Failure phenomenon
A 2011 BMW 525Li car (model code F18), equipped with an N52 engine, has a cumulative mileage of approximately 106,000 kilometers. Customers report that when the air conditioner is switched on for cooling, cold air blows from the central air outlet and hot air from the foot air outlet.
Troubleshooting
Test drive after receiving the car, start the engine, turn on the air conditioner switch, and adjust the temperature to the lowest. Cool air blows from the central air outlet, hot air blows from the air outlets at the feet of the main and co-drivers; adjust the air outlet mode and find that the air outlet of the front windshield is also blowing Hot air. Detect with a fault detector, no relevant fault code is stored; read the air conditioning data stream (Figure 1) and find that the evaporator temperature is 3.40 ℃, indicating that the refrigeration system is working normally; the temperature of the heating air heat exchanger on the right is 67.00 ℃, on the left The temperature of the warm air heat exchanger is 64.00 ℃, indicating that the coolant circulation in both warm air heat exchangers is abnormal.
Looking at the maintenance data, it is known that the warm air part of the car is controlled by the warm air heat exchanger coolant valve. The junction box electronic device (JBE) adjusts the opening of the coolant valve of the warm air heat exchanger according to the set temperature to control the amount of coolant flowing into the warm air heat exchanger, thereby adjusting the outlet air temperature. When the set temperature is the lowest, the cooling liquid valve of the warm air heat exchanger is kept closed, and no cooling liquid is allowed to flow into the warm air heat exchanger. Since the vehicle still has coolant circulating in the warm air 25 heat exchangers on the left and right sides when the set temperature is the lowest, it is inferred that the possible causes of failure are: the warm air heat exchanger coolant valve is stuck in the normally open state; The air heat exchanger coolant valve control circuit is faulty; the JBE is damaged.
The cooling liquid valve of the warm air heat exchanger of the vehicle is a normally open valve, that is, when the cooling liquid valve of the warm air heat exchanger is not working, the cooling liquid is normally vented to the warm air heat exchanger. It can be seen from Figure 2 that the cooling liquid valve terminal 1 of the warm air heat exchanger is often grounded. When JBE does not output power to the cooling liquid valve terminals 2 and 3 of the warm air heat exchanger, the cooling liquid valve of the warm air heat exchanger starts to open. ; When JBE outputs power to terminal 2 and terminal 3, the coolant valve of the warm air heat exchanger starts to close.
Connect the oscilloscope, and measure the signal waveforms on the 3 terminals of the coolant valve of the warm air heat exchanger at the same time (Figure 3). When the set temperature is the lowest, both terminals 2 and 3 are always powered by 12V, indicating that JBE is controlling the heating The heat exchanger coolant valve is closed, and the control target is completely closed, and the control signal is normal; when the set temperature is the highest, terminals 2 and 3 will be cut off intermittently for a short time, indicating that JBE is controlling the cooling of the warm air heat exchanger The liquid valve is open, but the control target opening is very small, and after a few seconds, both terminals 2 and 3 have been powered by 12 V (Figure 4). When the set temperature is the lowest, JBE controls the heater heat exchanger coolant valve to be completely closed, then when the set temperature is the highest, JBE should control the heater heat exchanger coolant valve to fully open, that is, JBE will not Power is supplied to terminal 2 and terminal 3 of the coolant valve of the heating air heat exchanger, but the power is cut off intermittently for a short time. Is there a problem with the JBE control signal during heating? The author does not understand this, but it is certain that when the set temperature is the lowest, JBE is controlling the heating air heat exchanger coolant valve to completely close, and the left and right heating air heat exchangers in the air conditioning data stream It can be seen from the temperature that there is still coolant circulating at this time, indicating that the coolant valve of the warm air heat exchanger is actually not completely closed, so it is determined that the coolant valve of the warm air heat exchanger is stuck (Figure 5).
Troubleshooting
After replacing the coolant valve of the warm air heat exchanger, test run, set the lowest temperature, and blow cold air at each outlet, troubleshooting.
Fault summary
After troubleshooting, after repeated tests, the previous confusion was finally revealed. When the set temperature is the highest, the coolant valve of the warm air heat exchanger will not always be in a fully open state, otherwise the temperature of the warm air heat exchanger will remain high, so that the air outlet temperature will also remain high, and the air conditioner The maximum setting temperature is 30 ℃, which does not require so much heat. Therefore, JBE will comprehensively analyze the setting temperature, ambient temperature and the temperature of the warm air heat exchangers on the left and right sides to control the opening of the coolant valve of the warm air heat exchanger , Just like when the air conditioner is refrigerated, the air conditioner compressor does not always work at the maximum cooling capacity. Because the coolant valve of the heater heat exchanger of the malfunctioning car is stuck, the temperature of the heater heat exchanger on the left and right sides is always high. Therefore, even if the air conditioner setting temperature is adjusted from the lowest to the highest, JBE will only control the heating for a short time. The coolant valve of the air heat exchanger is opened very slightly.
Expert remind
Through this case, I hope that you can develop the habit of "relating to the car based on the car" when diagnosing faults. On the one hand, it should be "based on the car's fault phenomenon" and on the other hand it should be "based on its working principle."
What is the "fault phenomenon based on the car"? Taking this case as an example, we cannot start to check the refrigeration system as soon as we see hot air blowing from the air outlet. As a qualified car repairer, you should be able to make full use of diagnostic tools, such as fault detectors, oscilloscopes, etc., to extend the observed failure phenomena, and you must be able to see the external "qualitative changes" (engine acceleration weak, air conditioning Symptoms such as poor refrigeration) and internal "quantity changes" (abnormalities in sensor signals, control signals, switch signals, etc.) can be seen, so that the scope of faults can be narrowed, carpet troubleshooting can be avoided, and diagnostic efficiency should be improved. In this case, Tang Gong made a good demonstration. He read the air conditioning data stream through the fault detector, and ruled out the possibility of failure of the air conditioning refrigeration system, and locked the fault point on the heating system; then passed The oscilloscope confirms that the control signal sent by the JBE is normal during cooling, and finally determines that the coolant valve of the heater heat exchanger is stuck.
What is "the working principle of car theory"? There are many types of models on the market now. Although some systems have similar working principles, sometimes if you can generalize the working principles of the same system on different models based on maintenance experience, you may make detours in the diagnosis without paying attention to details. Taking this case as an example, most of the heater heat exchanger coolant valve adopts negative control mode (there is constant power supply, the control unit is responsible for controlling the grounding), while the car adopts the positive control method (there is regular grounding, the control unit is responsible for Control power supply). Assuming that the warm air heat exchanger coolant valve repaired by the maintenance personnel before is all using the negative control method, when diagnosing the fault of the car, if you do not check the relevant circuit and judge based on the maintenance experience, then the heating is not measured. When the heat exchanger coolant valve is powered, it is likely that the heater heat exchanger coolant valve has no power supply, and the heater heat exchanger coolant valve is kept in the normally open position, resulting in constant cooling in the heater heat exchanger Liquid circulation.
To develop the habit of "car to car" is actually to be good at using diagnostic tools to understand the failure phenomenon as comprehensively as possible; it is to be diligent in viewing maintenance information and accurately grasp the working principle of the system. The German philosopher Leibniz once said that "there are no two identical leaves in the world". Similarly, we can say that "cars do not have exactly the same failure phenomenon." In short, familiar with the working principle of the system, and proficiently using diagnostic tools to find fault phenomena that can be used to narrow the scope of the fault, the fault will soon be revealed.