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A Look At Automotive On-Board Diagnostics You may not know it but your late-model car has an astonishing array of sensors to make sure that its engine and electronic systems all run at peak efficiency, while keeping emissions to a minimum. This increasing use of electronics in vehicles has also lead to improvements in the way a vehicle can be maintained. With On Board Diagnostics (OBD), the performance of critical engine components can be easily monitored. By JOHN CLARKE A LL NEW CARS sold in Australia from 2006 onwards are required to comply with the Australian Design Rules (ADR) to include On-Board Diag­ nostics. This is called OBDII and the “II” indicates that this is the second generation OBD standard. OBD is not new and has been around for more than 20 years. The first OBD standard adopted in California (USA) was introduced primarily to monitor the condition of vehicle emission sensors. Early OBD systems included a Malfunction Indicator Light (MIL). This used a rudimentary blinking light system where the number of blinks could be counted. This blink count could be cross-referenced against a list to find the problem indicated. Over the years, there have been many refinements and improvements. The latest OBDII standard is far 10  Silicon Chip more complex and includes a data link connector for connection of an OBDII scan tool which can be a dedicated hand-held unit. Alternatively, it may connect to a laptop computer via a cable that includes some form of signal processing and which is used in conjunction with special software. Either approach can be used to retrieve information from the vehicle. On-board diagnostics are possible due to the computer systems in modern cars. Most readers would know about the car’s ECU (Engine Control Unit) which is dedicated to controlling such components as fuel injectors, the exhaust gas recirculation (EGR) valves and the ignition timing. Computerised engine control is vital to ensure optimum fuel efficiency. The ECU is not the only computer system within a vehicle. There are others that control the anti-lock brak- ing system, stability control, cruise control, air-bag activation, climate control, central locking and even the sound system. Many of these computers communicate with each other, swapping data as required. With OBDII, communication is also available via the scan tool to provide vital information about the health of vehicle components. A diagram showing the input sensors connecting to the ECU and controlling the injectors and ignition is shown in Fig.1. The OBDII scan tool shows a sample of the diagnostics information available when attached to the OBDII data link connector. Modern engines are exceedingly complex, making it difficult to diagnose problems if anything goes wrong. There is an incredible array of sensors or monitors, including those for camshaft position, turbo bypass valving, siliconchip.com.au AIRFLOW METER COOLANT TEMPERATURE CRANKSHAFT ANGLE ENGINE RPM INTAKE AIR TEMPERATURE KNOCK SENSOR MAP SENSOR OXYGEN SENSOR THROTTLE POSITION VEHICLE SPEED MALFUNCTION INDICATOR LAMP (MIL) INPUTS ENGINE CONTROL UNIT (ECU) OUTPUT CONTROL IGNITION INJECTORS EGR VALVE CONNECTING CABLE OBDII DATA LINK CONNECTOR OBDII PLUG Fig.1: the engine management unit (ECU) accepts inputs from a range of sensors and controls the ignition timing, fuel injectors and EGR valve. The OBDII scan tool plugs into the ECU and displays a range of diagnostic information to aid troubleshooting. intake and outlet valves, fuel and oil pressure, injector operation, fuel pump operation, turbo boost, ignition misfire, ignition timing, cooling fan operation, air-conditioning refrigerant, battery charging, transmission, speed, RPM and automatic transmission functions. When there is a problem, the ECU can easily detect this because the engine sensor results will not be as expected. This is where on-board diagnostics becomes invaluable in making available the information from the ECU. Major problems are indicated with the Malfunction Indicator Lamp (MIL) or in severe cases, the lamp will flash. As mentioned, the scan tool plugs into the ECU’s Data Link Connector (DLC) which is located inside the vehicle. This connector is usually on the driver’s side of the car under the dashboard, behind the ashtray, near the steering column or in the central gear-stick console area. A workshop manual will show where the connector is located if it is not easily found. For the scan tool to work, the vehicle must be OBDII-compliant and use specified