72 280SE 4.5 A/C Gremlin

[Shep]

I have a ThermoKing A/C unit in my 108. It blows ice cold and everything works great. When it works, that is. My problem is the A/C will work fine for a few hours then it won't turn back on.

Typical scenario: crank up the car cold, drive it for an hour or so (up to and beyond operating temp), get out for lunch, get back in, crank the car with the A/C turned off, turn the A/C on and the blower nor the compressor will turn on. Try it several times while on the road, still no luck. Drive back to the house, let it sit overnight, crank it up cold, turn on the A/C and it works just fine. Until it doesn't work again, same scenario as before.

I've changed the fuses under the dash and the bigger fuses under the hood. I've checked most connections.

So when it's working, the compressor is on, the blower blows and all is right with the world. When it's not, the blower won't come on, the compressor won't come on (obviously) and it's frustrating, not to mention a little bit warm in Georgia in July.

Any insights?

[Mark DiSilvestro]

Did you check ALL the fuses? Often, in addition to the main fusebox, these cars have various small auxiliary fuseboxes under the hood containing one or two fuses for optional equipment. The AC fuse is usually the blue 25 amp one. Often the AC fuse contacts get weak or burned due to the high current. Did you check anything with a 12v test-probe or voltmeter? That's more reliable than just a visual inspection.

Happy Motoring, Mark

[andyj]

I had a similiar problem. Turned out the the aux fuse box under the hood on the firewall was only putting through 9 volts not enough to activate the system. Changed the fuse box and it works great.

[Shep]

I've checked the little fuses under the dash and the big square ones under the hood. No blown fuses. In fact, to be sure, I replaced all of them since sometimes the contacts look OK but are not. I'll look at the 12 volt issue and the fuse box itself under the hood. Very odd, though. If it was a voltage issue, why would it work and then not work and then work again? Seems like it would either work or not but not intermittently.

Thanks all. Helpful, as usual.

[Litton]

Mine does a similiar thing but it's just the switch with dirty contacts or some such. When it gets bad enough, I will pull it out or try to shoot it wih some contact cleaner.

[psfred]

The blower switch is a rheostat on this model, and mine has dead spots were it "disconnects" the blower. Try moving it around a bit to see if that is the case, it's a stepless speed control.

I'd also check the charge. If it's low, you can get evaporator freezeup on this car, and the thermostat will keep the compressor off until the ice melts.

Finally, when it refuses to run, jumper the clutch on the compressor and see if it runs then. If it clicks but won't spin the compressor, the clutch is bad and won't hold when hot.

Peter

[Tomguy]

I'd agree with the rheostat theory, most likely cause. However, also try this: In the fuse box under the hood, clean the contacts off (not the fuse but in the box) with fine sandpaper and contact cleaner. Mine were corroded enough on the windows so that they only operated some of the time!

[Shep]

Here's some more info:
I pulled the fuse under the hood and replaced it (again). It seems that the problem is presenting itself as a blown fuse. I cleaned the contacts on the fuse holder. There was a little corrosion on there.

However, I think this may be the source of the problem. The rheostat a/c switch on this unit was switched out by the PO to a three position switch rather than the infinite setting rheostat switch. So the green compressor light comes on when it's on high and low positions but not in the middle position. Perhaps it's this switch that's causing the fuse to blow repeatedly?

[psfred]

Most likely, since the original switch is in fact a dual one -- one switch is on/off for the compressor, the other a rheostat for the fan.

Probably not wired correctly, so it shorts in the medium position.

I believe the switch is still available.

Peter