Factory A/C question, '72 GMC

[harrydunn]

Good morning--

I am wondering if anybody can tell me if my '72 with factory air has a low pressure switch that would keep the compressor from kicking on. The compressor kicks on fine but I don't want to run it too long if it is low on R-12.

Which brings me to my next question...

I found one A/C port on the '72 last night but could not find the second. I believe I found the low side port. If anybody could tell me where the second port is, I would appreciate it. The truck is stored at a friend's house and we didn't have good light to look under the hood so it may have been right under our noses.

Anything different about charging these systems compared to other vehicles? I've charged both R-12 and R134A systems in the past. Pretty straight-forward.

Thanks,

Mark

[mrein3]

Originally Posted by harrydunn

Good morning--

I am wondering if anybody can tell me if my '72 with factory air has a low pressure switch that would keep the compressor from kicking on. The compressor kicks on fine but I don't want to run it too long if it is low on R-12.

Which brings me to my next question...

I found one A/C port on the '72 last night but could not find the second. I believe I found the low side port. If anybody could tell me where the second port is, I would appreciate it. The truck is stored at a friend's house and we didn't have good light to look under the hood so it may have been right under our noses.

Anything different about charging these systems compared to other vehicles? I've charged both R-12 and R134A systems in the past. Pretty straight-forward.

Thanks,

Mark

Good question on the low pressure switch. My 1972 Chevelle has factory air and has a "low pressure switch". On 70's GM vehicles this was a fuse that I always called a thermal limiter. This replace when blown component was there to sense the low charge. A low freon charge compromises both oil flow (oil is carried by the freon through the system) and compressor cooling.

My 1971 Chevelle with the same factory AC does NOT have a thermal limiter.

When I added factory air to my truck I did NOT add a thermal limiter but on my list of things to do is to find a wiring harness with that in the AC circuit.

To find if you have a thermal limiter in your truck, follow the green wire on the compressor. If it simply goes back through the firewall you don't have one. I'm guessing you don't.

[Ozark Tim]

There should not be a thermal limiter on these trucks. Compressor runs constant whe the system is turned on, POA valve controls refrigerant flow.
Low side port is on suction line just before the POA valve (sort of above the evap coil box on firewall)
High side port is on liquid line muffler over the top of compressor.
I would not convert this system to 134a, but that is just me.

[harrydunn]

Thanks Tim. I'll check it out. I have no plans to convert from R-12. My '93 Toyota Land Cruiser is R-12 and I chose to leave it alone. R-12 is expensive but can be found fairly easily. Blows very cold too.

[leddzepp]

if you have no leaks and no need to replace expensive components i would stay with r-12. it is expensive, but easy to find as stated, and it will freeze you out of the cab on the hottest day. the clutch bearing is finally starting to give out on my a-6 compressor and i am going to replace/upgrade with a pro6ten compressor from old air products, and staying with r-12.
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[mikemileski]

Mark, I also have a '72 GMC with factory air and absolutely do not have a high pressure port. If there was one on it originally, then a PO must have replaced it with something non-stock. I just watch for the flow of bubbles to stop in the window of the dryer to determine when to stop adding refrigerant. Seems to work fine.

Mike Mileski
Tucson, AZ

[SS Tim]

Mark,
It looks like you are going to be putting a factory service manual to good use!

[SS Tim]

Mark,
It looks like you are going to be putting a factory service manual to good use!

[harrydunn]

Originally Posted by SS Tim

Mark,
It looks like you are going to be putting a factory service manual to good use!

Ha, yep! I'm mechanically inclined but always prefer to line my ducks up before I start a project, if possible.

--Mark

[ole dollor]

harrydunn: the compressor on your truck should have an oil sump, it was called the a6 i believe , the newer ones dont have this, and have to have oil and freon going through the system , you could run it and check the freon level without worry of hurting the compressor , also if its out of freon you can check and add oil to the sump, before charging, the bottom of the compressor will have a bolt you can remove and use a oil can with the right compressor oil and fill it up...hope this helps...

[harrydunn]

A few questions.

1) Loosened the compressor sump plug and there was pressure inside. Normal? I tighened it back up without checking so I could post on here first without opening it all the way up.

2) Compressor is engaging so added a can of R-12 I had laying around. A/C line running into the firewall box was ice cold as I would expect but I never got any cooler air out of the cab vents.