communications protocols – see the section headed “OBDII Data Link Connector” for more detail. OBDII SCAN TOOL DISPLAY DIAGNOSTIC TROUBLE CODES (DTC) DATA: AIRFLOW CALCULATED LOAD COOLANT TEMPERATURE ENGINE RPM IGNITION ADVANCE INJECTOR DURATION LONG TERM FUEL TRIM OXYGEN SENSOR(S) PARAMETERS SHORT TERM FUEL TRIM THROTTLE POSITION VEHICLE SPEED INSPECTION & MAINTENANCE (I/M) READINESS Y N are a code that can be cross-referenced against a table that describes it. Better scan tools will show both the DTC value and its definition. DTCs are not all that can be shown by the scan tool. OBDII specifications include up to 10 modes of operation. However, not all scan tools provide for all of these and not all vehicles provide for all modes. Apart from the Diagnostic Trouble Codes, the other modes listed in the OBDII standard include showing ON-BOARD MONITORING VEHICLE INFORMATION pending DTCs, clearing DTCs, showing a list of DTCs cleared previously, showing current (real-time) data, showing freeze frame data (data captured at the time of a DTC), test results for emissions components, control operation of on-board components and vehicle information. Manufacturers may also include additional modes. Not all DTCs are necessarily supported or required by a particular vehicle model. Also some codes are manufacturer-specific rather than Diagnostic trouble codes The scan tool shows the details of any malfunctions. These are shown as Diagnostic Trouble Codes (DTCs) and siliconchip.com.au The OBD indicator light (or engine light) appears on the dashboard display of a modern car when the ignition is first turned on. If everything is OK, it goes out a few seconds after the engine is started. February 2010  11 A Look At Engine Control Systems The Engine Control Unit (ECU), as the name implies, controls the engine and it does this to provide optimum performance under all conditions. Its job is to also ensure that the exhaust emissions are as low as possible and in order for this to happen the ECU monitors various sensors. The ECU has control over fuel delivery, ignition timing and exhaust gas recirculation to meet these requirements. Fuel is delivered to the engine using injectors that open for a certain period to meter the quantity. The ECU calculates the amount of fuel to deliver to the engine, based on various sensors. These include the volume of the air intake using a Mass Air Flow (MAF) meter or indirectly via a Manifold Absolute Pressure (MAP) sensor, the air temperature and RPM data. Other sensors include fuel pressure and air temperature sensors, oxygen sensors to measure oxygen levels in the exhaust system, a tach­ ometer sensor for engine RPM, a throttle position sensor (TPS) and coolant and oil temperature sensors. For ignition timing calculations, the ECU monitors engine position, engine RPM, engine load and engine temperature. It also checks for engine knocking using a knock sensor. Exhaust Gas Recirculation (EGR) valve control is based on engine temperature, engine load and the required emissions. It is a system that recycles some of the exhaust gas back into the engine inlet in order to reduce nitrous oxide emissions. The engine can be run in closedloop or open-loop mode. In openloop operation, the ECU uses a predefined table of values for ignition timing and injector duty cycle versus engine RPM, air intake mass and engine temperature. When running open-loop, the fuel mixture will generally be rich, with the engine using more fuel than can be fully burnt in the combustion process. Open-loop operation generally only occurs when the engine is under heavy load, such as when the vehicle is accelerating. Under normal cruise and light throttle conditions, the engine normally runs in closed-loop mode whereby the fuel delivery is adjusted so as to maintain required air/fuel mixtures. In general, air/fuel ratios are maintained at “stoichiometric” where there is just sufficient oxygen for the fuel to be completely burnt. Under stoichiometric conditions, the catalytic converter can work best at converting combustion by-prod- ucts to less harmful compounds. Carbon monoxide (CO) is converted to carbon dioxide (CO2), unburnt hydrocarbons to carbon dioxide (CO2) and water (H2O), and nitrous oxide (NO) to nitrogen (N2). Many cars also include a diagnostic oxygen sensor that is mounted after the catalytic converter to check efficiency. Another benefit of the oxygen sensor is that the ECU can learn to predict the amount of fuel to provide under differing conditions. So the ECU compares the predefined table of values