A/C Compressor Disengagement

If faced with a customer complaint of compressor disengagement after turning the system on and everything appears to be functioning correctly, don't forget to check the evaporator thermoprobe/thermoswitch.

The evaporator thermoprobe is mounted in the fins of the evaporator and monitors evaporator temperature to prevent the evaporator from freezing up. It does this by cycling the compressor on/off as necessary. If the thermoprobe fails, it most likely will allow the compressor to engage initially but will open the circuit for the compressor, turning it off and then will remain open. The compressor can be engaged again by turning the A/C main switch off, then on again. However, the thermoprobe will again open the circuit and will not close to engage the compressor.

To check the thermoprobe is simple. Locate the connector for the thermoprobe under the dash near the glove box and disconnect it. Now jump across the harness side of the connector (refer to the correct Service Manual for the vehicle you are working on for the correct wires to connect). If the compressor now engages and stays engaged, you have most likely found the problem with the system. of course, there are other components of the system that can cause a similar problem but, this check is an easy one to perform and, in most cases, is the source of the problem.