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h27 engine control - diagnosis.pdf

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H27 ENGINE CONTROL DIAGNOSIS
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System Diagnosis and Troubleshooting An Overview of the Self Diagnostic System The ECU on all P7 and TCCS engines has a self diagnostic system which constantly monitors most of the electronic control system's input circuits. When the ECU detects a problem, it can turn on the check engine light to alert the driver that a fault exists in the system. At the same time, the ECU registers a diagnostic code in its keep alive memory so that the faulty circuit can be identified by a service technician at a later time. if the circuit fault goes away, the check engine light will go off. However, the diagnostic code will remain in the ECU memory even after the ignition switch is turned off. For most engines, the contents of the diagnostic memory can be checked by shorting check connector terminals T (or TE1) and E1 together and counting the number of flashes on the check engine light. After the problem has been repaired, the technician can clear the diagnostic system by removing the power from the ECU BATT feed. Fault Detection Principles The ECU fault detection system is programmed to accept sensor signal values within a certain range to be normal, and signals outside of that range to be abnormal. The normal signal range used to diagnose most sensor circuits covers the entire operating range of the sensor signal. As long as the signal value falls within this range, the ECU judges it to be normal. As a result, it is possible for the sensor to generate a signal which does not accurately represent the actual operating condition and not be detected as a problem by the ECU. The fault detection range graph shows typical THW signal parameters. Point A is normal operating temperature and falls within the fault detection normal range. Point B represents the freezing point of water and also falls in the normal range. If the engine is at normal operating temperature but the THW sensor signals the ECU that the coolant temperature is freezing (point B), the engine will operate excessively rich and may not start when hot. Because point B falls within the normal range, the ECU will not recognize this as a problem. No diagnostic code will be set for this problem. Limitations of the Self Diagnostic System The self diagnostic system provides an excellent routine to direct the technician to the heart of an electronic control system problem. There are however, several limitations which must be kept in mind when troubleshooting. ? The ECU must see a signal in an abnormal range for more than a given amount of time before it will judge that signal to be faulty. Therefore, many intermittent problems cannot be detected by the ECU. ENGINE CONTROLS PART #4 - DIAGNOSISPage 1 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. ? When the ECU stores a diagnostic code, the code indicates a problem somewhere in the sensor circuit, not necessarily in the sensor itself Further testing is always required to properly diagnose the circuit. ? Not all circuits are monitored by the ECU. Just because the ECU generates a normal code does not mean that there are no problems within the electronic control system. ? Occasionally, diagnostic codes can be set during routine service procedures or by problems outside the electronic control system. Always clear codes and confirm that they reset prior to circuit troubleshooting. Check Engine Lamp Functions The check engine lamp serves two functions in the self diagnostic system, depending on the status of the T terminal. When the T terminal is off (not shorted to E1) the check engine light goes on to warn the driver when a major problem is detected in the electronic control system. When the T terminal is on (shorted to E1) the check engine light displays stored diagnostic codes for use by the technician. VF (Voltage Feedback) Terminal Function The VF terminal also serves two diagnostic functions depending on the status of the T terminal. When the T terminal is off, the VF terminal voltage represents learned value correction factor. When the T terminal is on, the VF terminal will either display an emulated oxygen sensor signal (throttle open, IDL contact off) or indicate whether a diagnostic code is stored in the ECU memory (throttle closed, IDL contact on). ENGINE CONTROLS PART #4 - DIAGNOSISPage 2 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. Four Systematic Steps In Diagnosis Simply stated, there are four steps to follow when performing a methodical diagnosis from start to finish. Using this systematic approach will generally lead to reduced diagnostic time and a higher degree of success. The four steps are listed as follows. ? Routine Quick Checks ? Use of the Self Diagnostic System ? Troubleshooting by Symptom ? Quality Control Check Routine Quick Checks This step in diagnosis includes confirmation of the problem and routine mechanical and electrical engine checks. Confirmation of the customer concern is an excellent place to begin any diagnosis. It is important to gather and analyze as much information as the customer can supply and, if the check engine warning lamp is on, to retrieve and record the diagnostic codes. The conditions of the battery and charging system are critical to the proper operation of the electronic control system. Both should be routinely checked by measuring cranking and engine running battery voltage prior to proceeding with diagnosis. Depending on the problem or driveability symptom indicated, the following checks should be conducted under the hood: ? Inspection of the engine's mechanical condition (i.e., audible cranking rhythm and visual ignition secondary condition). ? Brief inspection of accessible electrical, vacuum and air induction system duct connections. ? Locate and inspect the condition of the ECU main grounds. ? Inspect for leakage in the EGR and PCV valves. ? Inspect for unwanted fuel entering the intake manifold from the EVAP system. The entire routine quick check procedure can be performed in less than ten minutes and will often save an hour or more of unnecessary diagnostic time. Use of the Self Diagnostic System Once you are satisfied that there are no routine problems causing the customer concern, use of the self diagnostic system is in order. This system is available on all P7 and TCCS equipped engines and is capable of indicating if certain faults exist in ECU monitored circuits. The P7 systems have limited diagnostic capabilities and can only display seven diagnostic codes, including a system normal code. This system will only indicate a fault if the circuit is open or shorted to ground. Late model TCCS systems have more sophisticated diagnostics which monitor more ECU related circuits with as many as 21 or more diagnostic codes. The latest TCCS ECUs have some special capabilities which make them more useful in diagnosis and prevent the check engine warning