for injector duty cycle against the actual duty cycle required to satisfy the required mixture, as gauged by the oxygen sensor signal. The ECU then sets up a table of trim values based on this feedback from the oxygen sensor. It can take some time for the ECU to learn the values and provide a full map of trim values. For this, it will require the engine to be run under a variety of conditions. These trim values are called “long-term fuel trim”. The ECU provides these trim values to allow the engine to run more efficiently. While driving, the engine RPM and load can change rather quickly and the oxygen sensor is not fast enough to respond to these changes. As a result, the mixture feedback to the ECU is not generic. Vehicle manufacturers may use manufacturer-specific DTC codes and imported vehicles may also use DTC codes different from generic DTC codes. We recommend that you do not fully rely on the DTC, especially for pre-2006 vehicles unless you can confirm that the generic DTC codes apply to your vehicle. (6) P0500-P0599: Vehicle Speed Controls and Idle Control System. (7) P0600-P0699: Computer Output Circuit. (8) P0700-P0899: Transmission. The full list of power train DTCs is far too long to feature here. You can find it at www.obd-codes.com/ trouble_codes/ Body codes include those for seat belts, lamps, solenoids, motors, ECU failure, climate control, air bags, central locking, doors, external mirrors etc. The list is at: www.obd-codes. com/trouble_codes/obd-ii-b-bodycodes.php Chassis codes include those for anti-skid braking (ABS) and traction control. For the full list see: www. obd-codes.com/trouble_codes/obd-iic-chassis-codes.php Fig.2: the OBD indicator symbol looks like a car engine. It illuminates if there is an engine malfunction. 12  Silicon Chip DTC categories DTCs are divided into four categories: “P” codes for the power train, “B” codes for body, “C” for chassis and “U” for network. P codes are the most common and include engine sensor malfunctions. These are further categorised into eight sub-sections: (1) P0010-P0099: Fuel and Air Metering and Auxiliary Emission Controls. (2) P0100-P0199: Fuel and Air Metering. (3) P0200-P0299: Fuel and Air Metering (Injector Circuit). (4) P0300-P0399: Ignition System or Misfire. (5) P0400-P0499: Auxiliary Emissions Controls siliconchip.com.au Table 1: List Of Toyota Car Teaching Manuals fast enough for rapidly changing conditions. However, using trim values based on past driving will have the engine run at close to the desired mixtures without waiting for the oxygen sensor response. When the oxygen sensor does respond, the ECU uses this information to make a short-term fuel trim adjustment to the current injector duty cycle. Ultimately, these short term trims are used to make up the long term fuel trim map. For more detail on fuel trim, refer to http://www.autoshop101.com/ forms/h44.pdf As mentioned above, in order to understand some of the data available from a scan tool, it is important to understand how the engine works. An understanding of car electronics is also required. The following links to teaching manuals will help. The USA branch of Toyota supplies them but most of the information is general and applies to any vehicle. The links are grouped into separate sections and a very good coverage is made of knock, oxygen, speed, pressure, MAF, position and temperature sensors in the sensors section. To access these, use the link http://www.autoshop101.com/forms/ hX.pdf but replace the “X” with one of the “h numbers” listed for each category. So, for example, replace Network codes are associated with communication between separate control modules within the vehicle or between the scan tool and the OBDII connection. For the list of these see: www.obd-codes.com/trouble_codes/ obd-ii-u-network-codes.php Many very basic scan tools will show trouble codes purely as the DTC value. For example, the display may show a DTC as P0130. This value would then have to be looked up to reveal the code definition. However, it is far easier to use a scan tool that also includes the DTC definition. So, for example, a scan tool that shows “P0130 Oxygen Sensor Circuit Malfunction (Bank 1 Sensor 1)” is easier to use than one that just shows the DTC value. A scan tool should also provide the siliconchip.com.au H1-H8: Basic Electronics http://www.autoshop101.com/forms/h1.pdf http://www.autoshop101.com/forms/h2.pdf http://www.autoshop101.com/forms/h3.pdf http://www.autoshop101.com/forms/h4.pdf http://www.autoshop101.com/forms/h5.pdf http://www.autoshop101.com/forms/h6.pdf