Need help troubleshooting A/C issue
 

1991 Silverado K1500. 5.7L. Automatic. 4x4.

I'm trying to get A/C working. Here's what I've done and observed so far:

- the blower is fine and all the dash lights appear to be in working order.

- I put refrigerant in the system up to recommended pressure. This was a couple hours ago and gauge shows it's still at the same pressure now, so hopefully this means there's no leaks

- took a test light to the connector at the Evaporator. Lights up when AC button is turned on. Does not light up when AC button is off.

- compressor clutch is not turning. took a test light to the connection at the clutch. It does not light up under any circumstance.

So I'm thinking there's an electrical problem because power is not going to the compressor clutch. Any ideas where I should look?

I'm also thinking it would be useful to test the compressor with a different source of power. Any thoughts on this? Is it a good idea and if so, how should I do it?

thanks

80skeys

A common thing on these systems is the low pressure switch going bad. It is plugged into the accumulator. You can test it by unplugging and jumping the 2 wires from the harness side together. This should send power to your compressor. If it does then replace the switch. They are fairly cheap.

Excellent suggestion. By "harness side" do you mean jumping the two wires that are unplugged and hanging loose?

Yes don't jump the switch itself because once it is unplugged it does nothing. Jump the 2 wires in the harness that you unplug from that switch. Seems like common sense, but I have had a guy that I told that to and he was jumping the switch not the wires.

Okay now I see. those two wires complete a circuit going to the compressor. Okay I jumped them and the compressor clutch came on. The low side pressure dropped to 0. I hooked up a can of refrigerant, but the low side pressure keeps dropping to 0 as I add more refrigerant.

It's taking the freon though?

I can't tell. When I depress the button on the can, the pressure temporarily goes up, then quickly drops down. I stood there for about 10 minutes putting (or thinking I was putting) freon in the system, but there still seems plenty left in the can afterwards.

It's hard to diagnose without being there. It is normal when the system is low for the low side to go to zero, but it could also mean that you have a blockage somewhere. It is also normal for it to take a while to charge. I just repaired my dad's Suburban which holds 4 LBS (dual A/C) and it took about 35 minutes for it to suck it all in. What is the A/C history on this truck? How long has it not worked? Have you added Freon before?

Oh gee, if that's the case maybe I didn't give it enough time. I did notice it seemed to be holding about 10psi after I was charging it.

After it takes a little freon you should be able to plug the low pressure switch back in and let the compressor cycle by itself. Unless the switch is bad of course.

I bought a new switch and replaced it. But it only powers the clutch for a second before shutting off. It does this several times in a row rapidly, then it quits, the control panel in the cab starts blinking, and I have to unplug the battery to reset it.

That should mean it is still low on freon. Do you have any idea how much you have put in so far. There should be a decal on top of the accumulator to tell you how many lbs it holds. Also remember that most of the little cans of freon are only 12 ounces not 1lb (16 ounces). Is it getting cold at all?

Hard for me to tell. The can still seems to have a lot in it. I'm going to try to put more in again today and see what happens.

I'll bet the switch was doing its job. :)

I've got an update.

It turns out something wrong with the can of R134. After several attempts trying to add it and it wasn't taking, so I went to the store and got another can: different brand and this one didn't have the pressure gauge. This one worked. It sucked it all in right away and now I'm getting cold air in the cab. I've driven it two days now and it's still blowing cold.

I've decided to leave the low pressure switch disconnected and the wires jumped together.

I would strongly suggest not leaving it jumped like that. It will burn up the compressor. A compressor needs to cycle on and off or it will freeze up the system. If it does not come on with it plugged in you are either low on freon or the switch is bad. I think you said you replaced the switch so you are probably low on freon. Also remember it can take several minutes for a system to take in a single 12oz can of freon.

I didn't realize this. Thanks for telling me. I'll connect it back up to the switch.

Okay I reconnected the low pressure switch and have been running the A/C and it's working fine.

question: why would the compressor burn up if I leave the wires jumped together? It seems to me that even with the switch connected, the compressor is always running anyway.

Your compressor will cycle on and off according to fan speed and outside temperature. If it is not allowed to cycle it will freeze the system up - usually at the orfice tube. Also if you got a leak in the system the compressor would continue to run possible with no oil and that equals trashed compressor and the entire system would be contaminated with the compressor locking up. Just a safety item that is there for a reason.

So the low pressure switch also does the cycling the compressor based on the temp and fan speed? The switch monitors all these things?