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Electronic Engine Management Pt.9: Fault Diagnosis – by Julian Edgar Electronic engine management systems rarely fail. In 10 years of driving five different EFI cars – including highly modified examples – I have been stranded on the roadside only once. The reason for that unscheduled stop was that the main EFI relay had corrosion on its pins where it plugged SENSORS SENSOR 1 into the wiring loom socket – it was as simple as that. As with most purely electronic devices, the ECM itself is likely to last for many years without internal failure – unless an output is short circuit­ed or it suffers physical damage. Problems which crop up in engine-managed cars CONNECTORS CONNECTORS ACTUATORS ACTUATOR 1 ELECTRONIC CONTROL UNIT SENSOR 2 ACTUATOR 2 30% 15% 30% 15% 10% Fig.1: most faults in engine management systems occur outside the electronic control module. It’s therefore usually wrong to initially assume that the fault lies within the ECM. TABLE 1: LIMP-HOME OPERATIONS Sensor Failure Diagnosis Method Fail-Safe Operation Water temperature Abnormal voltage Radiator fan kept switched on; specific water temperature value substituted Knock sensor Abnormal voltage Regular fuel map set; ignition timing retarded by 5° Turbo control Faulty sensor or pressure valve Fuel cut-off under full throttle & heavy load Airflow sensor Abnormal voltage Fuel injection controlled by rpm & throttle opening 6  Silicon Chip tend to be in the electrical/mechanical interfaces – the input sensors and the output devices. Fig.1 shows that 90% of the faults are caused by connector, sensor or actuator faults. But how do you find the problem? The first aspect to establish with absolute certainty is that it actually is the engine management system which is at fault. Poor fuel economy might be caused by a faulty injector, or it might be the brakes dragging. Or the engine might be hard to start because the hose to the MAP sensor is blocked; or it might be because there’s water in the fuel! There is a tendency – especially among people familiar with electronics but less used to mechanics – to start with the most complicated possible explanation first. However, all logical mechanical alternatives to engine management problems should be carefully examined first before turning to the elec­tronics. Limp-home operations All modern engine-managed cars run self-diagnostic facilities, where the ECM can be instructed to check for any fault codes held in its memory. It may be that an intermittent wiring problem exists with a sensor where, for example, the ECM loses its knock sensor input over large road bumps. Or a sensor may be returning a signal to the ECM which is well out of nor- If there’s a problem, where do you start? Most engine-managed cars have a built-in self diagnosis system to make fault finding much easier. This is a Daihatsu Mira Turbo engine. mal parame­ters and so the ECM may be using an internally-programmed limp-home replacement signal for that particular sensor. In both cases, the fault code for the circuit will be retained within the ECM’s memory. To indicate that an ECM problem exists, most cars illu­minate a “check engine” light on the dashboard. Some cars will show the light only briefly and then, if the fault is not major, douse the light so as not to cause driver concern. At the car’s next service, the fault codes can be accessed as a normal part of the procedure and the problem found and remedied. The fault code is then manually cleared from the memory. However, there are many cars which don’t run dash-mounted engine management warning lights. If your car is one of these, there are two possibilities: (1) your car’s engine management system doesn’t have self-diagnosis facilities; or (2) self-diagnosis is used but there is no indication on the dashboard when fault codes are logged. It is this last situation which prompted 1 CHECK EACH FUEL INJECTOR