chip-powell

I wound up having some other household projects take precedence, but I'm finally able to look into picking up a new a/c compressor for my '71, but I have a quick question. The parts site that I am looking at lists 2 types. One that has a switch, and one that is not. What are they talking about?


http://www.oreillyauto.com/site/c/se...eme&vi=5153912

Fun71

When I clicked on your link it brought me to a page asking for "with clutch" or "without clutch". Is that what you meant instead of "switch"?

If so, you want one with the clutch unless you are sure yours is good and you have the tools to pull the old one off for installation on the new compressor.

If not, I have no idea what switch you are referring to.

truckerlary

I think what they are talking about is a safety switch that is built into the compressor that protects it when you are using 134 freon as it packs a lot more pressure in the system than when using r-12. Someone correct me if I'm wrong but I think that is the way it was explained to me

Ozzie

Some A6 compressors used a device mounted in the rear head of the compressor called a "superheat shutoff switch". It was used in conjunction with a special fuse (thermal limiter fuse). The purpose was to disengage the compressor's clutch to protect the compressor from damage if the system's charge would drop to about 2 pounds of refrigerant. Normally the "switch" is "open". When the charge loss occurs, the switch closes, blows the fuse, and disconnects the clutch coil. It started being used in the early 1970s when R-12 was in use and was intended to protect the compressor from a low R-12 charge.

Rocketbrian

Good explanation, if I remember correctly, it began on the A bodies in 72 and the large cars in 71? I always wondered if it could be retrofitted to earlier years, as it seems like a worthwhile feature.

MDchanic

As far as I know, the superheat switch began in 1972, so the answer to your question is, No, you do not need the one with the switch.

That being said, it would be reasonable to look at you old one and see if it has a switch terminal on the back.

The superheat switch was a lousy solution to a real problem.
The earlier systems had no low-pressure protection, and also ran when the system was in Defrost (and some ran in all settings if you started on Defrost), so it was not uncommon for people to burn out their compressors without getting any warning at all (not something that GM really cared about, unless it was under warranty, at which point they began to care a lot. Since "normal" leakage was considered to be a pound a year, most cars could get just past their warranties before self-destructing).

The superheat switch detects low suction pressures, and the bass-ackward fuse arrangement allows operation for a few seconds before actually stopping the compressor. It does this because brief periods of very-low suction pressures are a normal part of operation, especially in cool rainy weather. The problem is that the fuse does not last quite long enough, and often blows under these conditions when nothing is wrong, which can be annoying to say the least.
A side effect of the fuse system is that the fuse was hidden in the engine wiring, so the average person who blew it would have to bring the car in for service, at which point the freon level could be checked and refilled, thus saving GM from an expensive warranty repair.

If you maintain your system, you have no need for the superheat switch.

- Eric

joe_padavano

Actually, all GM systems run the compressor in Defrost. That's intentional, for two reasons. First, it keeps the oil circulating in the system during the winter, when you don't normally use the A/C. Second, by cooling the air first, then heating it, you condense out the moisture so it's a much more effective defroster. My 2002 Volvo works the same way, by the way.

MDchanic

I didn't mean to imply that other A/C systems don't run in defrost, only to make it clear that these older systems do.

I personally prefer systems with a separate switch for the compressor, like my BMW and many Chryslers, but having the compressor activated on Defrost has been the common pattern for decades (this is why most have a setting labelled something like "Vent," which keeps the air doors open to the A/C settings, but the compressor off).

The thing that many do not understand (and did not understand "back in the day") is that most of the sixties Olds cars do not switch off the compressor once it has been switched on, so if you have run the defrosters for 5 minutes to clear some mist from the windows, and then drive for three hours with the heat on, the compressor will be on the whole time, unless you turn the system off and then back on again.

- Eric

chip-powell

Thanks again for the info guys. It has been a great help. Once again, you learn something new every day.

Fun71

That would be accomplished by turning the ignition off and then back on again. That was a real PITA on my '70 Supreme, so I wired in a toggle switch for the compressor.

BlackGold

I just internally modified the hold-in relay so that it wouldn't hold in. Compressor shuts off as soon as I deselect A/C on the dash.