What is the Low Pressure AC switch operating range

[WILL E]

My AC compressor is cycling. I hooked up a guage set. 100 degree's outside. At idle. Low side drops down to 25 and back up to about 70 (as the compressor cycles). High side about 175 to 225.



I think I can hear the low pressure switch 'click' with the compressor. Not sure it is being activated when the compressor is turned on/off or if it is signaling low pressure and turning the compressor off.



Anyone know the low pressure side where the switch will operate (turn off the compressor)

 

[Phil Frady]

Based on what you have stated, your system is functioning correctly. I would ask you, what is the cycle time which would actually tell alot more.



In a nutshell:

When you turn on your A/C, the compressor turns on (the click you hear) and the low side starts slowly going down till you say 25psi (low pressure switch), while the high side gets higher. Your compressor will then cut off and and both sides try to equalize again untill the low side gets to 70psi and turns the compressor back on.



If your low on re-frigerant, then the clicking will happen like every 10-20sec. If you have a good amount of re-frigerant, then it will be in the neighborhood of 1minute between click's. It is possible to over charge and defeat the purpose.



With Max-A/C selcted and the fan turned on High while another fan is blowing air into your radiator, you should see a temperture of 30degF coming out of your vents and in some cases 27.

 

[Todd Z]

Sounds Low on refrigerant to me..... The switches are working fine......



Ok, with the AC on max, with an ambient temperature of about 90F, the engine at 2000rpm ( NOT IDLE) and air moving through the condenser, Clutch fan is working properly or you have an extra fan blowing in the radiator, I would expect to see a Hi side reading of about 290 psi and a low side reading of about 35. I would also NOT expect to see either needle fluttering rapidly.



So You need to add refrigerant to the proper levels..



I also would let a pro look at it if your un comfortable just adding and re checking...



Todd Z

 

[WILL E]

Interesting, I guess I figured the low and high pressure switches were more to protect the system and not part of the normal operation. I am taking it back to the dealership (they did the compressor removal) I can't wait to hear what they say...

 

[scott degroot]

Funmobile ,6/5/2011 20:17 MT



With Max-A/C selcted and the fan turned on High while another fan is blowing air into your radiator, you should see a temperture of 30degF coming out of your vents and in some cases 27.

I have to disagree with this statement on a R134 system especially on a late model vehicle. Realistically about the best you can hope for is mid 40's, if the system were to get that cold than there would almost have to be a problem with the low side pressure switch. Another reason the system is not supposed to get that cold is due to icing up. If the temps in the evaporator case were that cold, obviously colder if you are getting 30 degrees out the vent, then the condensation that occurs would freeze the evaporator up and eventually shut cooling down all together because the low side pressure would be too low to close the switch. On some older Fords that seemed to have the best ac in the world we could get mid to high 30's but adjusting the low side pressure switch but this was on a R12 system that held 3+ lbs of refrigerant and even then it would cause freezing problems occassionally.

 

[WILL E]

Dealership told me they must not have put enough freon in. Yeah right. Like when you are adding it you don't notice that the compressor is cycling every 10 seconds or so... We will see how it goes.



As a general rule it shouldn't have to rely on the low side hitting a value low enough to turn off the compressor.

 

[scott degroot]

,6/11/2011 20:49 MT



As a general rule it shouldn't have to rely on the low side hitting a value low enough to turn off the compressor.

I don't understand what you are trying to say here. The low side pressure switch is what controls compressor cycling on a CCOT system. On systems that have expansion valves, which is a far superior system I believe, there is usually a temp sensor in the evaporator case that will interrupt clutch cycle if it gets to cold but the expansion valves usually don't allow this to happen.