I'm back with another question about my 1992 Honda Civic EX 4door, 5 speed manual with 252k miles.

This time it's about the behavior of the A/C Compressor. The original A/C system was an older R12 system. The system had a slow leak over time and many years ago (not sure when, but it was with a former mechanic I used to use) was converted to the newer rfigerant (not sure of the name). The original compressor blew about 2 years ago. The original compressor was replaced with a unit my mechanic sourced that was compatible with the newer refrigerant and came as a single part including an all new clutch and related parts. I remember from back in 2006 my mechanic had trouble getting the system to charge up properly. Now in 2008 everything seems to be running fine except that compressor will not turn on to run on sunniest, hottest, and most humid days. My mechanic seems to think that the system pressure switch is not allowing the compressor to kick on under these conditions. The sun does have something to do with it, because if I park in the shade or in a garage, the compressor will turn on. What do you think? If system pressure is causing this should I replace the pressure switch?... I'm not sure if the pressure switch was replaced when the system was converted.

Thanks for your help.

 

Have you recently measured the refrigerant (R134a) pressures on the low and high sides when the A/C is ON and also OFF?

 

^ditto.

Also, there is a relay near the condensor that controls the a/c compressor. If the relay is corroded or wet it will not work properly, and may work intermittently. It's possible that on the humid days, moisture has made its way into the relay and caused it to malfunction. Unplug the relay and inspect the terminals for corrosion. If the system pressure is not the issue, I recommend replacing the relay to see if that fixes it. It's only ~5 dollars and is worth a try if the pressure checks out fine. If it doesn't fix it, oh well, you have a new relay.

 

Thanks for the replies. I'll have my mechanic check the pressures. What is the range that should be looked for, and what type of readings would indicate a problem? The relay does sound like a possibility. The fact that the compressor won't turn on when parked in the sun is similar to a past problem I had where the engine wouldn't start after being parked in the sun... that turned out to be the main relay (a common issue with Civics from what I understand).

 

On a typical warm day with the A/C running, the low side should read 22-36 psi and the high side 220-295 psi. When the A/C is not running, the pressures should be equal on both sides (~70 psi).

 

When it is not working, press the A/C button with the engine idling and observe what happens. Look for these 3 things:

1 Condenser fan starts (the unit next to the radiator that is connected to the compressor)
2 Engine idle speed increases slightly
3 Compressor engages

If you don't get to 1, the controls inside the car or fuses are suspect. If you get 1 but not 2 that could be the pressure switch. If you get 1 and 2 but not 3 look to the relay as trustdestruction mentioned, also the gap of the compressor clutch. You can check for 12 volts on the wire going to the compressor. If you have that but it won't engage, it is either the clutch or the thermal switch on the compressor.

Expect 100-120 psi static pressure on hot days. The compressor should start at any pressure between about 50 and 350 so I doubt that that is the problem. Even without A/C gauges, you can simply unplug the pressure switch and check it for continuity.

 

Does the A/C have good cooling performance when the compressor clutch engages when the A/C is turned ON when parked in the garage? If so, I agree that compressor clutch relay could be faulty. If not, checking the refrigerant pressures might be informative.