I'm recharging the ac with R134a, after drawing a vaccum. I need to bypass the low pressure switch. I can see 2 wires going to the ac clutch and one or two snaking up behind the back of the compressor under the cruise control bellow. I figure if I provide +12v to one terminal at the ac clutch and ground to the other, then it should force the compressor on.

However with the ac/fan controls set to off in the dashboard, both are at ground and with the fan on, both terminals have +12v reading. And the clutch is not engaged.

Wasn't expecting that, figured I'd ask before I shorted something out

Also, how do I know if I've got a HSLP or superheat? It's a 88 XJ12 S3 and its been converted to R134a by po. AC has been inop ever since i got it

Thanks

 

Right.




That's probably OK



Hmmmm.


I'd have to do some mulling and diagram checking on that.


In the meantime simply remove the connector and use jumper wires on the terminals to engage the compressor.





My (recently departed) 88 V12 S3 was HSLP. Jaguar made the change at VIN 471xxx or thereabouts.

If you see one of the old 3-sprong thermal limiter fuses in the compressor wiring, you have the superheat type

Cheers
DD

 

This is interesting to me, because I am about to recharge my own system.

In one of the YouTube videos I've watched recently on how to charge into a vacuumed system, the person started by discharging part of the first can of R-134a into the system without starting the car. My understanding was, in this way he put enough gas into the system to avoid a low pressure situation, so the compressor would engage and he could start the car and fill the system the rest of the way.

Wouldn't that work? I hadn't thought of needing to bypass anything. Or am I missing something crucial in my planning?

Andrew.

 

Andrew, that was my plan as well, but on other cars my experience has been that "jumping the ac" is required. Reading the factory service manual, it goes along the process you outlined except that it calls for using a refrigerant source of at least 7.2 lbs. I'm using the smaller 12oz cans (with uv dye), so that may be the reason. Without the engine running, some refegerant has gone in, but more than half the can remains.


Doug, appreciate the pointer, my vin is 478xxxx and i dont see anything likeyour pic (hard to see whats behind the compressor though). My Series3 service manual only talks about superheat systems although other sections have references to late 80s cars, go figure.

If i pull the lead out from the ac and supply 12v and ground, the compressor clutch engages. With the leads plugged in and supplying 12v, the fan comes on but clutch does not engage. I assume this is due to the low pressure switch, so I'm just going to recharge without the leads plugged in and see what happens unless someone tells me otherwise.

 

So if I understand you, even with some fraction of a can in the system, the in-system pressure supplied via small can may not be enough to beat the low-pressure switch? And providing 12v and a ground to the terminals at the clutch (having removed the "usual" plug) forces it to run, and suck in more refrigerant? At what point will there be sufficient gas to plug the usual plug back in?

What about if I have a superheat switch? (Actually not sure how my wiring is configured – a shop recently "upgraded" my A-6 to low-p switch to fix a leak, but I still have a thermal fuse in the wiring.) does that make a difference? Or am I pulling that out of the loop in any case when I disconnect the clutch wires?

Thanks for any help. Please come back and let us know if and how your recharge works out.

Andrew.

 

How did it go? Was jumpering the a/c clutch plug successful? Do I have the procedure straight (see above)?

Thanks,
Andrew.

 

Well, pulling the leads to the ac clutch out and applying 12v/ground does engage the compressor, so you can always do that.

In my case I got an electronic sniffer and used 1/2 can of R134a mixed with UV dye after pulling a vacuum again. It does accept some referegerant, just not the whole can. I found out that I have two leaks, one behind the compressor (where the cruise control bellows are, at #1 and #4 in the pic) and the other is at the exp valve or where it meets the evaperator. (At #4 in the second pic of the expansion valve)

So whar are my options for the compressor? Rebuild or buy the newer GM A6 units for $300?
GM/Harrison A6 Replacement Compressor | RetroAir, Inc.

The exp valve has me worried, I'm unsure if it is the exp valve or the evaporator.

The sniffer goes haywire when it is actually against the square rubber (or whatever) gasket that is sandwitched between the firewall and evaporator and does not go off when I'm sniffing the valve body itself. It's a big *** 1 and 1/8" nut that does not want to budge. The service manual says to brace the valve while the union is unscrewed, but I'm not seeing any possibility of getting another hand or tool there while I'm unscrewing this. Any writeups or tips from those who have done this before?