ac system
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Not always - ECM driven, it depends on the temperature/pressure of the gas after the Condensor. For '85 to '89, a pressure sensitive switch signals the ECM when it reaches a threshold of about 230 psi. After that, a thermistor in the high pressure line sends a voltage that's proportional to pressure. Pressure is dependent on outside air temp or the temp of the air cooling the Condensor. Most designs drive the fan(s) to maintain an average pressure of 200 psi. Realistically, a fully charged system will make these numbers within seconds if it's 70 or better. Below that, they will cycle on/off and below 50, nothing may come on until there's some heat from the engine compartment. Troubleshoot with a manifold gage set (for pressure), scanner ('90 and above), and through '89, disconnecting the switch on the high pressure line should turn on the fan (meaning the ECM is working correctly and it's something else).