http://www.autoshop101.com/forms/h7.pdf http://www.autoshop101.com/forms/h8.pdf H12-H18: Advanced Electronics http://www.autoshop101.com/forms/h12.pdf http://www.autoshop101.com/forms/h13.pdf http://www.autoshop101.com/forms/h14.pdf http://www.autoshop101.com/forms/h15.pdf http://www.autoshop101.com/forms/h16.pdf http://www.autoshop101.com/forms/h17.pdf http://www.autoshop101.com/forms/h18.pdf H20-H27: Electronic Fuel Injection http://www.autoshop101.com/forms/h20.pdf http://www.autoshop101.com/forms/h21.pdf http://www.autoshop101.com/forms/h22.pdf http://www.autoshop101.com/forms/h23.pdf http://www.autoshop101.com/forms/h24.pdf http://www.autoshop101.com/forms/h25.pdf http://www.autoshop101.com/forms/h26.pdf http://www.autoshop101.com/forms/h27.pdf H31-H38: Sensors http://www.autoshop101.com/forms/h31.pdf http://www.autoshop101.com/forms/h32.pdf http://www.autoshop101.com/forms/h33.pdf http://www.autoshop101.com/forms/h34.pdf http://www.autoshop101.com/forms/h35.pdf http://www.autoshop101.com/forms/h36.pdf http://www.autoshop101.com/forms/h37.pdf http://www.autoshop101.com/forms/h38.pdf H39-H41: Ignition http://www.autoshop101.com/forms/h39.pdf http://www.autoshop101.com/forms/h40.pdf http://www.autoshop101.com/forms/h41.pdf H42-H44: Fuel http://www.autoshop101.com/forms/h42.pdf http://www.autoshop101.com/forms/h43.pdf http://www.autoshop101.com/forms/h44.pdf H46-H48: OBD http://www.autoshop101.com/forms/h46.pdf http://www.autoshop101.com/forms/h47.pdf http://www.autoshop101.com/forms/h48.pdf X with a number from 1-8 for one of the eight Basic Electronics manuals. For the first manual, the link is http://www.autoshop101.com/ forms/h1.pdf and so on: h1-h8: Basic Electronics h12-h18: Advanced Electronics h20-h27: Electronic Fuel Injection h31-h38: Sensors h39-h41: Ignition h42-h44: Fuel h46-h48: OBD Table 1 above shows the full list of the teaching documents available. means to clear the DTC. Clearing the code removes the trouble code from the ECU and switches off the MIL. Another way to remove DTCs is to disconnect the vehicle battery for a few seconds, to clear the ECU memory. But if a trouble code is cleared in this way, the car’s audio systems will require its security code to be re-entered and the radio station presets reprogrammed. The ECU may also lose its fuel trim values (see section headed Engine Control Systems). Note that a trouble code will reappear if the fault that caused the DTC has not been fixed. Scan tools can usually show pending emission-related DTC codes. These are problems that the ECU has detected recently during the current or last completed driving cycle. They may be cleared it the ECU finds that the problem has not recurred or a pending DTC may be transferred to become a DTC. Pending DTCs are useful as an indicator for revealing if a trouble code is likely to reappear, indicating that a fault has not been fixed. Parameter data Apart from DTCs, being able to access parameter data is a vital aspect for locating problems. If your scan tool does not provide for this diagnostic data then you are limited in how faults can be diagnosed. Diagnostic data includes real-time data, freeze frame data and emissions tests. Diagnostic data is addressed as a Parameter IDentification (PID) value. The PID listing can be found at http://en.wikipedia.org/ wiki/OBD-II_PIDs February 2010  13 The OBDII Data Link Connector The OBDII Data Link Connector is shown at right. It has 16 pin locations for contacts but only a few of these will be used, depending on the communication protocol standard used by the vehicle to send data to the OBDII scan tool. At present, there are five protocol standards. Since 2008, all new US vehicles are required to use only the Controller Area Network (CAN) protocol for OBDII communication and this standard is eventually likely to be adopted worldwide. In Australia, any one of these five protocols may be used: (1) CAN (Controller Area Network). (2) ISO (International Organisation for Standardisation) 9141-2. (3) ISO 14230 KWP2000 (Keyword Pro­ tocol). (4) SAE (Society of Automotive Engineers) J1850 PWM (Pulse Width Modulation). Some of the values that can be displayed in real time include airflow rate from a Mass Air Flow (MAF) sensor, absolute throttle position, calculated engine load, coolant temperature, RPM, fuel pressure, ignition timing, inlet manifold pressure, fuel trims, oxygen sensor voltages, vehicle speed, ambient air temperature and air/ fuel ratio. The data is continuously updated so any output changes from each sensor can be viewed. For