I’ve done some digging thru the forum but I can’t seem to find what I’m looking for. Please forgive me if I’ve simply overlooked it.
I’m in the process of setting up my 1986 GT with a ’92 Cad 4.9 and I’m attempting to take care of the a/c. The original Fiero compressor has what appears to be two (2) connections on the back (1 light colored mushroom and 1 red with a copper stem), whereas the new compressor has two (2) ports available on the back. The wiring harness that I had made contains one (1) connection which is labeled “A/C Pressure Switch”.
I realize that I’ll need to purchase the proper switch(es) for the new compressor, but will I need 1 or 2?
I am stumped. Thanks in advance!

[This message has been edited by Meanspiritdave (edited 07-14-2014).]

The original '86 system had two pressure switches on the compressor. One was to cut out the system (disengage the clutch) if the pressure got too high. The other was to turn on the fan in Defrost mode if the pressure got beyond a certain value. That's because, in Defrost mode, the AC is used as a de-humidifier, but the fan isn't turned on by the normal circuitry.
Your 4.9 compressor probably only has the cut-out switch on it. I put a "new" (rebuilt) compressor on my 86 SE, and it only had the cut-out switch on it, so I left the defrost fan switch connector disconnected. No problems so far.
Edit: Also, the new compressor switch had a different kind of connector on it, so I had to do some adaptive wiring. The old switch had two connectors, and the new one only had one. One of the connectors on the old switch was simply a ground, so I just grounded the wire that previously went to that connector.
There may be two switch ports on the back of your compressor. One or both may be filled with dummy plugs if it is a new or rebuilt compressor. If you have to buy and install a switch, you will have to remove the dummy plug (held in with a snap ring) and replace it with the switch.

[This message has been edited by TopNotch (edited 07-14-2014).]

Thanks. Makes sense. I'm still not sure how to wire this together. The harness was made with the intention of using the Fiero style compressor with the built in switch. But since there is only one connection on the harness, I'm not sure which switch is used, the red with the stem or the white mushroom? Thanks again!

One of our GA Fiero club members has a 4.9 in his Fiero. He hooked up his AC without using any of the switches on the back of the compressor, and it worked fine until one day, on the way home from a club event, his AC blew out a hose. That over-pressure cut-out switch wold have prevented that.

OK, another question. Is the red switch with the single copper stem the high pressure switch and the white mushroom switch the low pressure?

I'm not sure. Refer to the Helms manual for the colors of the wires to each switch.

You inspired me to dig a little deeper and I just found this on the 4 Seasons website,

"The original single terminal switch has been replaced by a dual terminal switch. Connect either one of the new leads back to the original lead. Then take the second lead and attach to a ground."

So, it appears that I need to purchase 4 Seasons part number 35961 which will replace my old style red switch. Thanks again!!!

[This message has been edited by Meanspiritdave (edited 07-14-2014).]

The 4.9 with the exploding high-side hose was mine. Made a hell of a noise and lots of smoke.

The stock 4.9 compressor has no provisions for either switch. Just a blank casting where the switches would be. (As installed in the caddy, it uses temperature sensors that phone home to the... BCM, I think. The BCM then switches the compressor on and off.)

Since the fan (as installed in the Fiero) seems to run whenever the A/C is energized, I'm content to use the pressure/cycling switch on the receiver dryer, and a high-side switch that I am going to add. I figure that the easiest place to add a switch is in series (electrically) with the pressure/cycling switch. I plan to mount it on the high side charge port in the front compartment, when I find the appropriate switch.

The HR6 compressor is supposed to have a physical safety valve to relieve the pressure, if it goes too high. Obviously, my high side hoses had a lower "release" pressure than the safety valve. They really shouldn't have blown first. (Yeah, I know. It's probably a "one shot", and would have scrapped the compressor, but my compressor is shot anyway. Oh well... at least I have new hoses now. )

[This message has been edited by Raydar (edited 07-14-2014).]

The red switch is the high pressure switch, and the white mushroom switch is the low pressure fan switch.

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

35961 would work fine.

You guys rock! Thanks!