Guys,
I have a 72 Cutlass Convertible, with A/C. The problem I am having is that when I turn on the air, the compressor will not fully engage. If I bring 12 volts to it, it will come on and keep running. I think it might be a relay or something. Any thoughts?

Thanks in advance!

 

Hmm this post is kinda old but since nobody ever answered I guess I'll throw this out there if any one else has the same problem...

Its my understanding that the a/c compressor clutch doesn't use relays, its power is switched directly by a mechanical switch. Off the top of my head I'm not sure if its integral to the vaccum switch that controls the vents or not, but if not it should be part of that same assembly.

As to a solution, I would think likely scenarios are that either the switch contacts are dirty/oxidized or the A/C wiring harness is showing its age.

If the switch is dirty you could clean the contacts with either an abrasive or isopropyl rubbing alcohol, though alot of times switches like this are sealed or riveted closed. Replacement might be easier.

I also recommend cleaning the harness firewall contacts and the plug and contacts of the compressor .

Problems in the harness stem from the copper wire either corroding or becoming brittle and breaking within the casing which causes higher resistance across the length of wire. Headlight wiring harnesses are notorious for this because it is easily noticed. When this occurs the headlights will slowly get dimmer as the wires in the harness degrade.

To check the harness wires unplug the wires from the coil when the compressor should be on and test the voltage across the plug with a multimeter. If its not 12V or more there is most likely something wrong.
Luckily, a new harness for just the A/C stuff isn't too terribly pricey from the repop places and is probably your best bet.

Since the clutch engages when you apply 12V directly I doubt that is the problem. (Though I have been wrong before)

 

The a/c compressor operates from the power relay mounted on the right fender near the hood hinge...try that

 

Not fully engage....is it slipping? Air gap possibly too big?

 

The system could be low on freon.

 

I would check the hold in rely(I can get pictures if needed) on the passenger side fender, or the freon. I am a noob tho when it comes to working on cars so I am just throwing out ideas.