I have an '88 with the electronic temp controls. I converted the system to R-134 on Saturday (I know that some like this some dont, however I have done it on several other cars and did not have any problems) and have a rebuilt compressor, new AC control unit and new Accumulator. The problem... I seem to have high low side pressure (60+ PSI when the compressor kicks on, over 100 when its off) and low high side (50 PSI). When I first charged the system the A/C worked fine (vent temps got down to 45 degrees) then a few hours later it would not cool and the compressor would not go on except for a few seconds at a time. Then I got in the car today put the temp to 60 and put it on Auto low and behold I have cold air (45 degrees at the vents), then I pulled into the driveway and the compressor shut down and would not go on again. I have installed leak check and cannot find a leak, and now that the system worked again I dont think the R-134 is leaking. Why do you think my compressor wont stay on?

 

my first guess would be one of the limit switches (high or low pressure) is shutting down the compressor so it doesn't burn up.

 

Static pressures on a warm motor can easily reach 100 psi, and as the motor cools, they will drop. They important thing is that the high and low side equalize, usually around 50 psi or so once everything is about 70 degrees.

There are two switches which monitor pressures. The low pressure switch will open at 25 psi, breaking the circuit to the clutch coil, and then, as the pressure rises, reclose at 50 psi turning the compressor back on. This maintains evaporator temperature as close to freezing as possible - otherwise it will turn to ice. Because R134 freezes a few degrees lower than R12, the switch should be adjusted so that it opens at 22.5 psi. If you remove the harness, there should be an adjustment screw between the terminals. Turning it clockwise reduces the setting. Adjust it 1/4 turn at a time until the compressor shuts off at 22.5 psi. If the system, at idle, won't pull down low enough to trip it, disconnect the blower motor. The reduced air flow across the evaporator will cause the pressure to drop.

The high pressure switch keeps the whole thing from blowing up if pressures get too high. It cuts electrical flow to the compressor at about 430 psi. Pressures can get this high if you mixed R134 with R12 or if you didn't pull a vacuum or if the system is restricted or if the main cooling fan isn't coming on at idle.

To help you further, you need to post your high and low side pressure readings with the a/c on. Also note the outside air temp, or more importantly, the temp of the air at the condensor. Obtain them at 1500 rpms, main fan on and with the a/c on high. To keep the main fan on, disconnect the fan switch on the high pressure or smaller line from the condenser to the evaporator. It's next to the high pressure switch and it's leads are usually green and black. You'll know you have the right one if the fan comes on when you disconnect it.

 

Sorry for the delay in this response I have not had a chance to check the pressures until yesterday. I have driven the car a few more times and the AC seems to work ok now, although sometimes at idle it does not blow cold air. The pressures at 80 degress outside temp are: Low side 45 / High side 175, much different then when I charged the system. But I did notice that my main fan does not turn on when the compressor comes on, infact it wont come until temps reach 225, the normal setting. I thought that the fan would come on when the compressor kicked in, is that not the case?

 

Sounds like you found the problem. I'm hundreds of miles away from my manual so I'm not much help. I believe the fan is controlled from a switch near or at the high pressure switch.
Someone with a memory will soon chime in

 

Low side is too high and the high is too low. Were these readings after the fan finally came on? High should rise to about 230psi and the fan should come on. It will then drop and once it reaches about 170psi, the fan will turn off. Low side should be about 28psi to 30psi. My guess is that it's undercharged or the compressor is shot. On average, you want the high side 2.2 to 2.5 the temp of the air over the condensor, engine at 1500 rpms. How much did you put in it? The fan switch is on the high pressure line next to the high pressure switch. Disconnecting it should turn on the fan, but you need to get the charge right to see if it's working as designed.

 

Did You Replace The Orifice Tube When You Were In There? I Suspect A Bad Oriface Tube Can Cause Simular Pressure Problems.