Model: C8, equipped with DPMA engine and 0CK transmission.
Driving mileage: 233 miles.
Issue: The customer reported that sometimes the fan will run automatically after the remote control opens the door or stops the car. The diagnostic instrument finds that there is a fault code "P069300 cooling fan actuation 2, short-circuit to ground" in the thermal management control unit J1024.
Troubleshooting: The car is a plug-in hybrid vehicle, the temperature of the air conditioner and engine as well as the temperature of the high voltage components and the cooling fan are controlled by the thermal management control unit J1024. Referring to the circuit diagram (as shown in Figure 1), it can be seen that the main and auxiliary fans have 1 main power supply, 1 main ground, 1 KL87 line and 1 signal control line respectively. KL87 is about 9.35V when the ignition switch is turned on. When the output duty cycle of the CTRL control line is 10%, the fan is stopped; when the duty cycle output exceeds 10% and is closer to 100%, the fan speed is higher.
According to the above inspection, the possible causes of the failure are as follows:
(1) The control line from the fan control module 2-J671 to the thermal management control unit J1024 has an open circuit or a short circuit to the ground.
(2) The fan control module 2-J671 has internal damage.
(3) There is internal damage to the thermal management module.
First, check that there is no virtual connection, open circuit and short circuit to ground from the No. 3 terminal of J671 to the T40/88 terminal of J1024. Then the fault at this time may be that one of the control units is faulty. Because the customer complained a lot and required a one-time solution to the failure, it was decided to repeatedly test the abnormal data when the failure occurred.
After a test drive, it was found that the fan did run automatically after a stop, but only fan 2 was running; when the data was further checked, the fan returned to normal. When the cooling fan of the Audi car series is running, two fans must be running or stopped together. The automatic operation of one fan in the car is initially considered to be a quality problem in the cooling fan 2 control module. To support the current diagnostic analysis on the data, further test runs were carried out. When a fault occurs, the cooling fan control duty ratio of the thermal management control unit J1024 is read in real time at 10%, that is, the fan operation is not controlled.
Based on this, it is judged that the cooling fan 2 control module J671 has intermittent faults. After replacing the cooling fan 2 assembly (including the fan control module), the fault is eliminated.
Fault summary: Although the cooling fan control of the car has been transferred from the engine control unit to the thermal management control unit, its original main control strategy is not. When the ignition switch is turned on, if there is a low potential or an open circuit in the control line, the fan will enter the emergency mode and automatically run at the highest speed to prevent high temperature. Originally, when only the engine is running, the cooling fan can run for at most 15 minutes after the ignition switch is turned off. After that, the main relay of the engine control unit is disconnected and cannot control the fan rotation again; while the plug-in hybrid vehicle may be turned off when the ignition switch is turned off and the door is turned off. When the locked high-voltage battery is charged, the air-conditioning system may also be activated to dissipate heat from the high-voltage battery; then the cooling fan must also run under this condition.
There are many vehicle control parameters, and the cause of the fault can be easily and quickly locked by analyzing the data flow. In the diagnosis process, the data flow should be used for analysis.