1994 A/C parts in 1988-1993 trucks

[5speed91]

Yes there is. But even if I installed said switch, how could I hook it up when that provision is not included in my factory wiring harness?

 

[SwatDawg15]

I honestly don't know. I would probably just retro fit your 91, or get a engine harness from the salvage yard. Its not that many plugs on these old pre 96 trucks.

 

[SAATR]

I found a schematic for the '91 trucks and, as you say, there is no high pressure cutout switch. The system will work just fine, as the high pressure cut switch is there for preventing damage to the system in case of excessive head pressure. The low pressure switch is what is actually controlling compressor cycling during normal operation. The low pressure switch is normally open, and closes above a certain pressure, depending on what the switch is designed for. The R12 switch in the diagram opens below 41 PSI, which is a bit too high for R134a. The proper switch should lower the cycling pressure down to the 20psi to 30psi range. If you want to add the high pressure cutout switch, all you have to do is splice it in series with the power wire going to the compressor. It is a normally closed switch, which opens above a certain pressure, opposite of the function of the low pressure switch. Think of it as a circuit breaker for pressure, that cuts the circuit only when the pressure goes too high. Should be ten minute job at most. Be sure to use a proper solder joint and heat shrink for the splice, as you don't want a butt connector causing issues down the road.

 

[IADC43]

Yes there is. But even if I installed said switch, how could I hook it up when that provision is not included in my factory wiring harness?

Is the high pressure switch attached to the back of your compressor the kind that uses one gray wire?
I messed around searching for the answer for mine, and found the new switches they sell now have two wires.
The instructions say to connect one wire to the gray wire, and ground the other one.

 

[7echo]

I need a new system as well and want to use a more modern compressor. I have read that the old R4 compressor failed quite often. I found this kit...
http://www.classicindustries.com/product/truck/parts/346002.html

I have been trying to find some comments about it but so far no luck. If anyone has used this kit please comment, or PM so as not to clutter the OPs thread.

When I get a chance I will look at the wire to my compressor switch.

 

[SAATR]

I'm leery of "Sanden-style" compressors from anybody. Some have great luck with them, but you really don't know what you are getting as they are generally no name Chinese copies. Sanden makes a great compressor, and they are the industry standard in heavy equipment and over the road trucks. If you could get just the kit sans the compressor, I would go that route and get a genuine Sanden compressor.

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[7echo]

Nice catch, thanks.
I hadn't noticed the 'Sanden style', just the 'Sanden AC compressor upgrade'.
I emailed them to see if they are Sanden brand and what model.

 

[5speed91]

Thank you to all that wrote in. I'm starting to get the picture. To clarify, the person on the previous page posted 1994 truck (came from factory with R134A) that had three connectors in the shot of the compressor. The connector that powers the compressor clutch, one in the metal AC lines just behind the compressor and one in the back of the compressor mostly out of view. I am aware of the low pressure switch on the drier.

Which one is the high pressure switch? The one on the AC lines going into the back of the compressor or the one in the back of the compressor. My 1991 R12 truck has neither, just the power connector for the clutch.

I looked at an R12 system in a 1993 Caprice police car I saw the other day and it had the switch in the back of the compressor but not in the lines.

I looked at a car I personally own, a 1994 Caprice LT1 which also came withR134A and it has the same three connectors as the 1994 truck shown earlier in the thread.

Which switch is which and how would an R134A retrofit be able to operate reliably without it? Is this the switch that in some vehicles turns off the AC at full throttle or is that a function of the ecm? Thanks.

 

[SAATR]

There is no provision for cutting the compressor off at full throttle on the '91 model schematic. The later trucks may have a provision for this, but I do not see it in their schematics. The switch that is mounted in the compressor is, as I said, wired in series with the low pressure switch and the clutch. It cuts power to the clutch if head pressure gets too high. All you have to do is install it, get the proper connector to hook it up, and splice it into the power wire going to the clutch. This will give you additional high pressure protection. The third switch mounted on the AC line at the compressor is more or less for diagnostics. It lets the AC control head know when the AC is working and how often it is cycling. It allows the controller to shut the system down if cycling is too rapid or there is insufficient pressure in the system. There may be more, but I can't locate a FSM to find the theory of operation. Your AC control head doesn't have this function, so the switch is unnecessary. Again, wire in the high pressure cutout and you will have all you need for safe operation.

Edit: I looked at the later model schematic again, and it uses a relay to control power to the AC clutch, instead of the control head powering it directly. The relay may be grounded by the ECM, and therefore shut off at WOT. I can't seem to dig up the grounding for the relay, so this is just speculation. If somebody has the schematics for both years, that would be helpful.

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