AC Compressor Constantly Running

[Kilian]

I was charging the AC system on my 83 GMC. System was empty so planning on putting in about 80% of the recommended R12 charge. Using 12 oz R134a cans. On my third can I noticed the compressor was no longer cycling, just running constantly. Using manifold gauge set but didn't notice any huge difference in the pressures. Is the pressure switch on the receiver/dryer the only control for cycling the compressor?

 

[gmbellew]

low pressure switch only. if it is really hot and humid, I dont know that it will cycle...

 

[Kilian]

low pressure switch only. if it is really hot and humid, I dont know that it will cycle...

Thanks for the reply. What shuts off the compressor and prevents things from freezing up or high system pressure?

 

[gmbellew]

Thanks for the reply. What shuts off the compressor and prevents things from freezing up or high system pressure?

the low pressure switch serves that function. it is on the drier.

 

[YakkoWarner]

In a properly working system, as it gets cold the pressure on the low pressure side should decrease, and then the compressor will cycle. If the system has a problem that doesn't allow it to cool down enough that might not happen, which is why newer systems also have a high pressure cutoff on the compressor. My truck doesn't have a high pressure cutoff either so it stays on. My system is not properly working so builds up too much on the high side, or I have to remove a very small amount of gas and then it short cycles. If its getting good and cold, with the pressures on-spec with the gauges, its probably working as designed.

 

[Conejo_K10]

Do you have an R12 or an R134 system?
I had heard that 134 is not compatible with a 12 system. At a minimum the receiver dryer will need to be replaced.
Also 134 doesn't run as efficiently in a 12 system, so this may be why the compressor is on 100%.

 

[75gmck25]

There is a metal temperature probe that sticks through into the evaporator and cuts off the A/C when the evaporator gets down to about 38 degrees (IIRC). It is inline with the low pressure cutoff switch.