minitech

I have a 77 coupe that and the compressor won't turn on for A/C. It has the ground connected and one other wire going to it.

Can I put +12 volt to the wire going to it to check to see if it is just the switch that needs to be replaced? See if the clutch engages?

REELAV8R

Yes that should work. That clutch is just an electro magnet coil. So if you apply a 12v source and a ground it should engage the clutch. Do this with the engine off and you should hear it click into position.
You could also use a multimeter and set it to ohms the horseshoe looking symbol and put one lead on each prong for the connector, with connector removed, on the compressor (engine off) and be able to read some resistance. If it shows open (infinite resistance) then the coil is burned out.

77C34SPD

It will also not come on if there is no charge left in the system

7t9l82

jump the pressure switch.

gcusmano74

Jump the pressure switch to test the clutch only. Chevrolet didn't want the compressor to engage in a low or empty system.

KapsSA

Probably low on Freon.
Pressure switch is the black/silver thing right side center in the pic with the green wire.
Unplug the black boot and jump with a paper clip or needle nose pliers,

ClassicGlass63

The system is set up so if the charge gets low, the system shut off. It is internally oiled so if freon stops going around, so does the oil. In which case the compressor would burn up.

If the clutch does come on when you jump it, go ahead and turn everything on. Look down the sight glass on the top of the receiver dryer ( receiver is a vertical canister on the passenger side of the radiator/condenser on the front side of the radiator support).

It will have two rigid lines going in the top. Between those two lines there is a sight glass under a small plastic cap. There should be a small window there. Once in a while there is no window. It is smaller than a dime.

Anyway, clean that small glass lens off, jump the compressor clutch. Give it a few seconds, 15 to 30 maybe and see if there are any bubbles going across at all.

No charge and you will see nothing. Some bubbles and your charge is low. As you add freon, the bubbles get fewer and fewer. When there are no bubbles or just the occasional bubble, you have a perfect charge.

When I say bubbles, sometimes it just looks milky. Pull the jumper off the compressor clutch.

When you go ta add freon, if the charge is just low, leave everything in the on position. Nothing will be happening, but, as you ad freon, the charge will get bigger and eventually the clutch will start going on and off as the charge fills up. It will go on and off less as the charge gets closer to full. It will eventually stay on. At that point watch the sight glass window in the receiver dryer until it clears up.

If you want to attempt this on your own, just let me know. I would be happy to talk you through it.