AC help

[89GMCJOHN]

So I am getting down to the finish line here on my 89 GMC shortbox build. I am now troubleshooting the ac system. When I turn on the ac the fan blows but the compressor clutch isnt engaging ...from what I gather you have a low freon pressure switch located on the dryer and a TOO high system pressure switch located somewhere else .

My questions are where is that too high freon pressure switch located on a 1989 ?
Is it located on the back of the compressors body ? I have a round hole with two terminals on it on the back of my compressor but I cannot locate the wiring harness connector that possibly connects to this. Can someone take a pic of their 89 compressor showing what the connector looks like going to the back of the compressor ? Or tell me where that high pressure switch is .

Where does the compressor pick up its circuit ground from ? From what I understand +12 goes through the low pressure switch then through the high pressure switch in series then to the green wire which is located on the left connector post when looking at it from the front of the truck on the compressors two post connections on top of the compressor ? Is that correct ?

What signal goes on the right post of the compressor connector -a permanent ground connection ?

Thanks in advance guys .....

 

[89GMCJOHN]

TTT help please

 

[89GMCJOHN]

I threw a pressure gage on the system tonight and its reading zero....I turned the ac on to max cool .....I then pulled the low pressure cutout switch connector off the side of the receiver dryer and jumpered a piece of wire across the connector terminals . Voila the compressor clutch engaged. So the compressor wasnt turning on because the system has no refrigerant in it . It threw rust dust off it and chattered a little then finally calmed down ...I dont think its spun up in yrs ..the ports on the system are the R134a quick connect type ...so I assume someone down the line converted it over to R134A ...I looked the compressor p/n up and its an R134A compressor ..I am trying the SUB ZERO brand 18 oz recharge kit ..1 oz of that is additives -leak sealer...so tomorrow night after work I will try to charge it for the first time ......wish me luck ........

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[deadbeat]

If you don't vacuum it down first you are wasting your time.

 

[studigggs]

No high pressure cut off switch on an '89. The high pressure switch on the back of the compressor controls only the fresh air door in the cab.

 

[89GMCJOHN]

No high pressure cut off switch on an '89. The high pressure switch on the back of the compressor controls only the fresh air door in the cab.

Good info tx ...just curious whats the reason / relationship with the high pressure switch and the compressor and the door control ? Does it thaw the system so to speak if pressures get too high ?

 

[deadbeat]

I didn't know that as well, but if pressure gets to high wouldn't the system blow hot air instead of freeze like it does on a low charge?

 

[studigggs]

its only true for 89-90 trucks without the "max/recirc" button on the control head. when high pressure hits b/t 210-255, psi the high pressure switch closes, and grounds the lead. the fresh air door moves to recirc, drawing cooler cabin air over the evaporator and lowering system pressure. When high pressure lowers to 165psi, the switch opens and you get fresh air again. If you switch to 134a, you will need a new switch (part number is in my previous posts), and if you dont hook up the switch, you will never get recirc air. The only high pressure safety on 89-90 trucks is the pressure relief valve on the back of the compressor...which I've experienced twice now.

 

[89GMCJOHN]

its only true for 89-90 trucks without the "max/recirc" button on the control head. when high pressure hits b/t 210-255, psi the high pressure switch closes, and grounds the lead. the fresh air door moves to recirc, drawing cooler cabin air over the evaporator and lowering system pressure. When high pressure lowers to 165psi, the switch opens and you get fresh air again. If you switch to 134a, you will need a new switch (part number is in my previous posts), and if you dont hook up the switch, you will never get recirc air. The only high pressure safety on 89-90 trucks is the pressure relief valve on the back of the compressor...which I've experienced twice now.

GREAT detailed post ...thanks !

 

[89GMCJOHN]

Well I charged it last night with the full 18 oz in the can and the good news is everything was in the green on the charge gage on the can and blowing pretty cold inside etc. ! I did have a tad of leakage on the back of the compressor at first but it appeared to stop soon thereafter. The bad news is the compressor sounds like their are rocks inside it and like it will grenade on me at any minute with any rpms . So it looks like I am in need of a new compressor. I ran it for about 15 minutes total runtime and it was still cold inside. I guess the previous owner must have stopped using the ac because of the horrible compressor noise then over time the refrigerant bled down to nothing.