thecarnut

The main radiator fan is not turning on when the AC is on, the fan works well otherwise.

I assume the main fan should turn on when the AC is on .... correct? What in the AC system triggers the main fan's relay?

This is a 1987 coupe., and it also has an aux fan. The aux fax also does not come on with the AC.

Aggravated4life

When this happened to my car before,it was a bad ECM.My a/c wouldnt get cold because the fan wouldnt come on for the command,but came on when the coolant reached fan temps.

RonsRed88

Mine did the same thing until I had the AC recharged. Before that, my AC was constantly cycling on and off. I'm guessing that a certain amount of pressure has to be built up before the fan comes on.

Ron

SunCr

a/c fan switch is on the high pressure or smaller line to the Evaporator - leads are dark green and black. Signals the ECM to turn on the fan when the high side gets to 240 psi. Turns it off at 190 psi. To rule out the ECM and harness, disconnect it. Fan should run. If it does, switch is either bad, charge is low, or outside air temp is too cold (like 50F & below) to get the high side to 240 psi. You need to hook up a manifold gauge set to check it out if ECM and harness are ok.

KurtK

I went through this problem last summer, and it was due to a low charge of refrigerant. You could hear the compressor's clutch cycling every few seconds.

:cool:

thecarnut

Thank for all the suggestions. The freon level appear fine, in fact the high pressure is too high do to the radiator fan not coming on.

I have the electronic AC control module (C68). Looking at the electrical diagram, the AC fan control switch is on the high side and sits right next to the high pressure cut off switch. The AC control fan switch is normally closed, so if disconnected the main radiator fan should start .... it did not. I checked continuity of the dark green wire between the control switch and the ECM pin D11 ... that is okay. Also checked that pin B8 on the ECM is getting voltage when the AC is on .... that is also fine. So it appears the ECM is getting the proper inputs but not commanding the radiator fan to start. A further test was to ground the diagnostic terminal on the ALDL, this tells the ECM to start all relays it controls (except the fuel pump) ... the main radiator fan works.

So, I am starting to conclude there is something wrong with the ECM. Is there any other possibility I am missing? There was a posting that said the outside air temperature had to be above 50 degrees ... where does the ECM obtain this information? Perhaps this sensor is bad ....

If the problem is indeed the ECM, can I replace ECM and simply move my old chip? What years ECM will work with my 1987 coupe?

SunCr

Refigerent has a static pressure based on temp. A can of R12 at 50 degrees has a static pressure of about 46.7 psi. At 70, it's about 70.1. The fan switch senses system pressure and will open when the high side pressure gets to about 240. Most systems won't get to this pressure below 50. If the fan didn't come on with the compressor engaged and the switch disconnected, it's either the ECM or the harness is shorted to ground. Rebuilds are available at your Dealer, most discount auto parts retailers or through the various vendors that cater to Vettes.