1972 Air Conditioning Problem

[Fred D.]

Re: 1972 Air Conditioning Problem

All,

I now have both a oOSOEZ print and AIM manual (neither of them have a clear schematic of the circuit but its the best info I can find).

What works (or tests ok): Blow motor at all speeds (rules out resistor pack), thermal switch, thermal limiter fuse, compressor (when jumped - 45psi).

I am suspecting the AC Relay. Does anyone know what the AC Relay circuit looks like?

For the AC Relay there is 1 connector for ground, and a 2nd connector with 3 wires. When the AC is on, the Orange (power in from Horn Relay) measures +12, the Purple (Blower Motor power) measures +12, but the Blue (Blower Motor switch) measure 0. Does anyone know if the Blue wire should have +12 when the system is on? I am thinking it should, but am not sure how to verify.

I'm open to all suggestions...

[Jack H.]

Re: 1972 Air Conditioning Problem

Now that you have the system wiring diagram (Chevy didn't publish schematic diagrams until the mi-70 era; the difference is a wiring diagram documents how the 'black box' elements of the electrical system are interconnected while a schematic opens up the 'black boxes' to show you what goes on inside), I advise the following:

Forget about the other aspects of the engine compartment flow of the A/C system, you already know the issue you're tracking is power to the compresor clutch.... Start at the compressor and trace the power lead back to its source. Now, walk that route with a multi-meter to find out where you lose power.

The general path is from the A/C system's ON/OFF switch, out to/through the freeze switch and on to the compressor's magnetic clutch. Plus, you understand that 1972 was the first year of designer safeguard(s) built into the A6 compressor; the super heat switch.

Prior to '72, if a compressor blew its main seal, lost refrigerant (along with the lubricating oil embedded), it'd continue to run blindly... They were known to get RED HOT, burn v-belts and self-destruct.

So, a thermal switch (super heat) was introduced in '72 which installs in the rear case of the A6 compressor. Power flows out to the compressor then takes a 2-way branch in the engine harness.

On branch runs directly to the compressor's magnetic coil. The other side runs through a current limiting resistor in the harness to the super heat switch on the back of the compressor.

In the event the compressor blows a seal and loses refrigerant + oil, the case heats up and the super heat switch closes. That provides a power path, through the wiring harness resistor to ground and effectively shuts off power to the compressor's clutch.

It wouldn't be the first time a super heat switch went bad OR a prior 'Bubba' had jury rigged the wiring harness because he didn't understand how the system was designed to work...

[Fred D.]

Re: 1972 Air Conditioning Problem

Found it, found it, found it!

The blower switch in the counsel was not pushed on all the way. It was barely handing on , but I couldn't tell by just looking at it. Once I pushed on it and felt it seat, I knew I had it. Such a simple thing, but hard to trace down.

Thanks to all who helped!!!

[Richard R.]

Re: 1972 Air Conditioning Problem

Glad you found it. Doesn't take much to put something out of commision does it. Later Reed