Long time reader, first time poster. I've solved many an issue with your forum. Thanks for all the help! However, I now have an issue I'm needing advice on.

99 Tahoe 5.7L 150k miles

a/c was cold last summer, but first fire up this season has it blowing warm. So far I've checked the fuses (interior and under hood) and verified that the clutch is not engaging. I don't have a gauge and didn't want to buy a refrigerant kit (with gauge) until I did a few more things. What I think I want to do next is consider the low and high side pressure switches. From what I've read, if I'm low on refrigerant, the low side pressure switch is not grounded. I can bypass the switch by removing the plug and shorting the contacts (assuming 2 wires, I haven't checked). If the a/c kicks on I either have 1) low refrigerant or 2) bad switch.

Can anyone offer advice on taking this route? If so, can anyone offer guidance on which switch is which. I'm thinking the low side switch is on or near the compressor, but I've got two switches in that area. There seems to be another one next to the firewall where the accumulator(?) is.

Thanks in advance!!

 

Welcome to the Chevrolet Forum. Not sure where the switches are but have you looked for any obvious leaks as denoted by the oil? If no leaks, then might just buy one can of 134a and see if that gets the compressor to engage. Best thing if you're going to work on your A/C is to get a set of gauges.

 

Finally got around to adding a can. Took a whole can of R134 and afterwards the clutch engages and blows cold. What I'm not clear on is when the gauge reading is relevant. When the clutch is not engaged, the pressure is around 60psi. Engaged (and blowing cold) it drops to 20psi. It ran steady for 10 minutes or so, then went into a pattern of cycling on/off. The gauge I have has 60psi in the alert range and 20 in the add range. The ok range is around 25-40 (from memory).

I have not seen any obvious sign of a leak (oil residue with dust or crud stuck to it) on the lines. However, I do think the intake valve is leaking. After I had replaced the dust cap, I took it back off to double check the pressure. It was difficult to get off and made a "pfffft" sound like it was under pressure. I spit on my thumb and covered it for a few seconds. Same thing "pffft". Can this valve be replaced? If so, does the system need to be evacuated and recharged?

Thanks!

 

The low pressure switch is the one on the accumulator. The reading is taken while the compressor is engaged readings when it is not are pretty much irrelevant. You can replace the entire high side fitting ( it bolts off even though it does not look like it) and for the low side you replace the schrader valve core.

 

If the schraeder valve needs replacing, I believe the system has to be evacuated and recharged. Sounds like you are still low. The low side should be around 35-40 psi w/ the compressor running. Might try adding another can and see how long it lasts. But if the valve is leaking, probably not long.

 

Yes the entire system must be drained the replace either one. The it should be vacuumed before recharging. The absolute by way to recharge it is to weigh in the charge using the amount on the ac tag under the hood

 

Thanks for the replies. I don't have the equipment to do the vacuum so I'll give my local mechanic a call tomorrow. If I'm looking to
vacuum the system
replace the schraeder valve
recharge the system

would anyone offer an estimate? I'm in eastern PA (Bethlehem). Father/son shop that I trust. Could it be as simple as some crud in the valve that's got it lodged open? And, since I'm 99% sure the valve is what's leaking, do I need to ask them to look elsewhere for leaks?

Thanks again!

 

I would