Audi A6 C5 reversing alarm system trouble (real case)

Audi A6 C5 equipped with ultrasonic reversing alarm system, commonly called as Parking Sensor.Contain the reverse alarm control unit J446, reverse alarm buzzer H15, rear bumper installed on the four reversing alarm sensor G203, G204, G205, G206, multi-function switch F125 reversing signal and other components.(picture 1)

It use to monitor obstacles rear of car 0.2 ~ 1.5m when you back up, in case of obstacles issued a buzzing alarm.

A Audi A6 2008 reversing alarm system get trouble.I test the car with owner,and observe parking sensors surface without damage.

Altrasonic reversing alarm system trouble possible reasons:

1.Parking Sensors fault
2.reverse switch fault
3.Control unit fault
4.Line fault

Dianoses:

Use VAS 5052 to check out address code 76 reverse alarm control unit J4467,and not found trouble code.When test car find reversing signal light off,it provided by the multi-function switch F125.If J446 will not receive the reverse signal,the reverse alarm system will be disabled.

According to circuit diagram(picture 2),fuse S231 provides multi-function switch F125 positive, if S231 fuse, F125 will not issue a reversing signal.I find fuse S231 has been blown,and circuit is not found on the ground short circuit fault.After repalcing the S231,reverse alarm system is working properly.But 30 mins later,S231 fuse again.It is found there exist ground short circuit under the S231.

There is a set point A38 in the circuit diagram.It provide positive voltage to legs line.It shows that several branches may exist on the ground short circuit. Take the exclusion method, one by one to disconnect the branch of the electrical plug,measuring successively.There is still short to the ground under S231 fuse.It indicating that the ground short circuit is located in the wire.
Continue to check the harness, find the engine compartment of the computer box plug was wet with coolant. After inspection,it come form the expansion water tank. Water droplets to the engine harness, wiring harness into the computer box plug.

Finally,replace the expansion tank and clean the plug at the computer box. Test, reverse alarm system back to normal.