light from becoming a source of customer dissatisfaction. ? To allow the diagnostic system to find more system faults, the electrical parameters which the ECU uses to set a diagnostic code are altered to find sensor performance faults like oxygen sensor degradation. ? Some minor TCCS system fault codes will set a diagnostic code in the ECU keep alive memory but will not turn on the check engine light and unnecessarily alarm the customer. ? To prevent false indication of certain system faults, some ECUs are programmed to use a two- trip detection logic which prevents the check engine light from illuminating, or certain codes from setting, until the problem has duplicated itself twice, with a key off cycle in between. ? Some ECUs have a special diagnostic TEST mode which causes the ECU to narrow its diagnostic parameters for the technician, thereby, making troubleshooting intermittent problems easier. ENGINE CONTROLS PART #4 - DIAGNOSISPage 3 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. Procedures to Retrieve Trouble Codes There are several different types and locations of diagnostic connectors which are used to trigger and, in some cases, read diagnostic code output from Toyota EFI engines. All late model TCCS applications, from 1988, use a multiple terminal diagnostic check connector. Earlier models use this same multiple terminal or a two- terminal check connector, all located under the hood. The procedure to examine the ECU memory for diagnostic codes is typically very simple regardless of which vintage engine being diagnosed. All engines equipped with self diagnostic systems have one terminal of the check connector identified as T or TE1. When grounded, this terminal triggers the self diagnostic feature of the ECU. The E1 ground circuit is also located in the check connector. To enter engine diagnostics: ? Locate the check connector under the hood and identify the T (TE1 on late model TCCS) and E1 terminals. ? Turn the ignition switch to the “on“ position and make sure that the check engine light is on. ? Confirm that the throttle is closed (IDL contact on). ? Jumper check connector terminals T (TE1) to E1. ENGINE CONTROLS PART #4 - DIAGNOSISPage 4 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. When the T terminal is grounded with the ignition switch in the “on“ position, the ECU sees the voltage at terminal T go low. Low voltage on T causes the ECU to enter diagnostic mode, producing diagnostic codes on the check engine light. On '83 through '85 Cressida and Supra models, the check engine light does not flash diagnostic codes. An analog voltmeter must be used to read the codes from the VF terminal of the EFI Service Connector. Depending on the vintage of the system being tested, the codes will be displayed in either one or two digit format. It is important to refer to the proper repair manual for specific information about diagnostic connector location, code format, and proper procedures for the vehicle you are troubleshooting. ENGINE CONTROLS PART #4 - DIAGNOSISPage 5 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. Super Monitor Display: On some 1983 through 1987 Cressida and Supra models, a Super Monitor trip computer was offered as optional equipment. This display can be used to read diagnostic codes by simply pressing and holding the monitor “Select“ and “Input M“ keys together, for three seconds, with the ignition switch in the “on“ position. When the “DIAG“ message appears on the display, pressing and holding the “Set“ key for three seconds will put the TCCS system into diagnostic mode. The display will indicate any diagnostic codes stored in the ECU's keep alive memory. Once Diagnostic Codes Are Retrieved Once diagnostic codes have been retrieved from the ECU keep alive memory, it is advisable to erase the codes and road test the vehicle. 'Me purpose of this procedure is to confirm that the problem(s) will be present during your diagnosis. If the diagnostic code re-occurs, the problem can be considered a hard fault and troubleshooting will be routine. If the diagnosis code does not re-occur, the problem is either intermittent or was inadvertently stored during a previous service procedure. If an intermittent fault is suspected, a physical check of the indicated circuit must be performed by flexing connectors and harnesses at likely failure points while monitoring the circuit with a multimeter or oscilloscope. If the problem is temperature, vibration, or moisture related, the circuit can be heated, lightly tapped, or sprayed with water to simulate the failure conditions. Attempting to troubleshoot intermittent problems using the normal diagnostic routines will likely result in a misdiagnosis and wasted time. Erasing Long Term Memory The procedure to erase stored diagnostic codes is as simple as removing a fuse or disconnecting the battery negative terminal for at least thirty seconds. Fuse removal is the method of choice because it will not disturb any other computer memories in the vehicle (ETR radio stations, trip computer data, etc.) The proper fuse to remove depends on application but will always be the one which feeds the ECU BATT terminal. The following fuses supply BATT power distribution to the ECU keep alive memory: EFI, STOP, or on some P7 applications, ECU +B. ENGINE CONTROLS PART #4 - DIAGNOSISPage 6 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. ENGINE CONTROLS PART #4 - DIAGNOSISPage 7 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. Monitored and Non-monitored Circuits Although the newer TCCS self diagnostic system is getting more sophisticated every model year, there are still many electronic control system circuits which the ECU does not monitor. Generally speaking, most input sensors are monitored for faults, but most output actuators are not. Exceptions to this are the Neutral Start Switch (NSW) and Power Switch (PSW)* input signals which are not monitored. Codes 25 and 26 monitor the air/fuel ratio rather than the status of a particular circuit. Troubleshooting After Code Retrieval The diagnostic code leads only to a circuit level diagnosis. A pinpoint test of the circuit indicated will be required to isolate the problem down to the component or wiring level. To find the appropriate diagnostic procedure to follow: ? Refer to the last column of the repair manual “Diagnostic Codes“ list. ? This will lead to one or more “Troubleshooting with a Voltmeter/Ohmmeter“ diagnostic charts which will facilitate circuit diagnosis. ? This may also lead to an “Inspection of Component“ procedure which will facilitate diagnosis of the sensor or actuator in the circuit. But what if you do not have a diagnostic code to help lead you to the cause of the customer complaint? What do you do next? Before we address the third step in the systematic diagnostic approach, the subject of an inoperative self diagnostic system must be addressed. ENGINE CONTROLS PART #4 - DIAGNOSISPage 8 ? Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
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