99 Tahoe AC Head Scratcher

[gadroon jones]

Our 99 Tahoe had good working air conditioning last year when we quit using AC in the fall, and this spring the AC will not get cold. It seems to be about the exterior air temp. I investigated, found that the system has 40 lbs of pressure without the compressor running, and the high pressure switch, low pressure switch, and the cycling switch (located on the accumulator tank) all have good battery and ground to them. If I short the wires at the cycling switch, the clutch engages, which it will not otherwise. I assume it's a bad cycling switch.... but it doesn't correct the issue. Did I get a bad switch? I verify that there is not continuity between the terminals on the switch with the AC on and engine running. I also verify that the valve behind the switch will pass a little gas when depressed slightly. I tested the new switch by pressuring it up with 30 LBS of air pressure, and the contacts close. Now why doesn't the switch close with the 40 PSI in the system?
I'm not sure what to try next! Any body have any ideas?
Thanks in advance for any help.

 

[gadroon jones]

Our 99 Tahoe had good working air conditioning last year when we quit using AC in the fall, and this spring the AC will not get cold. It seems to be about the exterior air temp. I investigated, found that the system has 40 lbs of pressure without the compressor running, and the high pressure switch, low pressure switch, and the cycling switch (located on the accumulator tank) all have good battery and ground to them. If I short the wires at the cycling switch, the clutch engages, which it will not otherwise. I assume it's a bad cycling switch.... but it doesn't correct the issue. Did I get a bad switch? I verify that there is not continuity between the terminals on the switch with the AC on and engine running. I also verify that the valve behind the switch will pass a little gas when depressed slightly. I tested the new switch by pressuring it up with 30 LBS of air pressure, and the contacts close. Now why doesn't the switch close with the 40 PSI in the system?
I'm not sure what to try next! Any body have any ideas?
Thanks in advance for any help.

As a PS, the relays have good voltage also and I traded relays to verify that the AC relay operates properly.

 

[drakon543]

well im not sure what pressure the system is supposed to be at but the high and low pressure switches may not be working. also your controls inside the truck may not be working. if you know someone with a high end diagnostic scanner have them hook up and engage the a/c through that. if the a/c engages through that your dash control is bad. or you can pull the dash apart enough to manually test the switches there with a multimeter.

 

[gadroon jones]

well im not sure what pressure the system is supposed to be at but the high and low pressure switches may not be working. also your controls inside the truck may not be working. if you know someone with a high end diagnostic scanner have them hook up and engage the a/c through that. if the a/c engages through that your dash control is bad. or you can pull the dash apart enough to manually test the switches there with a multimeter.

Thanks for the response.
Because the wires for the cycling switch have voltage and ground, and shorting them makes the compressor clutch operate, doesn’t that mean that the control panel is working and either the switch or pressure to operate it are the issue? Am I just missing something?

 

[drakon543]

id have to look at the schematic again however the relay for the a/c will always have power waiting until it gets a second 12v to flip the switch which comes from the dash controls. most of the higher draw components in a vehicle will be switch operated but the switch isnt actually driving the particular component it will drive a relay which drives the component. however actually pressure and the high and low switch are basically a safety circuit. if any of those aren't right it doesn't matter what else is working those will stop it from functioning. again if have to look at a schematic to be 100% but if it doesn't even try your low side could be at fault. the pressure switch will either be a n/o or n/c switch. turn your a/c on and unplug the low side if it doesn't do anything use a jumper wire in the prongs of the plug to bypass the low side switch. if that causes it to fire up your pressure is either too low or your low side switch is bad.

 

[OR VietVet]

Evacuate the system and charge with a full charge. Then see what happens. You can even just add refrigerant and see.

 

[gadroon jones]

I have added a small charge to the system, it took it from 35 to 40 PSI. Also, shorting the low pressure switch wires on the rear of the compressor does not kick in the clutch, but shorting the wires for the cycling switch does kick in the clutch. I may be wrong, but it seems to me that if the high or low pressure switches were bad, then shorting the cycling switch wires would not make the clutch engage.
I may be wrong about all this, but I can't figure out why a pressure switch that seems to be working, and has pressure behind it, would not close the circuit.

 

[YukonGTmaster]

I have added a small charge to the system, it took it from 35 to 40 PSI. Also, shorting the low pressure switch wires on the rear of the compressor does not kick in the clutch, but shorting the wires for the cycling switch does kick in the clutch. I may be wrong, but it seems to me that if the high or low pressure switches were bad, then shorting the cycling switch wires would not make the clutch engage.
I may be wrong about all this, but I can't figure out why a pressure switch that seems to be working, and has pressure behind it, would not close the circuit.

 

[YukonGTmaster]

I would recover the refrigerant in the system, do a leak test for at least 25 minutes, and if it passes the test, recharge with refrigerant. We use a Snap-On Polartek machine.

Check the sticker on top of your accumulator for proper amount of R134a charge

Only add as many ounces of PAG 150 oil, as is recovered in the process.

 

[drakon543]

the cycling switch is the main power switch to the clutch engagement i believe and is at the end of the circuit. so bypassing it will run it even if its empty and bone dry to the point you sieze the compressor. my 97 was very similar issue initially same symptoms and was the dash controls. then other problems arose that weren't worth it to me. dash controls go through the ecm at some point with signals from the low and high side to give the ecm the ok to trigger the ac clutch switch relay to trigger the clutch. above members have mentioned purging the whole system and recharging after vacuum. if the system got too low to even trigger while being charged a diy charge kit will not work as there isnt sufficient charge to keep the system open looped to get the proper pressure through the whole system. as above being properly evacuated and vacuum charged is the only way to properly bring the system back up to where.it needs to be.