A/C issues

[shegarty]

Hey all - I'm currently trying to get ahead on the warmer weather and get caught up on overdue maintenance on the truck (04 Burb LT). Last year on a road trip we lost a/c when the bearings went on the pulley. This fried the magnet and the clutch. I managed to get the whole front end of the compressor (clutch, pulley and magnet) replaced with a good unit from the salvage yard. I opted for this route since the system was still holding pressure and the compressor on the truck would still turn freely. I didn't want to evacuate the entire system to replace the whole compressor as the truck has rear a/c as well and the cost to refill is a bit prohibitive.

The issue now is the the truck will not engage the clutch either when directly called for (snowflake button) or when using defrost. I have checked resistance on the electromagnet (around 3.5 ohms) and continuity (passes). I have also tried jumping the clutch directly with 12V and it does engage. I have replaced the old blown 10A fuse and already tried a replacement relay. The low side of the system is sitting at around 30 psi which should be okay since ambient is only around 5ºC (41ºF) right now.

Any ideas on what I should be looking at next? Low/high pressure switches? Other wiring issues? Adding refrigerant?

 

[OR VietVet]

My first guess would be the low pressure switch at the accumulator. The high pressure switch is in the back of the compressor and typically hardly ever go bad. When you do supply the power to the clutch and the compressor does engage, what are the pressure readings? High and low side. I would not add refrigerant till you tell me what the high and low pressures are when clutch is engaged and the compressor is activated. It may just be seepage of refrigerant and the pressure is low. Typically the static low pressure and static high pressure can be the same when attach gauges at the lines with the engine off. It all correlates to the ambient temp outside. The higher the ambient temp, the higher the static pressure. For instance, if you look at a pair of a/c gauges you can see that if the ambient temp is about 85 degrees then the static pressure will be about 15-20 psi higher. This is all on a fully charged a/c system.

If, like you stated, the ambient temp is 41 degrees, the static low pressure should be a bit higher than the ambient temp reading. Based off all that, I would say the system is low. You may still have a pressure switch problem, but I doubt it. Unless the previous clutch failure caused switch damage that weakened it and is now also a problem. Low pressure switches are cheap. When you charge the system, put a new low pressure switch on the rig.

https://www.rockauto.com/en/moreinf...U6/ShSVVud2qRWhoP65Xzc0NjW5n0fcow5FK66+OC3pG6

 

[shegarty]

I didn't engage the clutch with the engine running as the connector is in an awkward location and I didn't have pins or alligator clips to make a secure connection. At the time I was more concerned with checking the functionality of the replacement magnet/clutch assembly. I will look at getting a low pressure switch and then think about topping it off when I do some a/c work on one of my other vehicles. To add to the problem the previous owner appears to have removed the schrader connection on the high pressure port so I can't get any readings from it. It looks like the valve was replaced by some sort of rubber seal.

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[OR VietVet]

Post a pic of that high side port. I have never heard of such a thing. The pressure at that port is a lot when system is running. A threaded shrader valve will hold it but have never heard of a different way to seal it.

 

[shegarty]

I've never seen anything like this before either. Looks like a rubber seal under the top part of the fitting.

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[OR VietVet]

Can you send a side pic lease.

 

[shegarty]

Outside looks just like a normal fitting

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[ks03]

I've never seen anything like this before either. Looks like a rubber seal under the top part of the fitting.

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I believe it’s a ball valve, if you look at the inside of the fitting on your gauge set, you should see a 1/8” or so post that will depress that black part in the fitting on your vehicle, when the gauge set fitting is screwed down

 

[OR VietVet]

Ok, so I went to look at my 2002 Tahoe and I found two fittings on the high side line. The furthest forward had a fitting like this one you have and the one closest to the accumulator, in the same line, had a normal shrader valve fitting. Do you have 2 fittings in the same line? Here are very poor lighted pics but in one of them you can see the two fittings in the same line.

 

[shegarty]

Ok, so I went to look at my 2002 Tahoe and I found two fittings on the high side line. The furthest forward had a fitting like this one you have and the one closest to the accumulator, in the same line, had a normal shrader valve fitting. Do you have 2 fittings in the same line? Here are very poor lighted pics but in one of them you can see the two fittings in the same line.

View attachment 245019 View attachment 245020 View attachment 245021

Yes there are two fittings on the same line but they are on opposite sides of what i understand to be the expansion valve making one the high side and the other the low. The high side is the one closest to the engine and the low closest to the firewall with the expansion valve in between.