some scan tools, you would have to enter the PID value and then wait for the response from the ECU to show the requested data. This can be in hexadecimal format and in a raw form that requires a formula to calculate a result. Some scan tools do this work for you and show all available data from the ECU and include the PID definitions. The data is also calculated to show the value with units such as Volts, %, °C etc, making the tool far easier to use. 14  Silicon Chip (5) SAE J1850 VPW (Variable Pulse Width). While CAN is becoming the preferred protocol from 2008, typically the ISO protocol is used by European and Asian cars and some Dodge, Chrysler, Jeep, Ford and General Motors vehicles. VPW is also used for General Motors, Dodge, Chrysler, Jeep, Toyota and Isuzu vehicles. Ford uses PWM. More details on these protocols is available at: http://en.wikipedia.org/wiki/ On-board_diagnostics Fortunately, what protocol a vehicle uses is not important because most handheld scan tools will automatically work with any of them. For OBDII compatibility, your vehicle needs to have one of the following pin sets: (1) CAN protocol – uses pins 5, 6, 14 & 16. (2) ISO and KWP protocol – uses pins 5, 7, 15 (optional) & 16. Freeze frame data is the information captured by the ECU concerning the Diagnostic Trouble Code (DTC). The information shows what data the sensors were providing at the time the DTC occurred. Emissions testing Another worthwhile feature of the OBDII standard is emissions testing or what is called “Inspection and Maintenance (I/M) Readiness”. These tests will reveal if the vehicle meets the emissions standards required when the vehicle was manufactured. The tests include monitoring of engine misfiring, fuel systems and comprehensive components (such as evaporative emission controls). These tests are made continuously. In addition, non-continuous I/M readiness emissions monitoring are made for the Exhaust Gas Recirculation (EGR) system, oxygen sensors, 1 8 9 16 OBDII DATA LINK CONNECTOR OBDII DATA LINK CONNECTOR PIN DEFINITIONS DEFINITION PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 Manufacturer's discretion Bus(+) SAE J1850 PWM & DAE 1859 VPW 14 15 CAN low SAE J2284 & ISO 15765-4 L line of ISO 9141-2 & ISO 14230-4 Battery positive 16 Manufacturer's discretion Chassis Ground Signal Ground CAN high ISO 15765-4 & SAE J2284 K line of ISO 9141-2 & ISO 14230-4 Manufacturer's discretion Manufacturer's discretion Bus(–) SAE J1850 PWM only Manufacturer's discretion Manufacturer's discretion Manufacturer's discretion (3) VWP protocol – uses pins 2, 5 & 16. (4) PWM protocol – uses pins 2, 5, 10 & 16. catalytic converter, oxygen sensor heater, secondary air, heated catalyst and the air-conditioning system. These monitors are tested during specific drive cycles where the full extent of the sensors’ operation can be checked. Testing of these will indicate a “Not Ready” status if testing is not yet complete. On-board monitoring of sensors is also available. These provide a minimum value, a maximum value and a current value for each non-continuous monitor. Oxygen sensors, for example, are characterised by Rich-to-Lean O2 sensor threshold voltage, Lean-to-Rich O2 sensor threshold voltage, Low sensor voltage threshold for switch time measurement, High sensor voltage threshold for switch time measurement, Rich-to-Lean switch time in ms, Lean-to-Rich switch time in ms, Minimum voltage for test, Maximum siliconchip.com.au voltage for test, and Time between voltage transitions in milliseconds. Multiple oxygen sensors OBDII-compliant vehicles will have at least two oxygen sensors. There will be one to monitor the air/fuel mixture at the exhaust manifold (the control sensor) and another to monitor gasses following the catalytic converter (the diagnostic sensor). The latter allows catalytic converter’s performance to be checked. Some engines will have more oxygen sensors, depending on how the exhaust manifolds are arranged. V6 and V8 engines will have separate control oxygen sensors for each side of the engine. Whether such an engine then has one or two diagnostic sensors depends on whether the exhaust is fed into one catalytic converter or two. Oxygen sensors are listed according to their number and bank. Sensor 1 is associated with the cylinder 1 side of the engine, while Sensor 2 would also be used in a V6 or V8 engine (ie, another sensor in the second exhaust manifold). Bank number refers to the sensor usage. Bank 1 is the control sensor that measures air/fuel ratio at the exhaust manifold. A bank 2 sensor