil is applied to the vehicle. (7) The sensor must not be exposed to strong mechanical shocks (eg, during installation or removal using an impact driver). If it is, the element could crack and destroy the sensor without there being visible damage to the housing. (8) Both 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 from sources such as windscreen run-off during rain or when using the windscreen washer. The resulting thermal stress could damage of the sensor. (10) Never switch on the sensor heating until the engine starts. This means that VR2 must be correctly adjusted to ensure heating does not begin until after the engine has started, as detected by a higher battery voltage (see settingup procedure last month). Using the S-curve output As mentioned last month, the S-curve output from the Wideband Controller can be used to replace the existing narrowband signal. However, the vehicle must be currently using a zirconia-type narrowband oxygen sensor. If the vehicle already has a wideband sensor, then this sensor’s output should not be replaced with the S-curve signal from the Wideband Controller. siliconchip.com.au 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. Once again, this type cannot be simulated using the S-curve signal from the Wideband Controller. Identifying the sensor leads In order to replace the existing sensor with the S-curve output from the Wideband Controller, you first need to identify the leads running from the sensor to the ECU. Basically, 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 3-wire sensor usually has Heater+ (H+), Heater- (H-) and sensor signal leads, 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 4-wire sensor is similar to a 3-wire sensor but with ground leads for both the signal ground and H-. 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. TAPPED WITH M18 x 1.5 THREAD Fig.20: this diagram shows the dimensions of the threaded boss that’s used to attach the sensor. It must be made of stainless steel and should cover the sensor’s thread completely. The tightening torque is from 40-60 Nm. The reason for this is that the current produced by the meter when measuring resistance may 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 1MΩ. Digital multimeters (DMM) generally have an input impedance much higher than 1MΩ but an analog meter may not have the required high impedance. The first step in identifying the leads is to set your DMM to DC volts, then connect the negative lead of the DMM to chassis. That done, it’s a matter of starting the engine and probing the sensor’s leads with the DMM’s positive lead (a pin can be used to pierce the wire insulation but seal any holes with silicone afterwards to prevent corrosion.). The sensor’s H+ lead will be at +12V, while its signal voltage lead will vary but should average over time at about 450mV. 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 of typically 5Ω (and usually less than 10Ω) to the previously identified H+ terminal (warning: do not connect the meter probe to the previously identified signal terminal when making resistance measurements). The signal ground terminal is the one remaining. In some cars, the ECU will check that the sensor is connected and August 2012  83 How To Remove The Original Original Narrowband Sensor W HEN REPLACING an existing narrowband sensor with the LSU4.9 wideband sensor, make sure you re­ move the correct unit. The required sensor is the one that’s between the exhaust manifold and the catalytic converter. A second oxygen sensor located downstream from the catalytic converter is there to monitor the latter’s operation. Removing the narrowband sensor may be difficult if you do not have the correct tools. The type of tool required depends on the sensor’s placement. With limited height access, you may have to resort to using an open-ended 22mm (or 7/8-inch) spanner. In most cases, though, you should be able to use a special oxygen sensor removal tool. This uses a 22mm socket that has a slit along one side to allow the oxygen sen- sor wires to protrude (see photo below). It’s also common for the original oxygen sensor to seize in the threaded boss in the exhaust manifold pipe. As a result, the nut will refuse to budge and will simply start to “round off” under spanner tension. The main difficulty is that the socket or spanner needs to be an open-ended type, as a ring spanner or standard socket will not fit over the sensor connector. And an open-ended spanner tends to spread open under tension. Even with the correct tool, removing a seized oxygen sensor can be difficult. In our case, we used “Loctite Freeze & Release Lubricant” (Part No. FAR IDH1024403) to help free it. This “shock cools”, penetrates and lubricates the screw threads and this allowed us This special oxygen sensor removal tool allows the sensor’s leads to exit via a slot in the side of the socket. Make sure that the sensor leads turn with the sensor as it is undone. produce an error code if it detects that anything is amiss. In most cases, however, the S-curve signal from the Wideband Controller will be accepted as valid but there are 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). However, the impedance the ECU will measure at the Wideband Controller’s S-curve output will be 150Ω and this may be incorrect for some sensors. This means that the 150Ω output resistor may have to be changed in 84  Silicon Chip some cases, to prevent an error code from the ECU. 