I'm all ears!

[ole dollor]

harrydunn: can you take a lot of pic's of all the a/c under the hood, all hoses, black box, compressor, every thing....maybe we can locate a high side hose connection and get some pressure reading on your system, that would tell us alot of what is going on in the system...also pics of the the condenser and rad. area....are both heater hoses very hot.....if so for right now plug them off.....and see if it gets colder inside....its to dang hot not to have an a/c not blowing cold air....

[harrydunn]

Took some photos but here is a video first. Let me know if you think some photos would help and I can post those too. Please note that the compressor is off for the first part of the video. It had been engaged but I turned it off right before I started shooting the video.

GMC A/C 1972 - YouTube

Thanks.

[ole dollor]

man...i cant see that fast.....slow way down....lol what i could see ..it looks like you have a new hose on the back of the compressor , like a made up one , not factory that may be where your valve went, they made a hose with out the valve. if you can take some still pic's of it and post....

[harrydunn]

Some photos I took this afternoon. I'll have to get some photos of the back of the compressor tomorrow...

http://s1065.photobucket.com/albums/u395/darryhunn/

[chewychevy67]

not to hijak but i added this switch to my 67 when i went thru the a/c. is this some kind of thermal limiter? it shuts the compressor off below 28 and above 380 psi. just had to wire it into the green wire that goes to the compressor. Rod.

http://www.ebay.com/itm/400288128124...84.m1423.l2649

[ole dollor]

in pic # 6 is that not a valve cap in the line at the back of the compressor?

[harrydunn]

I've seen that before and it is definitely a cap to the system but it does not seem to be the correct size to screw an A/C hose to it. What does the fact that the hose downstream of the orifice tube is ice cold telling me? I know it's supposed to be cold so I'm assuming my orifice tube is fine and not blocked/broken?

[ole dollor]

looking at the pic of the receiver dryer the little window looks like the inside of your system may be contaminated ... your dryer may be coming apart inside...the window is turned brown looking, a good sign it needs to be replaced, also you need to flush out the complete system when you replace the dryer .even the valves and lines at the black box....that is where i would start, it kinda hard to tell from miles away, pressure reading could help a lot in really seeing what going on inside the a/c system....maybe some one else has some better ideas....this is what i saw....good luck on the a/c fix....its hot out side.

[harrydunn]

I appreciate the assistance. I probably won't worry about it too much as it's not my daily driver. Just one of my toys. It would be neat to get it working again though.

[harrydunn]

How difficult is it to see if the blend door is working correctly? I popped my head under the dash last night and verified that the cable is connected to the 'COLD' rod on the dash HVAC panel but could not verify if the blend door was actually working correctly.

[harrydunn]

After confirming that the section of high-side hose containing the service port and muffler has been replaced at some point in the truck's life, I was only able to get a low-side reading, unfortunately. Static pressure reads 80 psi and, with the compressor kicked on, 30 psi. The pressures are a bit low but should not be responsible for the absence of cold air out of the vents I'd have to say. Tube past the expansion valve is ice cold so I would assume I have some sort of blockage in the evaporator or a blend door problem.

[Ozark Tim]

Harry,
I just saw a scenario where I had to use a hole saw in the top of the evap coil box to remove mouse nest and 40 years of oak leaves with a copper plumbing pipe tape'd to a shop vac hose, then flush with a spray nozzle and hose to clean thouroughly. In this scenario there was only 5" of evap coil left for air to pass over and pick up cooling.
I simply drilled a 1-1/4" hole and bought a plug at the hardware store to close it up after I was done.
FYI, I had to go on the i/s and do the same thing in front of the heater core. I completely filled my sears shop vac with mouse nest and crud. Amazing.
If you need a picture of where to make a hole let me know.
Sounds like an obstruction to me, or your heater/a/c door is not closing off properly. If there is enough junk in there that can keep the door from closing all the way and if the vacumm is not shutting the hot water flow to the heater core while a/c is running you could be mixing hot and cold as someone mentioned previously.

[harrydunn]

Thanks for the response. I'd like to see where you drilled those holes if you could post a photo.

Can you explain how I would troubleshoot the vacuum for the heater core. I did notice a vacuum line from the intake manifold running over to the evaporator box on the firewall.

[Ozark Tim]

Harry,
I will try to get some pictures of all of this for you tonight. Probably post something tomorrow if all goes well.