AC Compressor Cycling

[artbuc]

Re 1999 Toyota Avalon XLS

I have a hi side pressure switch but can not find a lo side switch. I assume compressor on/off is controlled by ECU and hi side switch is there only as fail safe. The reason I ask is because it seems like ac is cycling more than usual. I can see a few bubbles in my sight glass so could be a little low on refrigerant - will hook-up gages tomorrow. Getting about 44 degrees from vents. Does anyone know if there is a lo side switch on my car? Can anyone confirm that compressor on/off is controlled by ECU vs pressure switches? Thanks.

 

[Chris142]

Yes there is a low side port somewhere. Don't go by the site glass on a R134a system.

 

[Lubener]

If you are getting 44 degrees out, you are worrying over nothing.

 

[gabriel9766]

Originally Posted By: Lubener
If you are getting 44 degrees out, you are worrying over nothing.
+1 the AC in my malibu blows 38-45F on max eco AC. id say 44 is pretty good for a 99

 

[artbuc]

Originally Posted By: Chris142
Yes there is a low side port somewhere. Don't go by the site glass on a R134a system.


Yes, I know I have a low side port. I asked about a low side pressure switch. Bubbles in site glass are a good indication according to the FSM. However, as I stated, I use manifold gages for troubleshooting. I replaced the condenser 7 or 8 years ago and she has been working great ever since except this year seems slightly sub par.

So, what causes compressor to cycle on/off on my car? Is there a temperature sensor at the evaporator? Thanks.

 

[funflyer]

Your system should have a trinary switch on the high side, high/low to energize the compressor and fan relay.

 

[artbuc]

Originally Posted By: funflyer
Your system should have a trinary switch on the high side, high/low to energize the compressor and fan relay.


Got it, thanks! As I understand it, the high/low pressure switches come into play only to protect the compressor when pressures are out of range. If everything is working correctly, these switches do nothing. The compressor will shut off when the call for cooling is satisfied. I am not sure which sensor tells the compressor to turn off. I have a thermistor at the evaporator which I think is there to detect coil freeze-up but I am not sure. So, please tell me if I am on the right track.

I believe I am slightly undercharged allowing low side pressure to trip low side pressure switch. I have a fair amount of bubbles in my sight glass. I carefully weighed in the charge many years ago when I replaced the condenser. My notes confirm what FSM says. When I installed correct charge, bubbles diminished to almost nothing.

 

[funflyer]

Originally Posted By: artbuc
Originally Posted By: funflyer
Your system should have a trinary switch on the high side, high/low to energize the compressor and fan relay.


Got it, thanks! As I understand it, the high/low pressure switches come into play only to protect the compressor when pressures are out of range. If everything is working correctly, these switches do nothing. The compressor will shut off when the call for cooling is satisfied. I am not sure which sensor tells the compressor to turn off. I have a thermistor at the evaporator which I think is there to detect coil freeze-up but I am not sure. So, please tell me if I am on the right track.

I believe I am slightly undercharged allowing low side pressure to trip low side pressure switch. I have a fair amount of bubbles in my sight glass. I carefully weighed in the charge many years ago when I replaced the condenser. My notes confirm what FSM says. When I installed correct charge, bubbles diminished to almost nothing.



Sounds like you're on the right track and the charge is low causing the compressor to cycle more often. Since your charge probably isn't too low, I'd start by topping off the system with refrigerant/dye and looking for a leak with a UV light. You may have to drive it for several days or even weeks to discover where the leak is coming from.