This will be interesting and I'll probably leave out some valuable info, so please bare with me and I will answer any questions.

Toward the end of last summer, I replaced the AC compressor/drier, then charged the system after a vacuum. The system worked for about two weeks before the air lost all coolness. The system still had pressure though. So I found the pressure release valve was leaking and replaced it, but had the same problem. Since cooler weather was on the way and I had responsibilities elsewhere, I stopped tinkering with the system and drove through the winter with a good heater.

This past June when the weather got to hot to bare, I decided to troubleshoot the system to find the problem. I used a vacuum to discover a leak. Forgetting what I knew from last year, I took the car to a mechanic to find the leak. He told me the leak was in the evaporator and there was also an 'electrical' problem. He could fix them both for $XXX, and I wasn't interested in spending it, so I replaced the condenser, expansion valve, and the drier myself. Found a broken blower resistor and replaced it too, so now all 4 blower speeds work. Took several weeks to wait on parts and find time to replace it all. Once replaced, vacuumed the system again only to still have a leak. At this point, I'm upset because the mechanic I paid to find the leak, appeared to have misdirected me. So I took it to a 2nd mechanic with a sniffer, and he told me the compressor pop off valve was leaking. This is where I remembered the leak from last year and was mad because I had to spend money to help me remember. So I replaced it nonetheless, then vacuumed again. This time it held the vacuum.....thus leak fixed.

Now, all it seemed I needed to do is recharge the system. Upon attempting recharge (using coolant with sealer and oil combo, along with attached cheap 'in-line' gauge), the compressor wouldn't engage. I charged the low pressure line until it hit 30-35 psi, and the condenser would still not engage. I stopped adding coolant in fear of damaging the compressor pop off valve and having to replace it again. At this point, I remember the first mechanic advising of an electrical problem. So I straight wired the compressor to the battery and watched the low line pressure. When the compressor was on, the pressure dropped to zero. When it was off, it went back up to the previous pressure. I then tried adding coolant while the compressor was on and the pressure never went up, until the compressor turned off, at which point the pressure increased to the previous pressure + the pressure I added while the compressor was on. At this point I wasn't sure what to do, so I stopped adding coolant since the pressure was now around 45-50 psi in the low line.

I then ordered two new relays and a duel pressure switch in hopes the problem was electrical related. Reason I ordered two relays, is because I wasn't sure which one went to the condenser fan, and which one went to the compressor. So I replaced them both. Then replaced the duel pressure limit switch. After trying to recharge a last time, the compressor still would not come on when I added around 40psi of coolant into the low line. I didn't straight wire it this time.

Now I'm at a standstill. I've replaced the compressor, evaporator, expansion valve, blower resistor, duel pressure limit switch, and both relays. I have no idea why the compressor is not coming on, and I have no idea why the pressure drops to zero in the low line when I straight wired the compressor to come on.

Why is the compressor not engaging when I add 30psi to the low line? Is the problem pressure or electrical related? Is the pressure supposed to drop to zero in the low line when I straight wire the compressor so it will engage? If not, why is it dropping then returning to same pressure? If so, should I continue adding coolant only for the pressure to get higher in the low line when the compressor stops?

This is frustrating.

Thanks-
OSD

 

Stock R12 or converted R134a system?

Sounds like the compressor failed in the past. Did you flush the system to remove debris?

The A/C system must be charged by refrigerant weight not pressure.

When this^ is done, turn on the A/C and watch whether the condenser fan runs and the compressor clutch engages.

Also buy a proper manifold gauge set to measure the refrigerant pressures on both the low and high sides. These pressure readings are critical in diagnosing the problem.

 

whats high side pressure? get a set of manifold gauges from autozone from their loaner tool program.

 

How much freon did you put in?