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 the original heater connections to the old sensor and mount it in a convenient place away from parts that could melt (eg, against the firewall). Just make sure that the heated sensor cannot be accidentally touched, as it can run very hot. Alternatively, you can make up a to eventually loosen and remove the sensor. If you are not concerned about damaging the original sensor, its connector can be cut off so that a ring spanner can be slipped over it and onto the hexagonal nut. This can then be “tapped” with a hammer in the anticlockwise direction to loosen the sensor. Note, however, that this method will probably crack the ceramic material inside the sensor, leaving it permanently damaged. Which ever method is used, Freeze and Release Lubricant is still recommended because it makes removal much easier. It also helps prevent the sensor nut from being rounded off, which would then make removal extremely difficult. Note that special high-temperature grease should be used on the screw threads if you refit the existing sensor. That way, it will be easy to remove next time. A new sensor (such as the Bosch LSU4.9 sensor) will be supplied with this grease already applied to the thread. resistance box that has the same nominal resistance as the sensor’s heater element when hot. The hot resistance will be higher than the cold resistance and can be measured by disconnecting the sensor lead immediately after the engine has reached operating temperature and then measuring the heater resistance using a DMM. The alternative resistors should go in a diecast case and must be rated for to handle the power dissipation. In practice, the power rating is calculated by assuming a 14.8V maximum supply and a 50% derating. For example, if the heater hot resistance is 12Ω, then 14.8V2 ÷ 12Ω = 18.25W. In practice, a 40W resistor would be required and so the heater resistance could be simulated by connecting four 47Ω 10W resistors in parallel. Because the Wideband Controller’s S-curve output needs to simulate the original narrowband sensor, it’s a good idea to check the operation of the original narrowband sensor when the car is running. This can be done using a multimeter and an oscilloscope to monitor the sensor’s output. Alternatively, an OBD (On-Board Diagnostics) scan tool that shows live or real-time parameter data can be used to monisiliconchip.com.au Mounting The O 2 Sensor In A Tailpipe Extension EXHAUST TAILPIPE SENSOR CLAMP FOR ATTACHING TO EXHAUST PIPE Fig.21: follow this diagram to build a tailpipe sensor unit if you don’t want a permanent installation. MOUNTING BOSS EXHAUST OUT EXHAUST FLOW 150 ALL DIMENSIONS IN MILLIMETRES I f you do not wish to install the wideband O2 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 – see Fig.21. Note, however, that any readings obtained using this method will be affected by the catalytic converter and so won’t be as accurate. That’s because the catalytic converter reacts with the exhaust gas and changes the oxygen content. In addition, some catalytic converters include an air-bleed to feed oxygen tor the sensor voltage, if this feature is supported on your vehicle. When the engine is warm and idling, the sensor reading should oscillate above and below 450mV at a rate dependent on the sensor’s response rate and the ECU. By using the oscilloscope, the frequency of oscillation and the voltage can be directly measured (a multimeter will probably not respond quickly enough to show the full cyclic voltage range). A typical narrowband sensor response is shown in Fig.22. Connecting the controller The Wideband Controller must be connected to the vehicle’s 12V supply. The two ground wires are connected to chassis (adjacent to the lead from battery’s negative terminal), while the positive lead connects to the ignisiliconchip.com.au 100 250mm LENGTH OF 38mm (1.5") PIPE into the exhaust to allow full catalytic operation with rich gases. This won’t be a problem in older cars that don’t have a catalytic converter. Note that when the sensor is fitted to a tailpipe extension, TP2 in the Wideband Controller can be set for 4V. This will ensure that the sensor heater is immediately powered when the Wideband Controller is powered, without having to wait until the battery voltage rises when the engine is started (note: we don’t have to wait in this situation because condensation is no longer a problem). tion supply. Make sure this supply remains at +12V while the engine is started as some switched ignition supplies (eg, for the sound system) are disconnected during engine starting. Next, replace the existing narrowband sensor with the wideband sensor, then connect the S-curve output from the Wideband Controller to the sensor+ signal input of the ECU. That done, check that TP2 in the Wideband Controller has been adjusted