94 GMC Sierra AC problem

thx1138v2 · August 4, 2013

I'm losing refrigerant somewhere. I put in some of the flourescent leak check stuff and can't find anything in the engine compartment - dryer, compressor, lines and connections, and all sides of the condenser coil.

But I did notice that about 1/3 of the fins on the front of the condenser coil are bent to the point where they are closed. I'm wondering if this would cause an overpressure situation and there is some kind of relief valve that would vent the over pressure.

If that isn't the case, I guess I'm going to have to take the whole dash out to get to the evaporator coil. I looked at the condensation that drips out on the ground from the evaporator coil with the blacklight and it didn't show any detector.

Any ideas will be greatly appreciated.

thearborbarber · August 4, 2013

Look very closely in the a/c clutch. That's where mine leaked at it's a R4 compressor. It was hard to see even with die. You could only see a smidgen of die.

1997SierraSLT · August 4, 2013

My 97' was leaking at the compressor behind clutch as well.

thx1138v2 · August 5, 2013

That looks like it. I got the clutch off today and there's a lot of light green inside the electrical coil. It doesn't really show up in the ultraviolet lamp but where I am the sun is bright today and it's not very protected. I'll take another look at it after the sun goes down.

I went back through my records and this compressor is not the original. It is Delco but it was replaced around 2002-2003 time frame which makes it at least ten years old. I haven't been driving it much for the last few years so that may be part of the problem also. But heck, ten years service from an A/C compressor is pretty good in my books.

thx1138v2 · August 6, 2013

Yep, in the dark the leak detector really jumped out. I guess I have a kind of weak ultraviolet bulb. Many thanks.

Anyone have good links for compressor/clutch replacement procedure?

hollis092 · August 6, 2013

It's really easy to replace your compressor. You want to get a compressor (AC) replace kit - compressor, drier, and orifice tube filter - dont do the install without changing the drier (accumulator) and the orifice tube filter.

You will need a set of guages and a vacuum pump if you want to do it right.

I replaced mine on my 99' years ago - replacement kit came from an online company (cant remember name) - but I literally bought the rebuild kit (compressor, drier, and tube) for $180 - they are a bunch of GM retired engineers - you can probably find it out there.

thx1138v2 · August 8, 2013

It's really easy to replace your compressor. You want to get a compressor (AC) replace kit - compressor, drier, and orifice tube filter - dont do the install without changing the drier (accumulator) and the orifice tube filter.

You will need a set of guages and a vacuum pump if you want to do it right.

I replaced mine on my 99' years ago - replacement kit came from an online company (cant remember name) - but I literally bought the rebuild kit (compressor, drier, and tube) for $180 - they are a bunch of GM retired engineers - you can probably find it out there.

That looks like the way to go. Parts have gone up a little since then but I found a kit on the web for $205.

I've never done this before so I'll have to buy the vacuum pump and gauges but even with that cost it's about 1/3 of what I'm being quoted for a shop to do it.

Since I've never done it before I will still need some links to show me the procedure. And since I've put the leak detector in the system I probably need to flush the system also and need to know how to do that.

thx1138v2 · August 14, 2013

I got my parts and the tools I'll need and started tearing it down this afternoon. I ran into a problem with the nut that connects the dryer/accumulator to the the evaporator coil inside the firewall. I noticed when I turned the nut it didn't break loose but twisted the tubing. The nut is bound to the tube. Luckily I caught it immediately and didn't twist the tube very badly. So I took the dryer bracket off and unscrewed the dryer from the nut while holding the nut stationary.

I need to free the nut up to put the new dryer on properly but I'm not sure about the best way to free up the nut. I was thinking of a propane torch but don't know if that will work. There is only about 1/2" of aluminum tube showing between the nut and the rubber grommet in the firewall so I can't really get a grip on the tube either.

Any suggestions will be appreciated.

txab · August 14, 2013

First try soaking the hell out of it with PB Blaster or similar

thx1138v2 · August 15, 2013

That PB Blaster is some good stuff. Thanks for the reminder. I had some on hand but haven't used it in years and had forgotten about it. I let it soak over night and that loosened it up enough where I could turn it with a wrench and its pretty loose now after working it back and forth for 15 minutes.

Now I'm having a similar problem with the lower expansion coil connection to the high pressure line. It may be the same thing but I also can't find the correct wrench for the smaller fitting on that one. I tried different SAE wrenches and then went to a 20mm. The 20mm fits best but it's still loose. I'm worried about rounding off the nut. Maybe letting it soak in the PB Blaster will help. Is this some oddball wrench size just for R134a A/C systems?

thx1138v2 · August 16, 2013

The PB Blaster got it. I just had to wait longer to let it infiltrate. Luckily the nut on the lower expansion coil tube is afixed to the tube and doesn't need to be turned so the 20mm wrench gave it enough backup to get the larger nut loose.

I had a battle getting the compressor out of the bracket. I didn't know about the mounting bolt inserts in the back of the bracket and none of the videos I found said anything about them. So for others looking here, once you get the nuts on the back of the bracket mounting bolts removed use a hammer to drive the body of the old compressor toward the cab of the truck. That will loosen the inserts and make removal of the old compressor a lot easier. I'm not sure which models this applies to. Mine is a '94.

thx1138v2 · August 18, 2013

All flushed and back together. I drew 30 in Hg on the gauge and it held for 15 minutes so I charged it. Charge went fine until the end. 38 Low Side and 210 high side with 93F temperature. When I took the high side gauge coupling off of the service port it was hissing. Not strongly but audible and it can be felt. Thinking it was a Schrader valve I drained the system to replace it.

