AC Pressure Switch Problem

Hi all,

My AC recently stopped working, the compressor was cycling on and off but would not run continuously. I charged the system to the proper pressure but that did not fix things. I then bypassed the low pressure switch and voila, the compressor kicked on and I now have cold air and the system seems to work well.

My problem now is this - I replaced the low pressure switch but the system doesn't work with the new switch either.. the AC only works when I bypass the switch but not with the new switch.. what could be the problem? I don't really want to run the system with the switch permanently bypassed but not sure what the problem could be at this point.

Your advice is appreciated.

You might check to see if one of the pressure switch wires has voltage and one ground to complete the circuit.

Thanks for the reply, Chris. I bridged the contacts of the wires that clip onto the switch, if there was no voltage wouldn't this method not work?

Here's a link to a video that describes what I did.

https://youtu.be/3C4w7Ecrn4Q

Except for me, once I put a new switch on, the AC still won't work, so I've continued to bridge the contacts of the wires leading to the switch with a paper clip.

You answered your own question- you stated the system cycled on and off rapidly- with little or no cooling I'll assume- this means your original cycling switch is functional and doing its job and working as designed. You state the pressures are good- it is well known that you cannot judge system refrigerant amount by pressures- the ONLY way is to extract and weigh the refrigerant to know if your system is charged to specification.

Why you replaced the switch is beyond me....you need to assure your refrigerant charge amount is correct first- then diagnose from there. Chances are real good you are still undercharged- do you have a gage set on it to read static pressure? What is the suction pressure when you have it jumped out- If less than 20 psi, you can damage the thrust bearings in your compressor.

Also you failed to mention make and model of your vehicle- which helps in the diagnosis...

Correct, the compressor would cycle, ~2 seconds on, then shut off for 7-8 seconds before coming back on for another 2 seconds.

I charged the system with ~30oz of R134 until the pressure gauge on the low pressure port read 40lbs. How to measure static and suction pressure is something, quite frankly, I don't understand.. Just trying to diagnose and get my AC working without taking it to a shop that's going to take a look and tell me I need this and that and my bill will be $1000...

This is for my 2001 Bravada..


You answered your own question- you stated the system cycled on and off rapidly- with little or no cooling I'll assume- this means your original cycling switch is functional and doing its job and working as designed. You state the pressures are good- it is well known that you cannot judge system refrigerant amount by pressures- the ONLY way is to extract and weigh the refrigerant to know if your system is charged to specification.

Why you replaced the switch is beyond me....you need to assure your refrigerant charge amount is correct first- then diagnose from there. Chances are real good you are still undercharged- do you have a gage set on it to read static pressure? What is the suction pressure when you have it jumped out- If less than 20 psi, you can damage the thrust bearings in your compressor.

Also you failed to mention make and model of your vehicle- which helps in the diagnosis...

A bit of an update:

I think I overcharged the system. I put the gauge on the low pressure port without anything running (engine, ac, etc..). The pressure read ~65psi.

I released some of the refrigerant to bring the pressure down to ~50psi.

I then started the vehicle, ran the ac system on full blast with the pressure switch still bypassed and measured the pressure with the system running and had a reading of 28psi.

I then shut off the vehicle, re-connected the pressure switch, started the vehicle and ran ac system on full blast. The compressor engaged and I have cold air blowing out of the vents and a pressure reading of 28psi.

Outdoor temps today are around 70 degrees.

I had an orifice tube stop up on a '79 Malibu I used to own. After removing it, cleaning off the aluminum debris, reinstall, pull a vacuum & recharge, it worked fine for a coupla years till I got rid of it.

My daughter's '01 S10 currently has 95psi with everything off/cool & engine not running, 82F outdoor temperature.
It didn't cycle with Max-cool setting, fan full speed and had 29psi low side when I checked it out a coupla days ago..

Also you failed to mention make and model of your vehicle- which helps in the diagnosis...

I suggest you recheck his signature.

Without a set of manifold gages on both the hi and lo ports, sometimes you are just guessing, because the gages that come with those cans, usually are not accurate....

Rule of thumb is, with the compressor off, the high and low sides should be equal, and equal to ambient temp....for example, if ambient air is 70F, lo=hi=70psi....if the pressure is lower than 70psi, then you are low on pressure......

If the pressure in your can, is close to the pressure in the system, not much refrigerant will flow into the system.....so you have to build up the pressure in the can.....on way to do that is to immerse the can in hot water, this will build up pressure in the can and more will transfer into the system....

As Brcidd explained, your low pressure switch was working as intended, as did the new one....because you were low on pressure, when that compressor came on it pulled the low side below the low side threshold, and thus the PCM killed the relay......

Static pressure is the equalized system pressure with the a/c and engine off and engine cool- hasn't been run for a few hours. Again this pressure only tells you that you have enough refrigerant in your system to engage the compressor. you can have 5 ounces of refrigerant and read 70 psi static pressure or you can have 5 lbs of refrigerant and read 70 psi static pressure. There is simply more liquid present with more refrigerant.

Refrigerant is a condensible, not a compressible...your system is not like a tire-- where more air (compressible) means more pressure---with automotive a/c, refrigerant is measured by weight, not by pressure! This is why the specs foe weight are on the underhood stickers- anyone who states pressures to determine charge level is just guessing- there are too many variables to do that.

I'll admit, I am spoiled because I have a reclaim, recharge machine-that weighs everything it does. I don't hardly look at pressures, the first thing I do is extract and weigh the refrigerant in the vehicle- 65% of the time it is way below spec. Then I vacuum, add dye/oil and recharge to spec- run the system and then leak check. Not knowing how much refrigerant is in a system would drive me crazy- it is only a guessing game if you can't know the system is charge to spec...

A bit of an update:

I think I overcharged the system. I put the gauge on the low pressure port without anything running (engine, ac, etc..). The pressure read ~65psi.

I released some of the refrigerant to bring the pressure down to ~50psi.

.

Bad move....You've just wasted your time and refrigerant- see previous post.

Thank you for all the reply's guys, it's been educational and the system is working now, which is a step in the right direction from where I was a few days ago.



Bad move....You've just wasted your time and refrigerant- see previous post.

I'm no so sure. The system was not operating with the pressure switch connected after I had initially charged the system with 30-32oz of refrigerant. The compressor would not even cycle on/off. So I lowered the system pressure a bit and now the system works with the pressure switch connected - did I lower the pressure too much, maybe..but it works now and I'll probably take it somewhere have them evacuate/reclaim the refrigerant, weigh it, and charge it with the proper amount.

Thanks again for all the insights!