Gord's Green Z28

This one has been racking my brain for quite some time. 1989 GTA with dual fans. R-12 freon.

Driver side fan turns on at proper coolant temp (176 in my case) or immediately upon turning on the air conditioning. Good.

Passenger side fan is supposed to turn on at fan switch temp (240ish). It does.... Also supposed to turn on when the A/C head pressure hits 233psi/1600kpa.

I have never seen the fan turn on while running the air conditioning. Ran on the highway for 30 minutes, A/C on the whole way, pull into my driveway and pop the hood. Just the driver side is on. Let the A/C run while sitting in 90F stop and go traffic for half an hour. The coolant temp will eventually get to 240 and the passenger will turn on. But it never seems to turn on with A/C use.

So what I'm wondering is what are the regular A/C head pressures (I know this would vary according to outside temperature, but a ballpark number would be good). How often do our cars actually get to that high of a head pressure? Arizona desert in 120F summer conditions only? Or would driving to a corner store in New Hampshire in spring with the A/C on be enough for the pressure to kick on the fan?

I know I could put in a cooler fan switch and just avoid the ordeal. Might do that. I don't want a manual fan switch.

How does it work on your dual fan setups?

Stephen 87 IROC

Typical max AC head pressure is 250 psi. Some systems may climb to 300 psi. Not sure on the exact way an automotive system works. I work on heavy trucks.

Normally the AC compressor clutch will cycle depending on the outlet temperatures. It keeps the evaporator from freezing up. When head pressure reaches the limit set by the pressure sensor, either the clutch is turned off on older mechanical fan systems or the fan(s) are turned on to increase air flow across the condensor which will bring down head pressures.

Driving down the highway should provide enough air flow across the condensor to keep head pressure low enough.

Using the defroster will also turn on the AC because it will act like a dehumidifier.

Some modern computer controlled systems will turn on the fans to bring head pressures down however they will not turn off as soon as pressures reach the low limits. If computer controlled, the computer will keep the fans on for a few minutes longer.

In cool weather, there's enough heat being exchanged off the condensor that head pressure may not climb high enough to turn on the fans. Not having the proper charge in the system will also cause problems. Too much freon is just as bad as not enough. I fixed a truck a few years ago where the fan cycled too much when the AC was on. The system was designed for 4 pounds of freon and he had about 9 in the system.

If your system is contaminated and you have air in the system, it will also not work properly. Air can be compressed and will give abnormally high pressures.