I am looking for some info to start getting the a/c working in my 1972 Cutlass Supreme, it is all there and the compressor turns freely by hand.I had the hvac guy at work try to put R12 in it but it didn't seem to accept much as the compressor didn't turn on. So the question is where to start looking for the problem, if it is a power issue what should I be looking at?
One thing I noticed is that the blower will not run in the high position.
Thanks

 

High blower not running may be due to a blown inline fuse, but that should not have anything to do with the compressor not running.

As for the compressor clutch not engaging, have you checked for voltage at the connector that plugs into the compressor?
And is the connector actually plugged in?

Should look something like this:



Did the compressor turn or was it just the pulley? The pulley turns freely when the clutch is disengaged, and you said the compressor didn't run, so I am guessing the clutch wasn't engaged and the pulley was freewheeling. I'm just guessing, though.

 

If it's a 72, I think it has a thermal limiter on, or near the compressor. It's a little black plastic widget that's like a fuse. If that's blown, it won't charge. You need to jump that to get it to charge, then put a new one on. They are cheap. http://www.ebay.com/itm/Thermal-Limi...7a27c7&vxp=mtr

 

Thanks no the clutch does not engage, I meant I can grab it when the car is not running and turn it, I will try to jump out the compressor and yes everything is intact so all the parts are in place,wish me luck

 

Funny, someone else posted the same question, only 9 hours before you did.
It has been discussed here many times before that.
Here is a circuit schematic from the Chassis Service Manual ,which may help as well.





As noted, you mean the central clutch portion, and not the freewheeling pulley, right?



Are you saying that you and the A/C guy at work expected the compressor clutch to engage and the compressor to start running once a certain amount of R12 had been run into it?

Is your A/C guy aware that GM cars prior to about 1976 do not use a pressure-cycling clutch, like newer cars do, but use a POA valve system, which maintains a constant evaporator pressure regardless of compressor speed, without cycling the compressor on and off?

As noted, 1972 was the first year for the superheat safety switch on the back of the compressor, which triggers when the low side pressure is too low for too long (in practice, this means that it triggers on cold days in the rain), and causes a fuse in a small case or plug, near the compressor, to blow, rendering the A/C system inoperative until the fuse is replaced.

To quote myself from here in 2013:
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"The superheat switch is designed to sense the low-side pressure and to close (connect to ground) when the low-side pressure drops to -1psi to -5psi, which equates to a vacuum of 2" to 10" of mercury.

When the switch closes (grounds out), it shorts the power to the clutch coil to ground, through a fuse and a resistor, which are connected in parallel. This fuse / resistor combination creates a very-slow-blow fuse which should take about 2 minutes to blow, so that momentary drops in pressure won't blow the fuse.

The idea is to protect the compressor from two potentially fatal conditions:
1. low refrigerant, and
2. a blocked POA valve,
and thus save GM significant money on warranty repairs for these problems.
If the low-side pressure goes to vacuum for too long, the fuse will blow, disabling the compressor, and mandating that the customer bring the car in to the dealer to have it checked.

If your fuse keeps blowing, it could be for a number of reasons:

• The parallel resistor has burned out, is no longer connected, or is missing
• Your POA valve is misbehaving, and sticking closed for too long
• Your refrigerant is low
• Your superheat switch (SunAir part #MC-1316) is not working right.

You can test your switch with a gauge set, which will also let you see whether your low-side pressures are staying too low, and whether your refrigerant is charged.

I have no idea how an R-134 swap would affect low-side pressures, but if it makes them low, and you've switched to R-134, that may be your problem, too.

Bottom line: If you unplug the superheat switch, it will no longer trouble you, BUT if there is something wrong with the system, your sheep will be eaten after you have killed the little boy who cried 'Wolf!'"
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The superheat switch is also mentioned here and here.


So, you first need to ascertain whether your clutch will pull in if you want it to, then you can see if you have enough R12 in there.
Since your A/C guy seems to have added R12 to the system, it would be reasonable to have him suction the system out and try to recharge it to the correct level, or at least to slap the gauges on it and check its operating pressures, as he may have overcharged it trying to get it to start itself.

I would strongly recommend that you (and maybe your A/C guy...) read the A/C section of the Chassis Service Manual, which explains all of this clearly. If you do not have a CSM, I would strongly recommend that you get a genuine, original manual (not a scanned photocopy) as soon as possible.

- Eric

 

Eric, thanks for the info and yes I did think that a low charge condition would prevent the compressor from running and charging it might be the solution. My a/c guy is actually a HVAC guy as in buildings not cars so he was just doing me a favor. He did mention the POA valve as it is something he has seen before. So I will now be tracing power to see if and what switches/breakers/ thermal limiters ect. are bad and replacing if needed. Thanks again for the help, this car has never really been maintained and I would guess that when the a/c stopped working 30 plus years ago no one ever tried to get it working.
Marty

 

I seem to have screwed up when attaching the wiring schematic in the post above, but it's fixed now.

Just follow the schematic to see where the juice goes.
The Ambient Temperature switch is just there to keep the A/C off below about 40°, so it shouldn't be a factor.

It is completely reasonable to jump from the green wire right past the ambient switch, the superheat switch, and the thermal limiter, directly to the clutch, which is how all of the systems from about 1970-back are wired anyway.

- Eric