The high side service port on the refrigerant line does not have a Schrader valve. It appears to be a black plastic ball and I can move it with a long, thin allen wrench. It looks like the valve body should come apart. There is a 5/8" hex fitting welded to the tube that is the housing for the valve and it looks like two pieces but I can't get it to break open. I can fit a wrench on the bottom of the fitting and a socket on the top but I can't get it to break loose and I don't see how PB Blaster is going to get into those thread because if there are any, they are internal and are all covered with the housing.

Does anyone know if this really is a two piece valve housing? Otherwise, I'll have to replace the hole compressor fitting block and both hoses.

thx1138v2 · August 18, 2013

Well, duh! I looked at it more closely this morning and I was putting 5/8" the socket too far down on the fitting so the 5/8" wrench was on the bottom of the base and the 5/8" socket was on the top of the base. No wonder it didn't come apart. A wrench on the upper flats near the top opened it right up.

txab · August 18, 2013

Yep, it's two piece as you finally figured out. They designed it to replace those leaking valves. I hate those

thx1138v2 · August 19, 2013

Lessons learned the hard way by a rookie:

Get a can of PB Blaster and soak the fittings at the accumluator/dryer and condenser coil at least one day before you start the job. GM A/C lasts a long time. This is only the second replacement in 20 years. Some of the fittings I disconnected had never been disconnected before. I've bought the truck new off the dealer's lot so I know this for sure. If you bought used and don't know when it was opened last, do this anyway. It will not harm anything and it will save you waiting for the PB blaster to penetrate the threads. It's not fast but it is very good if you give it time to work.

You will need large wrenches. Get a set 1 1/4" down to 1/2" in 1/16" increments. Most of the hose fittings are sixteenth, like 15/16" , 1 1/16" etc. You'll also need a 20mm.

Breaking the system down and putting it back together is pretty straight forward but here are a few cautions.

Don't over tighten the low side clutch switch on the dryer. Tighten it as far as possible by hand and then only 1/8 or 1/4 turn by wrench. If it is over tightened the plastic case will crack and you'll have a vaccum leak when you draw the system down before recharging. It's not an expensive switch but it sure is aggravating to have to make another trip to the parts store. About $10.00.

Don't tighten the knobs on the gauge set port hoses that connect to the vehicle ports all the way down. Connect hoses first to the gauge set and then connect the hose port fittings to the vehicle ports and then only tighten until you get a "bump" in the pressure gauges. That means the fittings have opened the valves. Screwing the fittings tighter runs the risk of damaging the valves in the vehicle service ports. The first time I charged the system and removed the hose fittings from the truck service ports the high side port leaked because I damaged it by over tightening the connector. The problem is you won't know it is damaged until after you have charged the system and you remove the connector. The system must be evacuated again to replace the damaged valve. Another trip to the auto parts store and loss of 3 cans of refrigerant. About $40 for refrigerant and $5.00 for the valve.

If you damage the high side service port valve don't assume it has a Schrader valve. Mine did not. It has what GM calls a "high side service valve" and other people call it a "primary valve" or "high side port valve". The low side port does have a Schrader valve but the high side port is different. Look very closely into the exposed part of the high side valve. It looks like a small plastic ball where you would expect to see the top of the Schrader valve. You can use a small allen wrench to rotate it around when there is no pressure. Look into the back of the valve once it is removed and the difference will be clear. Don't waste your money on a Schrader valve replacement kit at the parts house because it doesn't work on the high side valve.

The base of the high side service port valve has a line around it about 2/3 of the way up from the tube it is welded to. It looks like that is where it separates. It is not. Near the top of the valve there are octagon shaped flats. That is where you put the wrench to loosen it and the size didn't fit any wrenches I had (standard or metric) so I used a Crescent wrench on it. It comes apart very easily so don't worry about rounding off the corners using a Crescent wrench.

I have had several bad experiences with Chinese parts so I went with AC Delco parts. That probably doubled my parts cost but both of the prior systems had AC Delco parts and they both lasted ten years. I had to replace the water pump after nine years and I used a Chinese part. It lasted eleven months so I shy away from Chinese parts when I can get American made. I could have maybe reused the electrical switches but they both appeared to be 20 year old original equipment so I replaced them. Replacing the high side compressor switch would require evacuating the system again so I thought it wise to replace it.

So the whole job with AC Delco parts and screw ups was $615. That includes the compressor with clutch, the compressor high side switch, compressor oil, dryer/accumulator, low side clutch switch, condenser and tube/orifice, service valve, flush for the lines and evaporator, high side service valve, and two fills of refrigerant.

I didn't include tools above because I didn't have to buy a vacuum pump. A friend gave me some discontinued and out of service medical equipment and one of the machines was biopsy machine that is used to suck up tissue for cancer biopsies. I had to disassemble it and do a little rewiring but it worked very well. So that's a place to look for a cheap vacuum pump if you can find medical equipment salvage.

One note on the gauges I bought - they didn't have an R134a dispenser valve in the set so I had to buy one of those at the auto parts house which was another $6.99 + sales tax.

Thanks for the assistance, guys.