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[shegarty]

I believe it’s a ball valve, if you look at the inside of the fitting on your gauge set, you should see a 1/8” or so post that will depress that black part in the fitting on your vehicle, when the gauge set fitting is screwed down

That was my thought but the inside of my manifold set does not engage far enough to depress the ball valve. It will for a schrader though.

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[OR VietVet]

When I got my 02 the a/c worked flawlessly and still does after 5+ years and have never looked. But, since the orifice/expansion valve is in the same line then yes, the high and low are in the same line. The gauges will only connect to the right port as they are different sizes as well. As ks03 said, it does look to be a ball valve. Attach gauges to both of them and get a pressure reading and list the ambient temp at the same time.

It may take a different set of gauges or an adapter for the high side gauge line to attach.

 

[OR VietVet]

Hopefully someone has more input on these valves. I watched a video and it said that the rubber valve can be damaged if screw the hose on too far. If I was doing the job I would insist on a new valve each time. Why would they change it from the old style?

 

[shegarty]

Hopefully someone has more input on these valves. I watched a video and it said that the rubber valve can be damaged if screw the hose on too far. If I was doing the job I would insist on a new valve each time. Why would they change it from the old style?

No idea why it was changed but I am also worried about damaging it. I'll look into some kind of adapter. I was able to too off the system two years ago and just didn't bother to monitor the high side as it was working fine.

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[wjburken]

No idea why it was changed but I am also worried about damaging it. I'll look into some kind of adapter. I was able to too off the system two years ago and just didn't bother to monitor the high side as it was working fine.

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@UmmScott might have some insights as to what you have going on.

 

[UmmScott]

Ok...those 2 fittings are the high and low side to the system. That compression fitting is where the orifice tube is for the front AC system.

Btw..your connections look normal with that black rubber stopper in there. Thats normal and what GM uses from factory.

2nd...lets address why your clutch wont come on.
When you have the lit snowflake the hvac control head sends a signal thru the BCM to the PCM requesting AC operation. The PCM is in complete control of the ac clutch.
There are 2 safeties are in this circuit.
There is obviously the one on the accumulator.
There is another and it might be on the backside of the compressor itself. Thats the high pressure cut out. Both these will keep the clutch off if theres a problem.

Easy peasy test. Unplug both sensors and let the truck be off for about 5 mins. Ohm them. They BOTH should be closed when the system is at rest. If they are closed, then you need to do voltage testing to see where you do and dont have power: fuse, relay, etc.

If the high pressure one is stuck open, its bad replace it. If the low pressure one is open, stop, you have a refrigeration charge issue and need that addressed. Carefully check your wiring around the compressor. And make sure you didnt break a connection going to the high pressure cut out

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[UmmScott]

Another thing after rereading your original post...

I know you meant well by trying to avoid opening the refrig system by just replacing the clutch BUT...

If the clutch bearing goes bad, alot of times that will destroy the front bearing of the compressor shaft and also damage the compressor shaft seal.
Thats a critical seal and if it gets slightly damaged kiss the refrigerant charge goodbye.

I would ohm that low pressure switch first. Its easy and right there and takes 30 seconds.

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[shegarty]

Another thing after rereading your original post...

I know you meant well by trying to avoid opening the refrig system by just replacing the clutch BUT...

If the clutch bearing goes bad, alot of times that will destroy the front bearing of the compressor shaft and also damage the compressor shaft seal.
Thats a critical seal and if it gets slightly damaged kiss the refrigerant charge goodbye.

I would ohm that low pressure switch first. Its easy and right there and takes 30 seconds.

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Yes, i am aware that the compressor bearings and seal can often be compromised when the clutch bearing goes. As I stated though the system is still holding a charge so I thought this would be the smart way to start.

If I am omh testing the low side pressure switch what exactly am I looking for? From my primitive understanding of electrical diagnostics is should see small resistance if the circuit is closed and infinite if it is open.

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[UmmScott]

Yes, i am aware that the compressor bearings and seal can often be compromised when the clutch bearing goes. As I stated though the system is still holding a charge so I thought this would be the smart way to start.

If I am omh testing the low side pressure switch what exactly am I looking for? From my primitive understanding of electrical diagnostics is should see small resistance if the circuit is closed and infinite if it is open.

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A 2 wire pressure switch should show only 2 readings..
Closed: 0 ohms or very very low ohms like 0.3 or similar.

Or open: OL reading, no continuity.


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[shegarty]

A 2 wire pressure switch should show only 2 readings..
Closed: 0 ohms or very very low ohms like 0.3 or similar.

Or open: OL reading, no continuity.


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Got it. I'll check this morning and post back with what I find.

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