FOR OPERATION OK CHECK FUEL PRESSURE NOT OK 2 CHECK VOLTAGE AT FUEL INJECTOR'S POWER TERMINAL NOT OK REPAIR HARNESS/CONNECTOR OK 3 CHECK FUEL INJECTORS NOT OK REPLACE FUEL INJECTOR OK 4 CHECK VOLTAGE AT EACH ECU TERMINAL NOT OK REPAIR HARNESS/CONNECTOR OK 5 CHECK HARNESS CONNECTORS BETWEEN ECU AND BODY NOT OK REPAIR HARNESS/CONNECTOR OK REPAIR ECU TERMINAL POOR CONTACT (REPLACE ECU) Fig.2: many factory workshop manuals provide flow charts for fault diagnosis; eg, if the engine won’t start. This particular flow chart is for the Subaru Liberty & shows the procedure to follow when checking the fuel injection system. June 1994  7 at least one mechanic to suggest that there are probably thousands of cars being driven around in constant limphome mode, their owners probably complaining that EFI cars use too much fuel, don’t start well, or have a poor idle! In fact, many limp-home or fail-safe operations are engi­neered so well that the driver will not notice any change in engine performance. Table 1 shows some of the limp-home capabili­ties of a Subaru Liberty RS Turbo. Accessing self-diagnostics DIAGNOSTIC MODE SELECTOR LIGHT-EMITTING DIODES Fig.3: self-diagnosis is triggered in different ways in different cars. In this Nissan system, coloured LEDs flash codes through an opening in the ECM case after the pot has been rotated to the correct position. Self-diagnosis in the Daihatsu Mira is activated by shorting two of the terminals in the under-bonnet diagnostic plug. This causes the “check engine” light to flash a coded output. 8  Silicon Chip The procedure undertaken to trigger the self-diagno­sis mode depends on the make of the car. In order to use the self-diagnosis mode, read the fault codes and carry out proper fault-finding, you need to have access to a workshop manual. These days, major new models are released with very extensive workshop manuals. The Subaru Liberty, for example, has a factory workshop manual comprising six volumes, with the engine management book alone being over 400 pages long! However, such manuals are expensive and often the manufac­turer will not sell them directly to the public. However, TAFE colleges which teach automotive subjects often have factory workshop manuals in their libraries, START ENGINE Protect your valuable issues Silicon Chip Binders WARM-UP ENGINE TURN IGNITION SWITCH OFF CONNECT TEST MODE CONNECTOR TURN IGNITION SWITCH ON (ENGINE OFF) CHECK I F CHECK ENGINE LI GHTS T UR N ON NO INSPECT CONTROL UNIT POWER SUPPLY AND GROU ND LI NE AN D CHECK ENGINE L IGHT LI NE DEPRESS ACCELERATOR PEDAL COMPLETELY. THEN RETURN IT TO THE HALF THROTTLE POSITION AND HOLD IT THERE FOR TWO SECONDS. RELEASE PEDAL COMPLETELY. START ENGINE These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. ★ High quality ★ Hold up to 14 issues YES CHECK I F CHECK ENGINE LI GHT I NDICA TES TROUBLE CODE ★ 80mm internal width ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover DRIVE AT SPEED GREATER THAN 11km/h FOR AT LEAST ONE MINUTE AND SHIFT UP TO 4th SPEED Price: $A14.95 (includes postage in Australia). NZ & PNG orders please add $A5 each for postage. Not available elsewhere. NO WARM-UP ENGINE ABOVE 2000RPM CHECK I F CHECK ENGINE LIGHT BLINKS YES TURN IGNITION SWITCH OFF Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. NO NO CHECK I F CHECK ENGINE L IGHT I NDICA T ES TROUBLE CODE DISCONNECT TEST MODE CONNECTOR CONFIRM TROUBLE CODE SELF-DIAGNOSIS SYSTEMS ARE OK. TROUBLE IS IN SOME OTHER SYSTEM Use this handy form ➦ YES Enclosed is my cheque/money order for $________ or please debit my MAKE SEQUENTIAL CHECKS OF TROUBLE CODE ❏ Bankcard   ❏ Visa   ❏ Mastercard Fig.4: this flow chart shows the self-diagnosis procedure for the Subaru Liberty. which are open to the pub­lic, while the Gregorys workshop manuals usually list fault codes and give a brief overview of testing procedures. Because each car is different, the examples used here relate specifically to