A/C Low pressure switch question

[BJM]

My system is fully charged and cools fine until the pressure switch cuts off. It cuts out at about 25 psi. The inside duct temperature rises a lot before it kicks back in. Does this mean the switch is tired? How rapidly should it cycle normally? Or better question, if it cuts out at 25 psi when should it cut in again. Mine seems to wait until 55-60 psi before turning on again which pumps out some pretty warm air. Just before cutting out the air temp start to fell quite nicely cool and just gets warm quickly until it turns on again.

 

[1970gsx]

I don't know about the pressures, but you can adjust the little screw in the front of the switch to change the pressure at which the a/c cycles on and off. It can be adjusted so it doesn't cycle as often.

 

[BJM]

I know about the little screw and I can set it to what ever I need. What I need to find out about is if the switch is allowing the pressure to rise too much before kicking back in.

 

[ijames]

Memory says the switch should cut off at 32-25 psi and come back on at 48-50 psi, so yes, it does have a lot of hysteresis. If it's over 80-85 deg F outside and the car is sitting still idling, with ac on max and fan on high, it really shouldn't cycle off - the load should be enough to keep the pressures up over the cutoff point. If you rev it up sitting still it might cycle after the inside of the car is cooled down. The purpose of the cutoff is twofold, to keep the compressor from running when the charge is low and therefore oil isn't circulating, and to set a minimum temperature on the evaporator of about 35-40 deg F to keep it from icing up. You say you have a full charge. Did you just put it in? How's the high side pressure? If the low side is holding in the 35-45 psi range with the compressor running, the high side should be over 200 psi and maybe up to 300 psi depending on the outside air temperature.

 

[S10xGN]

Hey Carl,

I have a slow leak on my system so it cycles off & on about every 5 seconds when it's low. When I charge it up enough so it no longer cycles, the low side is around 30# and the high side is only around 160#. You're saying it should be higher? What would cause the high side pressure to be low?

TIA!

 

[BJM]

ijames

The high side does reach about 300 psi and I did have the charge recently topped off. The A/C guy didn't seem to know how to set the pressure switch, luckily I had brought my service manual with me. From your description it sounds like my switch is fine but the variation in temperature inside the car is annoying.

 

[ijames]

Russ, the high side pressure is a function of ambient temp, rpm, and how much heat you are moving. It could be 160 on a moderate day at idle and 300+ on a scorcher with the engine at 2000 rpm.

BJM, there is a schrader valve under the switch so you can always go to a junkyard and get 2-3 to try out, or get a new one, and replace it without blowing the refrigerant charge. Maybe the band on yours is wider than normal, I don't know. I was mostly questioning why it was cycling at all on a hot day if it was fully charged.

 

[salvageV6]

Off 25 psi.

On 46 psi.

You want it set to 21 psi. or so with R134a.