compressor locking up

[grumpy old man]

I put an LT engine in my 63 Pontiac and as the original car did not have a/c I built a unit out of junk parts and adapted all the factory parts off of the donor truck. Compressor, condensor, evaporator and hoses. Logic was if the unit worked good in the truck it should work good in the car. It is a variable compressor which I know nothing about but was advised to just power up the adjustable vanes with the clutch circuit and it should work. Apparently it was originally controlled with the ECM but it now has a modified ECM and wiring.

The system was working fine until sunday when it started blowing hot air. Today I put the gauge on it and the clutch was on and off rapidly at idle and the low pressure gauge was at about 10 pounds. I tried putting some refrigerant into it and as soon as I added a small amount the compressor started locking up and slipping the belt, sounded like I had run over a cat!

With a minimal amount of refrigerant in it what would cause the compressor to lock up? Should I just replace the compressor and see what happens? I got out of auto repair in the late 70's and have not kept up.

Thanks for any knowlegable advice
Dwight

[JohnHere]

Dwight, You didn't mention the year, make, and model of the truck the A/C system came out of, or whether it called for R-134a refrigerant. Did the LT engine also come out of the truck? That will help us diagnose this.

For the moment, I'll take the liberty of "presuming" that this system uses a GM V-5 (possibly a V-6) variable-displacement compressor. You mentioned that it was originally ECM controlled, and that it's now being controlled in the '63 Pontiac by a modified ECM. Was the original ECM from the truck installed and reprogrammed, or was it some other ECM? That could be the problem—that is, the ECM might be erroneously processing the A/C system's pressures.

Also, if the compressor locks up on adding just a small amount of refrigerant, it sounds like the system might already be overcharged.

Did you take the specs from the truck and recharge the system according to those specs? Did you solvent-flush the components (except the compressor, receiver/dryer, and possibly the condenser), replace the used R/D, add any oil, and recharge the system yourself using the proper equipment? (Note: A parallel-flow condenser can't be flushed and must be replaced.)

In addition, a variable-displacement compressor needs to be "coaxed" into running at 100-percent displacement for the pressures to have any meaning and to recharge the system correctly.

[grumpy old man]

The truck was a 15 gmc sierra 1500 that was the worst wreck I have ever seen. It went off road hit somethhing unmoveable and did an endo. It must have hit hard because both front seat belts were locked in the extended position. Enough of that. The point being that the whole truck was a total wreck but the engine compartment was virtually untouched. It still had coolant in the radiator and a/c systems. The Pontiac and I have been together for a long time and the first thing I did was to build an ac system with junk parts which I had access to at the time. It worked sort of but was adequate in most So Cal weather. When the original 389 tossed a rod I decided to upgrade the running gear, so went on the search for a donor vehicle.

When I disassembled the truck I was careful in removing everything with the idea of being able to reuse most of it. When I took out the a/c parts I duct taped everything to hopefully keep out contamination. After installing the engine/trans I took off the a/c duct on the firewall side of the Indian and proceeded to modify the evaporator cavity to use the truck evaporator. I was able to use all the truck hoses with the exception of having a reciever welded into the return line to use a low pressure shut off to cycle the compressor. I also put fans in front of the condensor wired through the clutch circuit to maintain air flow through it because of stop and go traffic which is a staple of the LA freeway system. I have an acquaintence in up state New York who has a garage on his property and he gets into everything. He suggested powering up the variable vanes through the clutch circuit in lieu of any way to control them. As previously stated the system worked fine delivering a 40 degreee nozzle temp in 100 degree weather.

The day it quit working the nozzle went from 100 to ambient temp within a few miles. A few days later when I had some time I plugged the gauges on it and had 10 pounds on the low side. Unfortunetly I couldn't get a high side reading because for some reason my adapter would not connect.

This brings us to the present. First I did not add enough coolant to overcharge the system, it was less than a quarter can of r134. So my initial somewhat educated guess is a compressor malfunction. When I checked it both evaporator lines were equally hot.

My questions are:
Is it probably compressor failure or am I on the wrong rabbit trail?
Did my unconventional method of bypassing the variable system cause the issue. The system has been working since 2020?
If I am correct that it is a compressor malfunction can I just replace the compressor flush out the system recharge and go on with life?
In the 16 years that it has been working is there now an add on system to operate the variable vanes?
Is there a non variable compressor that will replace the existing and eliminate the variable vanes?
Do I just junk the whole system and go to a complete aftermarket?

Dwight