Troubleshooting Circuit Diagrams: ECU Power Failure Repair

With the popularity of electronic fuel injection systems, vehicles require more and more electronically controlled components, and the wiring of the entire vehicle is becoming increasingly complex. To learn the working principles of electronic control, one must first understand circuit diagrams and the working logic of the ECU.

1. (Unable to Start) ECU Power Failure Troubleshooting Process:

If the ECU is not powered, the starter does not work, indicating a possible wiring issue:

1. Turn the ignition switch to the “ON” position; if the fault light does not show self-checking or cannot connect to the diagnostic tool;

2. The operating voltage of the coolant temperature sensor is 0V;

3. The throttle pedal signal voltages for circuits 1 and 2 are also 0V;

4. The voltage at the vehicle connector terminals (power and ground) is 24V; (If this reads 0V, it indicates a break in the wiring from the battery; check if the 30A fuse is damaged and whether the wiring connection is broken.)

This indicates that the wiring supplying power to the ECU is normal, but for some reason, the internal relay of the ECU is not working! The relay is controlled by the ignition switch signal!

5. On the engine harness, piercing wire 140 (ignition switch) measures 0V; (This should be 24V)

6. The voltage on wire 130 is 24V (130 is the ground for the fault indicator light, which should be 0V. Measuring this wire aims to determine if the vehicle wiring harness is incorrect, indicating that 130 and 140 are reversed. Once this wiring is normal, the diagnostic tool can be connected to check for faults.

Generally, after the ECU is powered on, the corresponding pins will output voltage. Using an AS201 multimeter to measure the output voltage can basically determine whether the ECU is powered successfully or if it is normal.

The pins typically measured include communication pins (under normal conditions, the CAN line is around 2.5V, and the K line is slightly less than the power supply voltage), common high side (normally 24V or 12V), power supply module (normally 5V), and measurement unit power supply voltage (normally 24V or 12V); measure the voltage at the corresponding pins.