would be a diagnostic sensor that mounts after the catalytic converter. Vehicle information can also be displayed using a scan tool. This shows Vehicle Identification Numbers (VIN), Calibration Identification Number (CIN) and Calibration Verification Numbers (CVN). Using a scan tool One point that needs to be made regarding scan tools is that the OBDII features available for that vehicle will limit what can be done with a scan tool. So a scan tool may be capable of showing freeze frame data but it will not be able to display it if the vehicle does not provide the feature. While OBDII information appears to be complicated, when it comes to actually using a scan tool it is quite easy, especially with a good scan tool. Basically, you just plug in the lead to the OBDII data link connector located in the vehicle, switch on the ignition and wait for the scan tool to communicate with the ECU. You then select the functions you want to use, such as viewing DTCs, clearing DTCs and viewing data. The handbook supplied with the siliconchip.com.au Making Your Own Scan Tool There is a lot of information available about building your own OBDII scan tool using the ELM series of integrated circuits (ICs) to decode the OBDII protocol into a form suitable for a laptop computer. Although this is not an exhaustive list, here are some websites that may be of use in finding the data and applications for the ELM ICs and for software to run with them on the computer. (1) The ELM series of ICs has data at http://www.elmelectronics.com/obdic.html (2) For software, see http://www.obd2crazy.com/software.html, while commercial software is at http://www.obd-ii.de/screensm2.html An OBDII project is available at: http://courses.cit.cornell.edu/ee476/FinalProjects/ s2009/ama64_maa66/ama64_maa66/index.html (3) ALDL (Assembly Line Diagnostic Link) Interface: for Holden VR, VS and VT Commodores that use the ALDL interface see http://www.techedge.com.au/vehicle/ aldl8192/8192hw.htm For VN, VP and JE Camira see http://www.techedge.com.au/ vehicle/aldl160/vn_aldl.htm Note: a build-it-yourself OBDII scan tool interpreter using one of the ELM ICs is featured in this issue of SILICON CHIP. Is OBDII Available On My Car? Vehicles built in Australia before January 2006 do not usually have functioning OBDII. The Holden VR, VS & VT Commodores used a different system to OBD called Assembly Line Diagnostic Link (ALDL) standard. Details about this can be found in the “Making Your Own Scan Tool” panel – see above. AU and BA Falcons and VY & VX Commodores had the OBDII connector and some OBDII functions. The BA Falcon and VY Commodore did not include DTCs but included live engine data. The BF Falcon and the VZ Commodore do have full OBDII compliance. The FG Falcon and VE Commodore have a huge range of data available. Also, the locally built Toyota Avalon included OBDII compliance. Imported vehicles sold before January 2006 cars may have OBDII. US-built cars such as Chrysler, built from around 1996 onwards, do have OBDII compliance. European cars from about 2000 will also have OBDII while Subarus had OBDII from 2002. Other pre-2006 vehicles may have an OBDII-style connector or an alternative OBD connector. These tend to operate using a proprietary OBD system that is specific to either the vehicle’s manufacturer or to the model. Scan tools are generally available for use with these vehicles either from the manufacturer or a scan tool supplier. For example scan tools are available for 1990 and onward VW and Audi at www.theobd2shop.com.au. These are called VAG (or Volkswagon AG) readers. An extensive (but not exhaustive) check-list of pre-2006 OBDII compatible vehicles in Australia is available at www.scangauge.com.au/support/CompatibleVehicles.shtml scan tool will show how it is used. If you want to gain some experience with a scan tool, you could force a trouble code by unplugging a sensor’s connecting lead while the ignition is off. An easy sensor to locate is the Mass Air Flow (MAF) sensor, located between the air filter and the inlet manifold. A MAF sensor is used on many cars although others may have a Manifold Absolute Pressure (MAP) sensor instead. With the MAF sensor disconnected and after starting the engine, an almost immediate pending DTC should appear and will be shown when pending codes are accessed on the scan tool. A look at real-time data for the MAF sensor will show a zero reading. If you now switch off the engine and restart, the Malfunction Indicator Lamp will probably light and a DTC may be set. Finally, switch off the engine, reattach