Hi, Guys,

OK, if the A/C high pressure switch fails it applies a ground to the coolant fan relay and the fan runs continuously (my actual symptom on an '88GT). Unplugging the coolant temperature switch does not stop the fan from running, so the next step is unplugging the A/C high pressure cut-out switch (I have not yet done this).

My question is simply: what else happens if the A/c High Pressure Cut-Out Switch fails? Does it shut off the compressor clutch? Something else?

TIA,
Larry

If I recall right, the only thing the high pressure cut out switch does is not provide ground to the AC compressor clutch when the pressure is too high.

It would have no effect on the radiator fan.

The ground to activate the radiator fan relay for the A/C comes from the heater dash control unit, not from the engine compartment.

Mine did the same thing only the high pressure cut out switch was fine the wires to it just got twisted together during and after removing/replacing the engine. Untwisted them and fan shut off. Not sure what else it does or causes. I didnt have freon in the system to test.

[This message has been edited by Goldenfoot05 (edited 01-08-2012).]

Phonedawgz,

I cannot recall another time when you might be in error, but: From the '88 FSM:

"...the A/C High Pressure Cut-Out Switch...also energizes the Coolant Fan Relay by grounding circuit 335...";

also: the second step in trouble shooting continuously running coolant fan is unplugging the A/C High Pressure Cut-out swtich and replacing it if the fan continues to run.

So, does closing the high pressure cut-out switch also de-engergize the A/C clutch?

Larry

I would not be surprised if they wired the 88 differently.

Yes I could very well be wrong. I do believe I am right however for the pre '88.

And yes I do make mistakes. For sure.

This past summer I replaced the hp cutout switch on ours. The new switch came with two wires on it. The directions said to connect one of those wires to a ground and the other to the oem wire. I did just that, and the cooling fan then ran continuously. After a bit of PFF research I disconnected the grounded wire and the system ran perfect. By the way, the oem switch has only one wire on it.

Hope this helps.

quote Originally posted by imacflier: Phonedawgz, I cannot recall another time when you might be in error, but: From the '88 FSM: "...the A/C High Pressure Cut-Out Switch...also energizes the Coolant Fan Relay by grounding circuit 335..."; also: the second step in trouble shooting continuously running coolant fan is unplugging the A/C High Pressure Cut-out swtich and replacing it if the fan continues to run. So, does closing the high pressure cut-out switch also de-engergize the A/C clutch? Larry

Fan control is provided by the PCM on all modern GM cars. I believe that one of the inputs used for control of the cooling fan is the refrigerent pressure switch.The other is the ETS and A/C on function..

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

AC Compressor switches:

https://www.fiero.nl/forum/Forum2/HTML/117640.html

The low-pressure switch (white mushroom-looking one) is normally open, and will close when pressure reaches ~280 psi. If it fails closed, the fan will run. Unplugging it should turn the fan off. The red high-pressure switch has nothing to do with the fan, it disengages the clutch if pressures reach dangerous levels.

Does turning the AC controls in the car to off turn off the fan? What about unplugging the fan switch on the intake manifold?

RWDPLZ,

Per the 88 FSM, the High Pressure Cut-out Switch DOES control the fan....see about five posts up.

The A/C controls seem to have no effect and unplugging the sensor had no effect. The DPO (Dreaded Previous Owner) had tried to kludge a repair to the connector which promptly desintegrated when I tried to unplug it. Replacement pigtail on order from Rodney.

Larry

What page is that on? I have an 88 manual here. Is it in the V6 section, or the V5 compressor for the 4 cylinder?

RWDPLZ,

It is under coolant fan, V6 Vin9, page 8A-31-8

The exact wording, on page 8A-31-6 is

quote If equipped with Air Conditioning, the A/C High Pressure Cut-Out Switch and A/C Control Head also energize the Coolant Fan Relay by grounding circuit 335 whenever the A/C High Pressure Cut-Out Switch closes above 280 PSI (1931 kPa) and the A/C Function Selector is in MAX, NORM or B/L.

If you look at page 8A-31-0, circuit 335, the COOLANT FAN A/C PRESSURE SWITCH shares the dark green/white wire with the COOLANT TEMPERATURE SWITCH (aka the fan switch). The COOLANT FAN A/C PRESSURE SWITCH is the white switch on the compressor.

The red switch is not connected to the fan circuit. On page 8A-64-0, you can see exactly how the red switch, A/C HIGH PRESSURE CUT-OUT SWITCH is wired directly in series with the A/C compressor clutch, so that it opens the circuit (disengages the clutch) at 425psi.

RWDPLZ,

I think we are saying the same things: as I read it, the coolant fan relay can be activated by the a/c control head being in any a/c mode, OR the coolant temperature switch closing, OR the coolant fan a/c pressure switch closing OR any combination of the above. It will run continuously if the coolant temperature switch fails closed (can be tested by unplugging the coolant temperature switch) OR if the coolant fan a/c pressure switch fails closed (can be tested by unplugging the coolant fan a/c pressure switch on the back of the compressor....I do not know if it is red or white) OR if there is a short to ground in the a/c control head (can be tested by unplugging the a/c control head), OR if there is a short to ground from pin 4 of the coolant fan relay or anywhere along the dark green/white wire leading from pin 4. I believe those are the only ways the fan will run continuously.
The coolant fan a/c pressure switch, the coolant temperature swtich, and the a/c control head are all in parallel.

I was surprised to see that coolant fan does not run in the defrost mode since I thought the compressor ran in defrost.

Thanks for making me really look at it again.

Larry