1999 Mercedes-Benz ML320 Air Conditioning Evaporator Temperature Sensor Replacement Cost

The air conditioning refrigerant system is a large system designed to cool the evaporator core in order to cool the air in the cabin. This system works well, but needs to be monitored to ensure the refrigerant system is cooling the evaporator core sufficiently to cool the inside of the vehicle and ensure it does not get so cold it freezes. For this purpose, the air conditioning evaporator core temperature sensor is used to monitor the efficiency of the system.

Mounted on the evaporator core, this thermal sensor measures the temperature of the evaporator core constantly, and communicates the temperature back to the air conditioner control module. The control module will determine if the evaporator core is too cool, too warm, or just right. If too warm, the control module can adjust compressor output or expansion valve flow rates, which normally moves the evaporator core into the appropriate range. If the evaporator core does not move to an acceptable temperature range, the temperature sensor will likely stop function of the air conditioning system.

Like all air conditioning refrigerant system issues, an improperly functioning or failed evaporator temperature sensor will lead to decreased performance of the air conditioning system for the vehicle. Incorrect readings from the temperature sensor may cause the air conditioner to become extremely cold or warm. In either case, the air conditioner will likely stop functioning, and the a/c button in the cabin may begin blinking. The air conditioner may blow cold air intermittently, but will always return to a non-functional state.

There is no necessity to repair the a/c evaporator core temperature sensor immediately. Replacement is based on convenience and comfort alone. In some locations, the heat outside may be sufficient to consider using a vehicle with no air conditioner a health risk for children or seniors, so this would raise safety concerns.

Temperature sensors are typically long lived. They are installed in a safe location, have no moving parts, and include only simple circuits. If the temperature sensor reports a failure, it is most likely performing its job correctly, and there will be an issue in the refrigerant system causing the evaporator core to cool excessively or not at all.

The first step to identifying an issue with the temperature sensor is to identify if it is functioning properly. This can be done by measuring the air temperature coming from the vents inside the vehicle, and comparing those readings to the sensor readings in the vehicle. If the readings match, the sensor is likely functioning properly. However, if an incongruence exists between the readings, the sensor will be replaced once the circuit is verified to function properly.

Most often, the evaporator temperature sensor can be accessed underneath the instrument panel or behind the glove compartment. However, this is not always the case, and in some vehicles the dashboard will need to be removed partially or completely to access the sensor. Once accessed, the sensor can be popped out of place or unfastened, and disconnected electrically before being replaced and reconnected. Finally, the system will be tested in the same diagnostic manner to determine if the issue has been resolved.

Air conditioning issues tend to have the same symptoms, but there are many components that may fail and lead to these symptoms. This being the case, it is imperative that the air conditioning system be diagnosed properly before replacing components. This will prevent frustration and frivolous costs.

Not all vehicles have an evaporator temperature sensor. Some will only deactivate the system based on system pressure or temperature at other positions, so diagnosis is a must. It may be the most helpful to have the service manual for the vehicle on hand to allow proper diagnostic procedures to be taken.

Diagnosing the sensor is ideally performed with a diagnostic scan tool and a IR thermometer. However, since most people do not have the expensive scanning equipment, a multimeter can be used to diagnose the temperature sensor as long as the voltage specifications and vent air temperature are known. If this proves to be a challenge or impossible, we advise allowing a certified air conditioning technician handle the repair.