Failure phenomenon
An Audi A6L produced in 2009, equipped with a BPJ engine, has a cumulative mileage of approximately 265,000 km. The owner reported that the fuse of the air conditioner controller of this car was occasionally blown. Replaced with a new fuse. The fuse was blown again after the vehicle drove normally for about 2 days. The car has been repaired several times in other repair shops, but they have not been eliminated. If it failed, the car was driven to our factory for maintenance.
Troubleshooting
After picking up the car, according to the owner’s instructions, the occasionally blown fuse was found in the fuse box on the right side of the dashboard (Figure 1); check the description on the fuse box cover and found that the fuse is an air conditioner controller (J255). ) Power supply with a rated current of 10 A. Repeatedly test the air conditioner function, the air conditioner works normally, and the fuse is not blown. Looking at the related circuit (Figure 2), it is known that the occasionally blown fuse is the SC8 fuse, which provides the No. 30 power supply for J255, and the SB5 fuse provides the No. 15 power supply for J255.
Connect a current clamp to the SC8 fuse (Figure 3), and use an oscilloscope to measure the current passing through the SC8 fuse. When I turned on the ignition switch, I found that the air conditioner control panel suddenly went black; check the SC8 fuse, the fuse was blown; check the current waveform measured by the oscilloscope (Figure 4), it was found that there was a current of about 20A with a duration of about 344ms , It is inferred that this is the reason that caused the SC8 fuse to blow.
Replace the SC8 fuse, repeatedly turn on and off the ignition switch, the fuse no longer blows. Operate the switches on the air conditioner control panel (Picture 5) in turn. When turning the right temperature adjustment knob to the coldest position, it is suddenly found that the current waveform is abnormal (Picture 6), and the current flowing through the SC8 fuse will suddenly rise to 12.33A, exceeding the rated current (10 A) of the fuse, obviously abnormal, but the fuse is not blown at this time. Which actuators should be involved when adjusting the right temperature adjustment knob? Checking the maintenance data, we know that the main actuators involved in the work at this time are the temperature regulating damper motor (V197), the coolant circulation pump (V50), the right heating regulating valve (N176) and the air conditioning compressor regulating valve (N280). Since V197, V50, and N280 are common actuators when adjusting the left and right temperature, and the fault will not occur when adjusting the left temperature adjustment knob, it is inferred that there is a greater possibility of failure in N176.
Then connect another current clamp to the N176 control wire (black and yellow wire) (Figure 7), and then constantly adjust the right temperature adjustment knob, and found that when the current flowing through the SC8 fuse is abnormal, it flows through the N176 The current of the control line is also abnormal, and the current is basically the same. It is inferred that the fault of the car is caused by an occasional short circuit inside the N176.
Remove the N176 and the left heater control valve (N175), and measure the resistance of N175 and N176 respectively (Figure 8). The resistance of N175 is 10.1 Ω, and the resistance of N176 is 6.1 Ω. The comparison shows that there is a short circuit fault inside N176.
Troubleshooting
After replacing the heater control valve assembly (including N175, N176, and V50), the test was repeated, and it was found that the current flowing through the SC8 fuse was no longer abnormally high (Figure 9), so the vehicle was delivered. One week later, the car owner visited the car by telephone. The car owner reported that the SC8 fuse was not blown, and the car was in normal operation, and the fault was completely eliminated.
Fault extension
The car is equipped with automatic dual-zone air conditioning, and the temperature on the front left and right sides can be adjusted independently. As shown in Figure 10, the coolant is driven by the coolant circulation pump (V50), flows to the heat exchanger through the two heater control valves, and then flows back to the engine. According to the set indoor temperature, J255 controls the compressor of the air conditioner for cooling on the one hand; on the other hand, it controls the opening angle of the two heating control valves to adjust the coolant flow into the heat exchanger.
As shown in Figure 11, the second area of the 40th group of data in the J255 data stream is the opening angle of N175, the third area is the opening angle of N176, and the fourth area is the working state of V50. The opening angle of the heating control valve varies from 0% to 100%. When the indoor temperature is set to the lowest, the display is 0%, indicating that the heating control valve is closed; when the indoor temperature is set to the highest, the display is 99%. Indicates that the heating control valve is at the maximum opening angle; after turning off the ignition switch, the two heating control valves are both at the maximum opening angle; the working status of V50 is divided into two types: "off" and "on". Note: The data in Zone 2 and Zone 3 both indicate the opening angle of the heating control valve calculated by J255 according to the set indoor temperature, not the actual opening angle of the heating control valve, for example, the heating control valve is stuck in the closed position , The data of Zone 2 and Zone 3 will not always display 0%, but will still change from 0% to 100% with the change of the set indoor temperature.
