Hi, I have recently purchased an 1985 300 SD and the AC does not work. The compressor does not come on. In goofing around with it, I was able to short the lower switch on the dryer (? - this is the one without pigtails) and get the compressor to come on. When this ran for a minute or so, I was not getting cold air from the vents. The switch is getting 12 V to one side.

I'm not sure how the switch is designed to work and I haven't had any luck searching the forums for answers to this particular issue.

So here are the questions...

1. Is the switch faulty?
2. Or could the system be empty, therefore making the switch malfunction (i.e. if I can get someone to check the refrigerant, will I find that charging the system will make the switch work again, and all is well?)

It will be another week before I can get gauges put on it, so I don't have an answer to that yet.

I appreciate any input.

Sounds like the switch is working as designed. It is a low pressure switch, designed to prevent the compressor from turning on if there is low/no refrigerant in the system.

There is another version of the switch, designed for R134A applications that is both a low pressure and high pressure cut-out. This prevents overpressuring the system from the interently higher system pressure R134A refrigerant.

When you get gauges on it, I'll bet it shows an empty system.

__________________
Bob
'82 300D Petrol B-G Metallic

Thanks for the reply.

If the lower switch is the low pressure switch, what is the upper switch on the dryer? It has two pigtails about 4" long coming out of it. Is it a high pressure switch or something else?

Also, for my possible future reference, what is the typical or estimated cost of switching the car over to R-134 if I have to. My plan is to stick with R-12, but who knows what that may cost me to recharge.

Thanks.

The switch is a low-pressure switch in the high-pressure line. The other is the temperature sensor, which turns on the auxiliary fan.

There is no separate high-pressure switch, but the switch is incorporated into any replacement low-pressure switch. If yours is original, you have no high-pressure cutout.

If you want your a/c to work, do not convert to R-134.

Forgive my ignorance. Do you mean that it won't work literally, or that after the conversion, it won't work well?

Thanks.

It will work, but not well. R12 is more expensive than 134a, but not so much as to prohibit its use. You do have to buy it in larger cans, but there's less waste that way.