Bill Ball

I've done a lot of AC, but I ran into a weird one this time. I thought I understood the system well enough for most issues.

What could cause this? In a 90 S4 with an R-134a conversion there is power at the low pressure switch and the clutch engages just fine with the car not running. As soon as we start the car, there is no power at the low pressure switch and the clutch does not engage. I have jumped the freeze switch leads.


This happened as we were charging the system. The compressor was running and pressure was building. In fact, after getting a good 2/3rds charge of R-134a system but seeing no cool air at all, we stopped charging. The high end pressure got very high and low was not up that far yet. So, I suspected a plugged expansion valve. Not sure why, as the system worked fine before I replaced the clutch after the clutch broke. So, this is a used clutch we installed and it and seemed fine for 20 minutes or so. It will engage if I give it 12V, so there is something once the car starts that is blocking the power to the clutch even with the freeze switch and the low pressure switch bypassed. Probably something simple, but I'm not getting it. I don't think it is the head-unit relay due to the response when the car is not running , and I replaced the stock unit with a heavy duty relay a while back.

dprantl

Ok, so you're saying all was good until you charged to 2/3 weight of R134A, then the clutch would not engage? And the high pressure was very high? Sounds like the high-pressure switch is cutting the compressor off. I think the later 928's have both a low-pressure cut off switch and a high-pressure cut off switch.

Dan
'91 928GT S/C

Bill Ball

OK, I will look at that. The pressure went down to 80 after we shut it off. From then on, the clutch would not engage at all with the car running. The pressure switch did not shut off the compressor when we were charging and pressure exceeded 400 briefly! It never produced any cold as we charged even with the compressor running and compressing, so we have a bigger problem.

Bill Ball

The wiring diagram shows the low and high pressure as one switch with a single wire being fed power from LH pin 14. OK, I'm lost!

Ketchmi

I would suspect that your relay in the control unit gave up the ghost with the new clutch, they will show 12v when disconnected but apply a load and it goes away. Not just a WAG...have encountered it a few times.

funkyboogalooo

Bit of a long shot but it wasnt something failing inside the compressor that caused the clutch to go and then went on to block somewhere in the high pressure side?
M

Bill Ball

I kind of doubt it. This is a new relay, high rating, but I can replace it easily enough. I can take the belt off, so there is no load, if that is what you mean, and see if the same thing will happen.

Bill Ball

I agree there is a blockage, most likely at the expansion valve. I guess I need to address that first.

AO

Does it have rear air? If so, it would be pretty rare for both expansion vales to be dead. But not impossible.

Bill Ball

That occurred to me last night - rear air should work. Thanks for reminding me.

But I still need to figure out why 12V to the low pressure switch is not there anymore when the car is running. All the AC guides are pretty sketchy on the way power is provided, just mentioning the freeze switch as being in the path. But the wiring diagram shows pin 14 on the LH feeding the switch, labeled high and low pressure for MY90. Once you put the LH in there, I can't trace the flow. I don't know how to determine where LH pin 14 gets its power. I MUST BE READING THE FINE PRINT ON THE WIRING DIAGRAM INCORRECTLY. Let me chack that again...

AO

Bill-
Have you look at Wally's write up. It's on Specialist's website. 928GT.com

I just went through this recently: No cold air - fron or rear. Found the clutch would not engage even when I jumpered the pressure switch. Read Wally's writeup which told me to start by looking for power at the freeze switch. Low-and-behold, the connector was off the freeze switch. Plugged it in and BAM! cold air.

Good luck.

Bill Ball

Here is the pressure switch feed. The jump to coordinate H-89 (the LH) is from M-38. Very hard to read, but the wire color matches up. So, the LH is involved. Where is Alan!!

AO

Bill-
I just looked at the wiring diagrams, and it appears to me that the freeze switch passes the power directly (Not through the LH) to the pressure switch which in turn passes it to the AC clutch by way of the 14-pin connector. It does appear that the freeze switch passes through G21 on the CE panel, however.

First check for power at the freeze switch - Let's assume you have it, Then power at the pressure switch (make sure you check both terminals). If no power, then check continuity between it and the freeze switch. If nothing , then look to the CE panel at G21.

Bill Ball

Wally's comprehensive write up is sketchy on this. I jumped the freeze switch plug.

Bill Ball

OK, I MUST be reading the wiring diagrams incorrectly. I must be reading the jump box coordinates incorrectly. But it sure is a coincidence that the diagrams show jumps and the wire colors match, as I show above.