◆◆Mercedes-Benz E200 Air Conditioning Not Cooling◆◆
(1) Failure Symptoms
The chassis model of this vehicle is W211, equipped with an M272 engine. The air conditioning system is not cooling, and the two air outlets in the center of the dashboard are not blowing air, while the side outlets are blowing hot air.
(2) Diagnosis and Repair
All buttons on the air conditioning control panel can be operated normally, and there are no warning messages on the dashboard, but the fault does exist. A diagnostic tool was used for self-diagnosis, and the fault information for the automatic air conditioning system indicated a fault in the LIN bus. The working data of the automatic air conditioning system is as follows:
Refrigerant Pressure/bar: 12.80
Evaporator Temperature Sensor Signal/°C: 7.40
Air Conditioning Compressor Working Current/A: 0.95
External Temperature Sensor Signal/°C: 27.50
Outside Temperature/°C: 26.50
Inside Temperature Sensor Signal/°C: 36.50
Coolant Temperature/°C: 95.00
Engine Speed/(r/min): 750
The above data shows no obvious abnormalities. Checking the circuit diagram, it was found that the air conditioning system has a total of 5 air door motors, which are connected in series on the LIN bus. The LIN bus comes from the automatic air conditioning control module and passes through the defrost air door motor, air distribution air door motor, left mixing air door motor, right mixing air door motor, and internal/external circulation air door motor. Using the diagnostic tool for adaptive learning of the air door motors, the diagnostic tool indicated that it could not be completed.
Checked the power supply and ground lines of the automatic air conditioning control module, which were normal. Measured the LIN bus voltage at the connector of this control module, which was 13.5V, indicating that the voltage was too high and abnormal. Sequentially unplugged each air door motor’s connector for inspection, and found that when the left mixing air door motor’s connector was unplugged, the air conditioning system started blowing cool air, and the two air doors in the center of the dashboard opened. This indicates that there is a fault in the left mixing air door motor or its associated wiring, but the fault point is still undetermined. Continuing the inspection, it was finally determined that the internal/external circulation air door motor was damaged. After replacement, the fault was completely eliminated.
(3) Summary
The cause of this fault is the internal damage of the internal/external circulation air door motor, which caused a short circuit between its power supply and the LIN bus. The power supply voltage is around 13.5V, while the LIN line voltage is around 11V, so when measuring the LIN bus voltage, it becomes 13.5V, causing the automatic air conditioning system to enter a protection state and unable to cool normally, and the air door motor cannot learn.
◆◆Mercedes-Benz S350 Air Conditioning Not Cooling◆◆
(1) Failure Symptoms
The chassis model of this vehicle is W221, equipped with an M272 engine. During the use of the air conditioning system, the airflow from the air conditioning outlets suddenly decreased, with a slight breeze from the side outlets and no airflow from the central outlet. Adjusting the airflow control button had no effect.
(2) Diagnosis and Repair
Confirmed the failure symptoms and found that when the fault occurred, no matter how the airflow was adjusted, there was no airflow from the central outlet, and the airflow from the side outlets was very small, with no change in the blower motor speed, while the airflow level displayed on the central screen could be adjusted.
Used a diagnostic tool for self-diagnosis of the air conditioning system, which indicated two current faults: one was a communication fault with the blower motor regulator (A32n1); the other was the LIN bus being closed. It seems that there is a problem with the LIN bus of the air conditioning system, so a oscilloscope was used to measure the LIN bus signal, which was abnormal and in a short circuit state to ground. Referring to the air conditioning system circuit diagram, it was found that the system has a total of 3 LIN buses, which are connected to terminal 24 of connector B of the air conditioning control module, and terminals 3 and 5 of connector D. The LIN bus connected to terminal 24 of connector B is connected to the blower motor regulator and the front SAM control module through node Z68/3. The LIN buses connected to terminals 3 and 5 of connector D are used to control all air door motors in the front area. Additionally, terminal 24 of connector B is internally connected to terminal 5 of connector D in the air conditioning control module.
Removed the air conditioning control module, connected the oscilloscope to terminal D, and then unplugged terminal 24 of connector B. At this point, the oscilloscope displayed a normal LIN signal waveform. Then reconnected terminal 24 of connector B and disconnected terminal 13 of connector C5 of the front SAM control module, and the LIN signal waveform was also normal, and the air conditioning system returned to normal.
Could it be that the front SAM control module has a problem? What role does the LIN bus connected to the front SAM control module play? Referring to the circuit diagram of the front SAM control module, it was found that the rear air conditioning system’s outlet motor is controlled by the front SAM control module. Measurements on the actual vehicle confirmed that the outlet motor of the rear air conditioning system is also controlled by the LIN bus.
At this point in the repair, the fault became clearer; it should be a problem with the outlet motor or wiring of the rear air conditioning system. Inspected the 4 outlet motors of the rear air conditioning system, and when the outlet motor at the right rear B-pillar under the front passenger seat was removed, it was found that the connector of that motor was corroded, due to the front evaporator’s drainage pipe falling off, causing condensate to flow under the seat and short-circuiting the LIN wire of that motor. Cleaned the wiring and connector, replaced the right B-pillar outlet motor, and after a test drive, the fault symptoms disappeared, concluding the repair work.
(3) Summary
The LIN bus of the air conditioning system connects many components, and the network structure is relatively complex, making repairs quite challenging. It requires the use of relevant materials to analyze the circuit path and to find the fault cause through diagnostic and measurement results, ultimately eliminating the fault.
This content is sourced from:
“Automotive Bus Fault Diagnosis and Typical Cases” (Edited by Xu Xiaoqi)
This book mainly introduces the basic structure of automotive buses, including the power CAN bus, comfort CAN bus, information CAN bus, LIN bus, serial data interface, body bus protocol, gateway, byteflight bus, and MOST bus structure and working principles, as well as fault codes, data streams, oscilloscope, multimeter detection and diagnosis methods, etc. It includes over 100 typical cases of buses from brands such as Mercedes-Benz, BMW, Audi, Volkswagen, Haima, Honda, Toyota, Ford, Nissan, Citroën, and General Motors.
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