1972 Air Conditioning Problem

[Fred D.]

1972 350/200

Just got my car back from storage and my AC has quit working (worked last fall). Here's what I checked.

I checked the AC fuse in the fuse box and it is good but no power to the compressor. If I unplug the thermal limiter fuse and check voltage to the socket, I do not measure any power on any of the 3 pins. Shouldn't I measure power on at least one of them with the AC on? Is there a way to ohm this part out to check it?

I check the low-side pressure and it was good. I don't really trust my gauge but it said I had about 60psi (this was on the valve closest to the engine - low side I presume). I jumpered +12 VDC to the compressor - it came on and blew cold.

I also see that the compressor has a line coming off the back of it (black w/ yellow stripe). Is this a low or high pressure switch? If it is, what monitors this line and cuts power if it is low? It this also done in the thermal limiter fuse?

I have to get my hands on a schematic as I don't have one at the moment. The wiring diagram I have doesn't show the AC system.

Any idea what might be wrong here (or a good source for a wiring diagram w/ AC)?

[Bryan M.]

Re: 1972 Air Conditioning Problem

Fred,
Along with a 1972 Chevy Service Manual and the AIM, I use a Dr Rebuild oOSOEZ schematic. I like it. Easy to follow.

[Jim T.]

Re: 1972 Air Conditioning Problem

Fred do you still have R12 in your air conditioning system?
Do you have a sight glass on your dryer?
If you do have a sight glass at the top of your dryer, jump the system again and read what the sight glass will tell about the status of your R12 if you have it.
If you have full charge, the sight glass will run clear.
If you are low on R12 you will have consistant bubbles in the sight glass.
My 70 does not have the low pressure switch, I use the sight glass to add R12.

[Jack H.]

Re: 1972 Air Conditioning Problem

Dust off your copy of the AIM book. Turn to the options section in the back and find air conditioning (C60). One of the last pages in that section is an electrical wiring diagram specific to A/C cars...

You'll find there are TWO fuses for the A/C system, one in the fuse block inside the cockpit and the other in-line in the engine wiring harness coming off the B+ bar on the side of the horn relay.

It's the second (engine compartment) fuse that provides power to the high draw components of the A/C system (compressor clutch and blower fan). The cockpit panel mounted fuse simply controls the low current drain components of the system (control relays, Etc.).

Now, study the power routing from the in-line fuse in the engine compartment. You'll see it routes to the A/C control relay (mounted on the Harrison air box attached to the fire wall). And, for the path to the compressor, it routes to and through a thermal switch...

The switch is there to PREVENT the compressor from engaging if outside ambient temperature is too low (no need for A/C). It just might be you've got a 'cockpit' problem--expecting the A/C to run on a cold day when it's specifically designed NOT to run...

[Fred D.]

Re: 1972 Air Conditioning Problem

Bryan,

I have contacted Dr Rebuild for details on ordering their schematic. I can't locate pricing on their web site. Hopefully this will help me figure things out as I don't think I can fix w/o an understand of the circuit.


Jim,

I do have R12. I do not have a view window that I see, but believe there is a low pressure switch (described earlier). With power jumped to the compressor and the air blowing cold out the vents, the low side measure 45 (+/- 2) psi as it cycles.

Thermal Limiter Fuse:
I did find that my thermal fuse was bad but it did not fix the issue. I bought a NAPA part ( http://www.napaonline.com/MasterPage...l+Limiter+Fuse ) and it ohmed out < 12 ohms to any pin. My original part had an open on one pin. The new part was labled S, B, & C. S went to the low cutoff switch (at least thats what I think it its), B went back to the harness, C went to the compressor V+ pin. My old part was open B-C.


Jack,

I do not have AIM, please elaborate on this book. I was planning to order the Dr Rebuild schematic but I'd like to hear about AIM - perhaps its a better way to spend my $.

This 2nd fuse - is it AGC style? I am going to run out to the garage and look near the horn relay right now...

[Richard R.]

Re: 1972 Air Conditioning Problem

I had the same problem on my 70. It turned out to be the thermal switch in the air box. It's kinda hard to get to. If I remember right I accesed it by removing the side grille and it was right there. Hope this helps. Richard

[Fred D.]

Re: 1972 Air Conditioning Problem

Richard,

I believe I found the part your referring to (see pic) but mine test as 0 ohms which should be ok.

Fuse2.jpg

Please keep the ideas coming...

[Richard R.]

Re: 1972 Air Conditioning Problem

Fred that is the thermal switch I was talking about. Your next step should be to check if you have 12 volts to one side of the switch when you turn on your AC, that will check everything back to the AC control. The next stop is the compressor. Hope this helps Richard

[Pat K.]

Re: 1972 Air Conditioning Problem

Fred,

The AIM is the Assembly Instruction Manual. It's what the worker's used to assembly your car. It's cumbersome to find information but once you kinda figure it out it's an invaluable resource for working on your Vette. I think I bought mine from Corvette America, I'm assuming many of the Corvette vendors carry it.

Pat

[Fred D.]

Re: 1972 Air Conditioning Problem

Richard,

I checked that earlier, and no power at either pin on that connector when the cars running and the AC controls are turned on. The problem must be further up the line. Since my schematic and shop manual don't cover the AC system, I don't know if the which component is before this point (perhaps the console switch?).

Neither my shop manual or schematic show the AC system. I must get a schematic. I'll look for the AIM suggested above or order the Dr Rebuild one.

Keep the ideas coming and thanks for all the help...

EDIT:

Could this be the correct AIM:

http://www.ecklers.com/product.asp?pf_id=38157&dept_id=226

[Richard R.]

Re: 1972 Air Conditioning Problem

Fred that is the AIM that you should have. You should also invest in the wiring diagram from Dr. Rebuild. It is color coded with the same colors as your harness. It is easier to use because it is on one sheet rather than several pages as the AIM. Hope this helps. Richard

[Terry M.]

Re: 1972 Air Conditioning Problem

I guess I am puzzled as to why a member of NCRS would not buy the AIM from the NCRS store that is just a click at the top of the page. We ought to support our own organization. This "free" site is supported from not just membership dues.

[Ridge K.]

Re: 1972 Air Conditioning Problem

I guess I am puzzled as to why a member of NCRS would not buy the AIM from the NCRS store that is just a click at the top of the page. We ought to support our own organization. This "free" site is supported from not just membership dues.

Very good point, Terry.
I bought my copy of the AIM from the ncrs store, and my copy of Doc Rebuild's excellent wiring diagram from the good Doctor at Carlisle. I second or third both purchases. Ridge.

[Jack H.]

Re: 1972 Air Conditioning Problem

The only reason I can think of Terry, is some of the NCRS supplied AIM books have poor copy quality compared to what might be available from another source...

[Richard R.]

Re: 1972 Air Conditioning Problem

I agree with Terry 100%. I buy all my books and manuals from the NCRS store, also I have several CD's from the store. I was only agreeing that it was the correct manual. Fred will also notice it will be cheaper from NCRS. Good thinking Terry. I should have thought of that. Richard