1994 A/C parts in 1988-1993 trucks

[5speed91]

This kind of goes along with another question I asked but after searching for awhile, I could not find the answer that I am almost certain others want to know also and many others probably have been through this.

There are a lot of people of who say that R134A conversions don't work well because the vehicles were not designed for them. But what if you are in the unique situation as we are where 1988-1993 owners where there way an identical version of your vehicle that was designed for R134A such as the case of 1994 GMT 400s.

Could it be as easy as ordering 1994 parts and putting them in an older truck to have a factory functioning AC system. I think R134A systems have an additional cut off pressure switch in the lines that R12s don't. How do you get around this. Do you also need a 1994 engine harness and computer or can you wire in the switch yourself?

 

[SAATR]

The R12 system will still have a low pressure cut off switch after the orifice tube. A basic AC system with a fixed displacement compressor has to have a low and high pressure cutout switch to maintain proper system pressure by cycling the compressor on and off. Rockauto shows that the 94 evaporator is about 7mm thicker than the 91 evaporator, but appears to have the same design as the older evaporator. If you already have the system torn down, go ahead and use the newer design. Also, be sure to change the condenser out for one designed for use with R134a. These will be either a serpentine or parallel flow design, which are much more efficient that the tube and fin style that was OEM in R12 applications. This is critical, as R134a doesn't reject heat ad readily as R12 and needs the most efficient heat exchanger you can get to work well. Be sure to get new high and low pressure switches designed for R134a, as well as the proper 134a orifice tube. Flush the lines with a good AC flush, and purge them with either clean, dry shop air or dry nitrogen to remove all the flush from the system. Have a deep vacuum pulled on the system for at least 2 hours, and charge the system with a properly calibrated set of scales. This is critical, as trying to charge 134a systems off of pressures will give poor cooling results at minimum, and system damage at the extreme end of things. If you do all of that, you should have a good working system. I would also recommend a new, not reman, compressor filled with fresh PAG150. Ester oil does not work with R134a and should be avoided.

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[5speed91]

Wow, thanks for the reply. Very good information. I was planning on buying everything new including the lines. I was hoping that I could install new evap, condenser, compressor, lines, etc all from a 1994 model spec then deliver to a shop to actually install the compressor, the oil, refrigerant etc? I expect it to operate as GM designed the 1994 R134A system in these trucks when new. Am I over looking something?

Also, I cant find that second switch on my original R12 system for the life of me. Do you know where it is. The only one I can find is the switch on the drier. Thanks.

 

[SAATR]

It's mounted to the back of the compressor. It's a high pressure cutout switch, to kill the compressor when head pressure gets too high.

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[5speed91]

I don't see it or the wiring harness for one. Ill keep looking. Thanks.

 

[SwatDawg15]

I don't see it or the wiring harness for one. Ill keep looking. Thanks.

Just getting off work, I will take some pictures of my 94 k1500 later today and post them for you. I just replaced everything except the evap, but I did clean it, and installed a new AC Delco blower motor. It upped my air flow allot. My AC works well. but standby for pictures so you can make sure your getting the correct parts, as the lines are different I believe.

 

[SwatDawg15]

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[5speed91]

Thanks for the pics. I definitely do not have the switch mounted to the lines at the rear of the compressor on my R12 system. Question is, if I buy 1994 AC lines with that switch, how do I hook it up without having the plug in my wiring harness?

Also, the switch on the drier, I noticed the connector has been changed. The green wires match the ones on my 1991 but the wires on the connector are different on your truck. Did you have to change to a different plug?

 

[5speed91]

Also, it looks like there is a small connector on the other side of the compressor on your truck just out of view with green wires. I don't have that either and main connector on the top of my compressor looks different than yours. I may just have to buy 1991 stuff that is purposed for R134A retrofit if there is such a thing. I don't want to get into changing my whole engine harness. I wonder if those other switches on the 1994 can be wired into a 1991's existing AC harness or if they have to go all the way to the ECM.

 

[SAATR]

Every R4 compressor I have seen has the provision for a high pressure switch on the back of the compressor. Is there a metal plug held in by a snap ring in that area?

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