to 4.33V to ensure that the engine is must be started before the sensor is heated (see setting-up procedure last month). To do this, first switch on the ignition without starting the engine and check that the LED on the controller is only dimly lit. This indicates that sensor heating has not yet started. Conversely, if the LED lights brightly, it indicates that the sensor is heating After use, make sure that the sensor is stored upright in a dry environment, to prevent moisture forming in the unit. Fig.21 should be followed quite closely if you intend mounting the sensor in a tailpipe extension. By 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 materials can be made of steel or brass but use a stainless-steel boss for mounting the sensor. and so VR2 will need to be adjusted to give a higher voltage at TP2. In practice, you may have to experiment to get the best setting for VR2 (as measured at TP2). If TP2 is too low in voltage, sensor heating will start before the engine starts. Conversely, if TP2 is too high, sensor heating will not start immediately after engine-starting and will not kick in until the battery voltage rises sufficiently. You can confirm this by revving the engine a little until the battery voltage rises high enough to start the sensor heating. Note that you will need to switch off the Wideband Controller via the ignition and then back on again to have any changes to the TP2 voltage read by the controller. That’s because this voltage is only checked at power-up, so always switch the controller off and on again each time you adjust VR2. August 2012  85 Using A Wideband Sensor In A Permanent Installation A S A TEST, we installed a wideband sensor in place of the original narrowband sensor in a 2004 Holden Astra. The S-curve output from the Wideband Controller was then fed to the car’s ECU (in place of the output from the original sensor). This worked well, with no error codes produced by the ECU provided that the heater connections to the original narrowband sensor remained in place. In operation, the narrowband signal from the Wideband Controller enabled the engine fuel mixture to cycle correctly The correct setting for VR2 is critical to prevent sensor damage. Basically, it prevents the sensor from heating before the engine exhaust has blown out any condensation. Note, however, that condensation only occurs after the sensor has cooled. If the Wideband Controller starts heating the sensor when it is already warm but before the engine has started, then that’s OK. In practice, this means that VR2 should be set when the battery is at its normal resting voltage – ie, after the engine has been off for some time. For example, the battery voltage may be above 13V when the engine has just been switched off, but it will eventually drop to below 13V. Once VR2 has been correctly adjusted, start the engine and monitor the S-curve output. It should cycle above and below 450mV in a similar manner to the original narrowband sensor. If the S-curve simulation proves unsuccessful, either because the engine runs poorly or the ECU logs a fault code, then the narrowband sensor will have to be reinstalled. The Wideband above and below the stoichiometric value. In short, it proved to be compatible and the Holden Astra’s engine ran normally. This is in marked contrast to the Wideband Controller described in September and October 2009. With that controller, the overall response to the air/fuel mixture was too slow compared to that from the original narrowband sensor. As a result, the engine RPM constantly varied at a fixed throttle setting as the air/fuel ratio varied above and below stoichiometric. This in turn varied Sensor will then have to be installed in a separate position. Often, fault codes can be cleared by disconnecting the vehicle’s battery for a minute or so. Otherwise an engine-code reader will be required to clear the fault. Note that disconnecting the battery may affect a security-coded sound system on some older cars, which means that and the security code will have to be re-entered. Disconnecting the battery or clearing a fault code using an engine-code reader could also reset some of the learned parameters stored in the car’s ECU. These parameters include such things as engine timing (to prevent pinging) and fuel-mixture trim. These are tabled values made by the ECU during normal operation to improve engine running and fuel economy based on oxygen sensor readings and knock sensing. As a result, the engine may take a while to restore these parameters if they are cleared. Pressure connections If you wish to monitor the exhaust pressure, it will be necessary to drill 0.55V TIME 0.45V 0.35V 1.25sec Fig.22: a typical narrowband sensor response when the engine is warm and idling. The output oscillates above and below 450mV as the ECU maintains a stoichiometric mixture. 86  Silicon Chip the vehicle’s road speed at constant throttle settings. As a result, the original Wideband Controller Mk.1 was unsuccessful as a permanent installation, at least in the Holden Astra. By contrast, our new Wideband Controller Mk.2 using the LSU4.9 sensor produces a much lower range of RPM cycling at constant throttle above idle and with no load (transmission in Neutral). In fact, it’s no more than occurs with the original narrowband sensor in place and is completely unnoticeable when the vehicle is being driven. 