the MAF sensor and start the engine again. The DTC may automatically clear or you may need to clear it with the scan tool. A look at the MAF live data should now show readings February 2010  15 The Autel MaxScan GS500 Scan Tool L et’s take a look here at the Autel MaxS- can GS500. This is a hand-held unit that weighs in at 300g and measures 95 x 180 x 35mm. A 54 x 35mm backlit LCD shows the information, while four pushbuttons provide the numerous functions. On the top of the unit is a DB15F connector which is connected to the OBDII connector on the vehicle via a 1.4m OBDII to serial cable. A separate serial-to-USB cable is provided for upgrading the inter- This view shows the OBDII connector fitted to a 2005 Honda Accord. It’s mounted under the dashboard, near the driver’s side kick panel. The same type of connector is fitted to other late-model cars. above zero that will rise in g/s (grams per second of air mass) as more throttle is applied. In order to get the best from your scan tool, a comprehensive understanding of how a modern vehicle engine operates is required. Included here is some engine control information located in the panel headed “A Look At Engine Control Systems”. We have also provided some useful tips on fault diagnosis in the following section. Diagnosis using a scan tool An OBDII scan tool should only be used as an aid to assist in diagnosis of a fault condition. When a Diagnostic Trouble Code (DTC) is found, it only indicates where the ECU has found a problem but not the cause. In other words, the ECU indicates the effect of the problem rather than where the problem may be. As an example, a fault code for a 16  Silicon Chip nal DTC definitions library via a computer connected to the internet. A padded black Nylon carry-case is also supplied to protect the OBDII scanner, its operating manual and the leads. A CD is also included, providing a library of over 8000 DTC definitions. The CD also includes a USB driver and a manual for the GS500, as well as manuals for other Autel scanners. The GS500 works with all OBDIIcompliant vehicles and supports CAN and all other OBDII communication protocols such as ISO 9141-2, ISO 14230 KWP2000, SAE J1850 PWM and SAE J1850 VPW. Power comes via the OBDII connector. There are no internal batteries and it will not power up without connection to the OBDII port or the USB port. This is typical of scan tools. The list of GS500 functions is numerous. These are: (1) Read and clear generic OBDII diagnostic trouble codes, including pending trouble codes. (2) Read and clear manufacturer-specific trouble codes, including those for GM, Ford, Chrysler, Toyota and many more. (3) Turn off the Malfunction Indicator Lamp (MIL). (4) Show Freeze Frame data. (5) Display Monitor and I/M readiness status. (6) Read live data streams. (7) Display oxygen sensor test data. (8) Perform Modules Present test. (9) Retrieve Vehicle Information (VIN, CIN and CVN). The GS500 includes real time (live) data display. This is an invaluable aid to diagnosing the cause of trouble codes. In addition, the internal trouble code definitions library is an excellent inclusion so that Diagnostic Trouble Codes are shown alongside their definition. This saves looking up a long list of trouble codes versus their definitions. For live data, the Parameter IDentification (PID) values are shown by their definition rather than the PID number and the data is shown with units. That means that the scan tool is easy to use. Any data that cannot all be shown on the screen at the same time can be accessed by scrolling to the next screen. Alternatively, there is the option to customise what data is shown. Operation of the scan tool to access information involves a simple 2-button up or down scroll through a menu system. An extra two buttons are used to acknowledge or exit (using Y or N buttons). Having just four buttons makes the GS500 easy MAF sensor (DTC #P0100) or MAP sensor (DTC #P0105) could just mean that there is no power to the sensor. This may be due to loss of either the ground or positive supply rail (or both) and the sensor itself may not be at fault. With this fault, probing the sensor leads with a multimeter should be done to check for power to the sensor. Continuity from the 0V supply to ground (chassis) should also be tested. Another example is for an oxygen sensor DTC. This DTC may show as P0130 [Oxygen sensor circuit mal­ function (Bank 1 Sensor 1)]. It refers to an incorrect control oxygen sensor output for a narrowband sensor. Note that wideband sensors (as used on some late