I recently purchased a 1992 Toyota Celica. The A/C was not working when I purchased it. I am a proficient residential A/C mechanic, however, I know only a little about car A/C’s. I put a set of gauges on the system. When I turn on the A/C, the compressor clutch clicks and in the blower starts. The suction pressure begins to drop to about 30 lbs, the high side increases to about 160 lbs. (R134 Freon) The compressor runs for almost 5 seconds then drops out. The dash indicator light for the A/C then begins to flash in one-second intervals. If I shut it off for a few minutes, it will re-start and repeat this process. Any clue?
When checking the high/low side pressures of an automotive A/C system, the engine idle speed should be at about 2,000 RPM’s with a large fan blowing at the condenser at the front of the vehicle. Because the system was converted from R12 to R134a refrigerant, you can’t go by the standard spec’s when the system had R12 because R134a runs at a higher pressure. So what is done is install a thermometer into one of the outlet vents, and charge the system until you get a temperature reading of 40-50 degrees out of the vent and call it good.
Thank you for responding. My problem is that the compressor clutch will not remain engaged long enough to charge the system. It engages for 5 seconds and drops out. There must be some type of sensor or something cutting the compressor off. I have tried jumping out the low pressure switch - no difference. It still engages for 5 seconds and drops out??? One person told me the system may have a restriction somewhere and the high side is getting too high and shutting it off. I am out of ideas.
How cold is it where you are? Some car A/C’s (many/all?) will cut out if it’s too cold to prevent damage or freezing of the lines.
South Louisiana. 70’s - 80’s when we were working on it.
Well, so much for my theory…
Sounds like you have an electrical problem.
If you’re proficient at home HVAC, you understand how A/C works, the principles of refrigeration are all the same. Your pressures are nowhere near high enough to cause a safety shut-off of the compressor and nowhere near low enough to indicate a restriction or stuck expansion valve. If your compressor can create pressures of 30/160 in 5 seconds the compressor is good and the system charge is likely near full.
Rule of thumb–at 2500 rpm and the condenser fan operating properly you can expect high side pressures 3 times ambient temperature and low side less than half ambient temperature, with a dash vent temperature 30 degrees below ambient. If your system has a sight glass use it to determine state of charge, otherwise rely on your gauges and your sense of touch and feel. Discharge line should be too hot to hold, liquid line should be quite warm, suction line should be cold and dripping.
But your problem is electrical. Your car uses a compressor speed sensor (mounted in the compressor), which sends a compressor rpm signal to the A/C amplifier (control unit). If this signal is absent or erratic, as a safety measure the control unit will disengage the compressor clutch assuming belt failure, clutch failure, or internal compressor failure. I’ll bet this is your problem. Unfortunately I don’t think there’s a way to bypass this sensor. You could try to jumper 12 volts to the compressor clutch and see what your operating pressures/temps are. Good luck.
Sounds like an undercharge situation to me and that can certainly cause the compressor to go in and out.
Connect the gauges and check the static pressures, meaning the engine and A/C off and after they have been off for quite a while.
Both high and low sides should be the same and while it will vary a bit based on a number of things, you should see about 115-125 PSI on both sides if the system has a full charge. If it’s down in the 100ish or lower range then it’s probably low on refrigerant due to a seal leak and some leakage could be considered normal on an aged system.
It’s a '92 so this means someone has probably converted the system if it takes 134.