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 on the downstream side, so that 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 burning. If you don’t wish to monitor the pressure, just leave the port open (or leave the pressure sensor out and install the links as described in Pt.2). Wideband controller tests If you strike problems with your Wideband Controller, the best way to troubleshoot it is to first alter it to measure the oxygen content in air. That way, you can check the operating voltages in the circuit while the sensor monitors a known “mixture”. The necessary changes to the circuit are as follows: Step 1: Disconnect the sensor and add a 560kΩ resistor in parallel with the 560kΩ resistor between pins 6 & 7 of IC3b. You can use TP7 and TP6 to terminate the leads of this resistor. Step 2: Add a 560kΩ resistor in parallel with the 560kΩ resistor between pin 5 of IC3b and the Vs/Ip connection. You can use test points TP1 and TP5 to terminate the leads. Step 3: Remove the 510Ω resistor in series with the 62kΩ resistor for the 20µA reference current and install another 62kΩ resistor in its place (ie, so that the total resistance between Vs and the +5V rail is 124kΩ). Step 4: Apply power and adjust VR3 siliconchip.com.au so that Vs/Ip is at 2V, as measured between TP1 and TP GND. Step 5: Adjust VR4 so that TP4 is 2.343V. Having made these changes, you can now troubleshoot the Wideband Controller as follows: Step 6: Gently rest the sensor in a Pyrex bowl, connect it to the controller and apply power. Step 7: Wait until the sensor has heated and the indicator LED flashes at a fast rate. Now check the various operating voltages on the circuit. The voltage between Vs/Ip (or TP1) and Vs should be 450mV. The voltage between TP11 and TP GND should be 2.5V. However, there may be small variations from these values as the controller continually adjusts the current to maintain these voltages. Step 8: If you have an oscilloscope, check that the 684mV p-p square-wave (used for sensor impedance measurement) is present at TP11. Alternatively, by inserting jumper JP1, the wideband output (as measured between the sleeve and tip connections of a stereo 3.5mm jack plug) will indicate the impedance of the sensor instead. This should show 684mV DC for the sensor impedance to be kept at 300Ω. This may vary by ±2% or so as the controller maintains temperature, due to the resolution of the impedance measurement. Alternatively, if you have the Wideband Display unit connected (and set to show lambda), it should show a reading of 0.85 or 0.86. Step 9: Check that TP12 (or TP7) is at 4V (ie, the Vs/Ip voltage of 2V plus the amplified voltage across the 62Ω resistor between Rcal & Ip). To explain, the Ip current through the 62Ω resistor should be 2.54mA when measuring 20.9% oxygen (ie, the oxygen content of air), so there should be 157.5mV across this resistor. IC3b operates with a gain of 12.73 (560kΩ//560kΩ ÷ 22kΩ), so this adds an extra 2V to the Vs/Ip voltage at TP12 to give a total of 4V. Step 10: Check that the wideband output (between tip and sleeve) is at 2V with JP1 out. The Wideband Display should show 1.15 (if set to display lambda). Note that if the air pressure is less than 1013hPa due to atmospheric conditions or altitude, the readings specified above may differ slightly. However, if VR3 has been correctly adjusted for altitude as detailed in Step 4 above, the error will be corrected. siliconchip.com.au This photo shows an original narrowband sensor at left and the Bosch LSU4.9 wideband sensor at right. The original sensor has exhaust gas entry slots in the side to provide faster exhaust gas access compared to the access at the base of the LSU4.9 sensor. This OBDII diagnostic tool readout shows how the simulated narrowband (S-curve) output cycles about stoichiometric when the Bosch LSU4.9 wideband sensor and the Wideband Controller were installed on a 2004 Holden Astra. The horizontal scale is 10 seconds. The blurriness is due to display update movement as the trace moves leftward. Resistor tolerances will also cause the voltage reading to differ. If necessary, the unit can be calibrated to give an exact 2V wideband output by changing the 62Ω resistor. To this end, the PCB has extra mounting holes so that a multi-turn 100Ω trimpot can be fitted instead. However, this modification shouldn’t be necessary. Configuring the controller to measure the oxygen content in air is done to test the circuit’s operation rather than check the sensor calibration. Step 11: Once all checks are complete, restore the circuit to normal operation by undoing the changes outlined in Steps 1-3 above – ie, remove JP1, remove the two extra 560kΩ resistors and replace the added 62kΩ resistor with the 510Ω resistor. That done, disconnect the sensor, apply power to the Wideband Controller and readjust VR3 to give 3.3V at VS/Ip and VR4 to SC give 3.92V at TP4. August 2012  87