1990 Celica GT A/C turns on and off

[ssfiggy]

Have 1990 Celica GT 2.2L and the a/c compressor (clutch) turns on and off at random times. When it stays running it cools fine, but the after a few minutes it start kicking in and out. The Radiator Fan and the A/C Fan are both running. Car not running hot. Added some freon R134 to try that. Previous owner said it had been converted - how can I tell if it has? The local auto parts store said it may be the A/C relay. I cannot find where the thing is located to replace it with the new one. I found a square relay under the hood, but this is a round relay. When the A/C stops working and kicks in and out the clutch engages and disengages and can hear clicking that seems to be from somewhere near the passenger firewall or under passenger dash. Is that the relay I hear and where is the thing? Can someone tell me where this relay is? I would also like to hear any other ideas about what could be causing this problem? HELP!!!

[93celicaconv]

So you already added some R-134a to the system?

A proper conversion from R-12 to R-134a requires the 2 taps that Klapa mentioned to be changed, to avoid confusion and possibly putting the wrong refrigerant into the system in the future. A conversion should also have a label clearly stating the conversion from R-12 to R-134a in the engine compartment. If this wasn't done, I would really question if your car was ever converted to R-134a, and if it was, if it was done properly. R-134a does not work if adding it directly in an unconverted R-12 system, as it will damage the entire system.

The A/C compressor does not turn on and off without getting an input to do so from your A/C amplifier. From what you are describing in that it runs normally for a few minutes, then cycles on & off rapidly (every second), without doing any testing, it sounds to me that your refrigerant charge is low and your 3-way pressure switch is working just fine. The pressure switch on the low side piping will shut the compressor off it the pressure on the low side is too high or too low. Since you can start your A/C fine initially, the low side pressure is in the normal range at the start. It can only get lower after the compressor kicks in. As the evaporator coil cools, if the refrigerant charge is low, the pressure on the low side continues to drop until it gets too low (too low refrigerant pressure means the refrigerant is too cold and could ice up your evaporator).

You have a sight glass on your receiver/dryer at the front of your car, just ahead of the condenser & radiator. If you see a lot of foaming within that glass while your A/C compressor is running, that is also indicative of a low refrigerant charge.

If you have already added R-134a to the system, yet the system has the old screw-on taps, I would have a complete A/C system service done at this time before using it. You should find out what kind of oil is in the system (mineral, PAG, POE, etc.) so the place doing the service will know what kind of refrigerant to put back in. If R-134a, they need to put the right tap fittings in and the label, so there is no question in the future. If R-12, you will need to determine if you want to stay with R-12 moving forward at this time.