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A Chevrolet Cruz car with a mileage of about 15 thousand km and a 1.6L engine. User response: the vehicle starter can operate normally, but there is no sign of engine running. After taking the car: connect GDS2+MDI check, engine control module ECM set 6 fault codes: P080A, unlearned clutch pedal position; P0643, 5V datum 1 voltage loop high voltage; P0653, 5V reference 2 voltage loop high voltage; P0108, manifold absolute pressure sensor circuit high voltage; P0688, engine control open Turn off relay to cut off current and voltage; P2108, throttle actuator position performance. Check the description of the fault codes for the maintenance manual, P0688: the engine control switch relay feedback circuit; check the circuit diagram of the engine control module (as shown in Figure 5) that the engine control switch relay (the code code on the circuit diagram is KR75) is an electrical switch and is played under the control of the engine control module. Open and close. The engine control switch relay switch is kept in the open position by spring tension. The battery positive voltage is always supplied directly to the relay coil and relay switch contacts. The engine control module (ECM) provides an iron path to the relay coil control circuit through the output driver module. The output driver module of the engine control switch relay also contains a fault detection circuit, and the engine control module continuously monitors the circuit. When the engine control module directs the engine control switch relay to be energized, it will switch the battery voltage to the fuse in the fuse box under the hood. The switching battery voltage provided to the engine control module is supplied to the internal circuit of the engine control module related to the throttle actuator control (TAC) operation. The engine control module also monitors the voltage on the switch battery circuit to confirm that the engine control switch relay contacts have been closed. When the engine control module detects the engine control switch relay control circuit opening, short circuit or short circuit voltage, P0688 is set. Because the engine control switch relay not only supplies the engine control module through the F10UA (15A), but also supplies the fuel injector and the ignition coil through the F9UA (15A) fuse, so if the relay is not closed or the circuit has problems, it will directly cause the engine to be unable to start because of no oil and no fire. Therefore, the engine will not be started because of no oil and no fire. Decide to check the engine control switch relay KR75 and its circuit first. First, the rear window demogging relay KR5 with the same part number is interchanged with the engine control switch relay KR75, which is not valid; then the power supply and the iron lap of the ECM are measured to be normal, the 4 foot of the X2 terminal is measured as B + battery voltage, and the No. 4 foot is normal in the case of the ignition switch open, and the X1 plug 20 is normal. The 35 foot and the X2 plug 1, 3, 5 leg tie lines are normal; the ignition switch is placed in the OFF position, the KR75 engine control switch relay is removed from the fuse box under the engine hatch cover, and the voltage between the engine control switch relay base terminal 2, 3 and the iron lap is higher than 11V, normal; the test of the KR75 engine control The voltage between the switch relay base terminal 1 and the iron lap is less than 0.3V, and there is no normal short circuit. The test lamp is inserted between the terminal 2 and 1 of the switch relay base of the KR75 engine control, the ignition switch is placed in the ON position, and the result of the test lamp is not lighted. Test KR75 engine control switch relay base terminal 1 and K20 engine control module X1 plug twenty-first foot control circuit between normal, no circuit breakage / excessive resistance phenomenon, indicating that the cause of failure is that the engine control module is damaged, can not pass the output driver module to the KR75 engine control switch The electrical coil control circuit provides an iron lap path. In order to verify the above judgment, the ECM of another car is interchanged with the car, the ignition switch is opened, the X1 plug 50 foot of ECM and the X2 plug No. 6 foot are the battery voltage, normal. Troubleshooting: replace engine control module ECM and program, troubleshooting.