Open Road

Attention all 928 Air Conditioning Experts!
Whilst troubleshooting a non-functioning A/C system (88 S4), using all the usual guides an historical threads, here's what I found:
1. PO had a receipt showing new compressor and dryer. They appear new. No mention of any other work done.
2. I discovered one of the wires in the harness from the pressure switch cut off and "substituted" for a new wire back to and disappearing through the firewall.
3. A little DMM work and found a bad freeze switch, and the wire from it to the pressure switch being the one cut.
4. Traced the new mystery wire back to the console and found it spliced into the A/C on-off switch.
5. Checked and repaired "new" wire splices, and A/C started working.
Conclusion is a bad CCU relay and freeze switch caused someone to bypass both, undocumented of course. After further research and wire diagram study, I decided to put a 5 amp fuse in the compressor line from the pressure switch (cheap insurance), and use the A/C (being summer and I just got the car on the road and have other more urgent issues needing attention)
The CCU on this car has a date from June 1985. I'm thinking it may have already been replaced?
The relay is the Gruner 970. The RL posts on the relay fix - on or off-board, are all quite old and show old Radio Shack parts (remember them?) Does anyone have a recommendation for a new "modern" relay replacement - on or off board?
Any other newer fixes out there to this known issue?
Any concern with running this way temporarily (knowing coil-freeze protection is disabled)?

Thanks all for any advice for this 928 newbie...

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm … decided to put a 5 amp fuse in the compressor line from the pressure switch (cheap insurance), …

I always put in a weather pack style sealed fuse holder in the plenum with spade connectors in series with the freeze switch.

I use a 4A fuse since the clutch draws almost exactly 3A.

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm The CCU on this car has a date from June 1985. I'm thinking it may have already been replaced?

Why? If yours is an 88 wouldn’t you expect a relay with a date of 88 or later if it had already been replaced?

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm Any other newer fixes out there to this known issue?

People have done all kinds of (IMO dumb) shit to ‘fix’ this issue. I prefer to do something that doesn’t require a new wiring diagram. So, replaced on-board relay with a 5A and a 4A fuse on the freeze-switch is what I prefer.

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm Any concern with running this way temporarily (knowing coil-freeze protection is disabled)?

As long as the low-pressure switch is still in the circuit, perhaps no. Oh, if your A/C works like it should then it will freeze the evaporator shut, but you can simply turn off the A/C for a few minutes. This, though, with a 4A fuse. The 2A OE relay often acts as a high-dollar fuse, but it typically fails open and thus the huge current draw from a seized compressor has less chance of frying the PCB. So, I’d use the smallest available fuse in the circuit: 4A

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm The relay is the Gruner 970. The RL posts on the relay fix - on or off-board, are all quite old and show old Radio Shack parts (remember them?) Does anyone have a recommendation for a new "modern" relay replacement - on or off board?

I have a stick of these (see below) that I got on e-bay years ago, for whenever I decided to get good at soldering or rebuilders stopped rebuilding. Assuming your plate isn’t lying, you could hop down south of Nashua buy me a burger and I’ll give you one to mess with.

I made a video of the relay repair, if it can help (see below). I've done this job on a couple cars and have had no issues since.
Someone asked about not being able to find the exact relay. The relay is a basic general purpose DPDT 12VDC relay. The TE relay in video appears NLA. On the Digikey website a substitute part is listed, K10P-11D15-12. Haven’t tried it but looks similarly robust, industrial, rated for 15A. Amazon sells NTE brand 10A R14 series and Radio Shack has something similar. There are lots of relay options. The compressor may draw up to 3A, so suggest that as a min rating.

worf wrote: ↑Wed Jul 24, 2024 9:38 pm

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm … decided to put a 5 amp fuse in the compressor line from the pressure switch (cheap insurance), …

I always put in a weather pack style sealed fuse holder in the plenum with spade connectors in series with the freeze switch.

I use a 4A fuse since the clutch draws almost exactly 3A.

I used a 5A cause I had it on hand in a watertight holder. Put it near the Press switch because of the bypass. If/when I replace the freeze switch, I will definitely put one there. We're on the same page on this!

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm The CCU on this car has a date from June 1985. I'm thinking it may have already been replaced?


Why? If yours is an 88 wouldn’t you expect a relay with a date of 88 or later if it had already been replaced?

The date is labelled on the head itself. I think the whole thing was replaced, relay and all, with a used unit from a 1985-86 car. No reason known if/when/why this happened. So I need a relay for that older unit to match. I see no reason to change the head back to a correct dated one for any functional reason.

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm Any other newer fixes out there to this known issue?


People have done all kinds of (IMO dumb) shit to ‘fix’ this issue. I prefer to do something that doesn’t require a new wiring diagram. So, replaced on-board relay with a 5A and a 4A fuse on the freeze-switch is what I prefer.

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm Any concern with running this way temporarily (knowing coil-freeze protection is disabled)?


As long as the low-pressure switch is still in the circuit, perhaps no. Oh, if your A/C works like it should then it will freeze the evaporator shut, but you can simply turn off the A/C for a few minutes. This, though, with a 4A fuse. The 2A OE relay often acts as a high-dollar fuse, but it typically fails open and thus the huge current draw from a seized compressor has less chance of frying the PCB. So, I’d use the smallest available fuse in the circuit: 4A

LSHK928 wrote: ↑Wed Jul 24, 2024 7:52 pm The relay is the Gruner 970. The RL posts on the relay fix - on or off-board, are all quite old and show old Radio Shack parts (remember them?) Does anyone have a recommendation for a new "modern" relay replacement - on or off board?


I have a stick of these (see below) that I got on e-bay years ago, for whenever I decided to get good at soldering or rebuilders stopped rebuilding. Assuming your plate isn’t lying, you could hop down south of Nashua buy me a burger and I’ll give you one to mess with.

Plate is real. I am in Merrimack, near Nashua. Would love to meet-up for a 928 "play-date".
Thanks for the input!


[/quote]

Sorry, messed up the quote / reply thing. Newbie at this forum stuff too...

MFranke,
Thanks, that's great information.
I've seen your videos - and am a subscriber!
I've got a YouTube car stuff channel as well, and started putting some 928 stuff on there.
Stoneface Garage
Check it out:

https://www.youtube.com/channel/UCzfd1U ... m_vKLraP9Q