the models being discussed. Most cars are similar but the codes and procedures will vary from car to car. Another initial problem in access- ing the self-diagnosis codes is that manufacturers often don’t give direct instructions on how to trigger the system’s operation. In one Daihatsu work­ shop manual for example, the listed procedure is to plug the under-bonnet check connector into Special Service Tool number 09991-87201-000. In fact, all this grand “tool” does is short the ECM self-diagnosis trigger wire Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ June 1994  9 0.5s TABLE 2 Flashes GLOWING EXTINGUISHED 4.5s 0.5s MALFUNCTION CODE 3 Fig.5: this chart shows how the “check engine” light flashes the diagnostic codes in the Daihatsu Mira. Faulty Circuit 2 MAP sensor 3 Ignition signal 4 Water temp sensor 5 Oxygen sensor 6 Engine rpm sensor 9 Vehicle speed sensor 11 Airconditioner switch TABLE 3 to ground! A bent paper clip used to connect two of the check socket’s terminals together does exactly the same job. A close examination of the workshop manu­al or a chat with a friendly mechanic at the local dealership will usually reveal all. Reading the codes If the car doesn’t have a “check-engine” light but still uses a self-diagnosis system, then LEDs mounted MALFUNCTION DISPLAY 01 02 03 04 05 OUTPUT SIGNAL (SELF-DIAGNOSIS CHECKER) ON OFF ON OFF ON OFF ON OFF ON OFF 10  Silicon Chip Possible Causes Engine difficult/fails to start 1, 2, 3, 4, 5, 7, 8, 9, 11, 12 Engine starts, then stalls 1, 2, 3, 4, 8, 9, 11, 12, 13 Acceleration flat spot 1, 4, 6, 8, 14 Engine surge 1, 4, 5, 6, 7, 8, 9, 11 Variable idle speed 4, 6, 10, 11, 12, 13 Engine misses 1, 2, 5, 7, 8, 12, 15 Lack of power 1, 4, 6, 7, 8, 9, 13, 14, 15 High fuel consumption 1, 4, 5, 7, 8, 9, 13 1 = ignition module; 2 = control relay; 3 = fuel pump; 4 = airflow meter; 5 = ECM; 6 = air intake system (leaks); 7 = injectors; 8 = fuel pressure; 9 = coolant temp sensor; 10 = throttle valve; 11 = idle air bypass valve; 12 = loom, earth points; 13 = throttle position switch; 14 = air filter; 15 = fuel filter. Limp-home modes will allow the engine to run near-normally, even with the coolant temperature sensor wire removed from the sensor as has been done here. MALFUNCTION CODE Symptom in the ECM and visible through a small window are usually used as the communica­tion interface. On the other hand, cars with a “check engine” light usually flash this dash-mounted light to communicate the codes. As an example, the Daihatsu system flashes its check engine light at 1Hz, with a 0.5 second duty cycle. Two flashes indicates the digit ‘2’ for example, with a 2.5 second delay until the next fault code is flashed. When all the fault codes have been flashed, there is a 4.5-second delay and then the fault codes present are repeated. If no faults exist, then the light will SENSOR OR SUBSYSTEM SELF-DIAGNOSIS IGNITION PULSE NO IGNITION SIGNAL NE SIGNAL NO NE SIGNAL FROM CRANK ANGLE SENSOR G1 SIGNAL NO G1 SIGNAL G2 SIGNAL NO G2 SIGNAL KNOCK SENSOR AND KNOCK CONTROL UNIT OPEN OR SHORT CIRCUIT flash ‘1’ at 4.5-second intervals. Table 2 shows the other fault codes. Note that it is the circuit which is faulty and not neces­sarily the sensor itself. Any wiring loom problems obviously also have to be checked. Older ECM systems Finally, what if your car is one which doesn’t run any form of self-diagnosis? These are more likely to be older cars and there­fore are more likely to suffer problems because of their age. With these systems, there are really no quick answers, however Table 3 makes SC a good starting point. FAIL-SAFE NEITHER G1 NOR G2 SIGNAL: ENGINE STOPPED RETARDS IGNITION TIMING 6o I N HE AV YLOAD CONDITIONS WASTE GATE OPENS EARLIER Fig.6: part of the selfdiagnosis section in the Mazda MX-6 turbo workshop manual. The malfunction code indicates a problem with that particular circuit, and can include connector, wiring, sensor and ECM faults.