model vehicles) have a different set of Diagnostic Trouble Codes. Their output response is very different to the narrowband sensors. The P0130 fault code means that the ECU has found that this sensor is not providing the expected output. It does not necessarily mean that the oxygen sensor is at fault and you may need to look elsewhere to locate the cause. A rash approach would be to purchase a replacement sensor only to find that this does not clear the fault. To trace where the fault is, use the scan tool to read freeze frame data or the data stream. Freeze frame shows what the parameters were at the time the DTC occurred but this feature may not be available in your scan tool or from the OBDII in your vehicle. Real time data can show the current sensor readings but make sure you use a scan tool that can show real time data. It’s a matter of setting the scan tool to monitor the oxygen sensor readings and viewing the sensor readings. The correct output from a Zirconiabased narrowband sensor when the engine is idling is where the voltage cycles above and below a nominal 450mV. If the voltage stays well above this voltage (say >800mV), then the siliconchip.com.au The Autel MaxScan GS500 Scan Tool comes with a black carry case, a CD with over 8000 DTC definitions, a manual and connecting cables. The GS500 works with all OBDIIcompliant vehicles and has a long list of features. Power comes from the OBDII connector. to drive and it should prove invaluable in diagnosing vehicle engine malfunctions. The Autel MaxScan GS500 hand-held OBDII scanner can be purchased from Engine Code Readers Australia – www. theobd2shop.com.au Alternative hand-held scan tools are also available from automotive suppliers such as Repco. Check that the scanner’s features suit your application. See also www.scangauge.com.au/index.html and www.jaycar.com.au mixture is rich and if the voltage stays well below (say <100mV), then the mixture is lean. These voltages are for a sensor that is in working condition. If the voltage remains low, it could be that there is an air leak to the inlet manifold between the MAF sensor and inlet manifold. This leak will cause the mixture to be leaner than normal because the MAF is not measuring all the air entering the engine. In other words, the air leak is providing extra unmetered air that the ECU does not know about. A split in the crankcase ventilation hosing is a likely cause. Another cause for an oxygen sensor DTC is an air leak in the exhaust manifold or oxygen sensor gasket, causing a lean reading. If there don’t appear to be any air leaks, then check the sensor’s operation when it is removed from the engine. You will need to determine which leads are for the oxygen sensor element and which leads are for the heater and this can be found by referring to the vehicle’s wiring diagram. Measuring the output voltage of the sensor when under the flame of a butane blowtorch can test a narrowband sensor. Heat the tip of the sensor under the flame until it has a red glow. The output voltage should rise to >800mV due to the rich mixture and drop to <100mV when the sensor is removed from the flame. The sensor may be faulty if it does not produce any voltage. Another scenario for the fault code may be that the sensor voltage stays at a fixed voltage. It is fairly normal for the bank 2 oxygen sensor (located at the output of the catalytic converter) to show a relatively fixed voltage of about 450mV. But a fixed voltage is not normal for a control oxygen sensor at the exhaust manifold. Yet another possibility is that the sensor’s output voltage (as measured using an oscilloscope or multimeter) always shows a rich (>700mV) reading. For a sensor with only one signal lead, the reading would be taken between the sensor signal and the body of the sensor. The >700mV reading would suggest that the sensor is OK. Instead, the fault in this case may be that the negative (-) sensor terminal or the sensor body for the single output lead sensor is not grounded correctly. This could be due to a wiring fault or a broken connection inside the ECU. Either way, grounding the negative (-) sensor terminal to chassis can restore sensor operation. An alternative solution would be to find the negative (-) sensor wiring for the second oxygen sensor used in the vehicle and connect the floating negative sensor terminal to that. With a single-wire sensor, the sensor may be isolated from chassis due to corrosion on the sensor or a faulty earthing strap connection from the SC engine to chassis. siliconchip.com.au February 2010  17