Jadz928

Alan,
Let me make sure I understand this correctly...
You suggest adding a relay/fuse to 78-79, where there is not...
And for later 928s, separating the clutch circuit from the existing relay/fuse (which feeds the HVAC head) by adding a completely new relay/fused circuit?

Thanks!

Alan

Yes - I'd do that (and have done that).

'78-'79 Have no relay - switching is the circuit track wiper switch (creates issues over time)
'80-'83 Have a single pole relay in the HVAC head unit (this is the least overloaded stock config)
'84-'95 Have a dual pole relay in the HVAC head unit

For '78-'79 - Add a Bosch style SPST/SPDT relay & fuse on the CE panel using the existing clutch output wire to drive the bosch coil

For '80-'83 - Add a Bosch style SPST/SPDT relay & fuse on CE panel in place of the integrated relay use the existing coil wire to drive the bosch coil (via the exiting clutch wiring).

For '84-'95 - You still need a working existing relay (or a replacement) because it also drives a separate circuit. Add a Bosch style SPST/SPDT relay & fuse on the CE panel using the existing clutch output to drive the bosch coil*

* In the case where the integrated relay has failed on an '84-'95 car:
I'd replace the existing relay with a mini SPST relay to drive the second pole and add a Bosch style SPST/SPDT relay & fuse on the CE panel using the existing coil wire to also drive the bosch coil (via the existing clutch wiring).

I think these are the best fixes for this issue - a bit more work, but neat, tidy, well protected electrically, environmentally & mechanically and easy to document. The fuses & relays are then easily tested, swapped and are interchangeable with others already in use.

Swapping a failed later head unit relay with just a mini-SPST makes the rewiring easier and relay selection easier too (ratings are less important and there fewer & lower power connections to make).

Alan

Jadz928

Wow, Alan... I think I actually get what you're saying! Thank you for the concise response.

Boiled-down a little...
So in all cases, wire the clutch exciter circuit independently to an open relay/fuse on CE panel.
(I may need a little help on the backside... but I kinda get that too, just have never done it)

For 84-95, swap out SPDT for SPST relay on HVAC head.
(I may need help understanding what pcb terminals to use for the low pwr stuff).

Alan

BTW its important to have protection diodes on the coil circuits - this means the relay coils and the clutch coil.

The stock head unit coil driver circuit already has a protection diode (reverse biassed diode to ground with reverse breakdown voltage of ~25v) so mostly that is covered. Later cars also have a supressor on the CE panel on the clutch driver line, the part that is important to the clutch coil driver is also just a reverse biassed diode.

These help protect the relay & switch contacts but also are needed on the "AC on" indication line to the ECU for idle load compensation on cars so equipped... This line MUST be protected in this way, ensure these don't get dropped or moved relative to the coil load they are protecting.

Alan

Pierpressure

Wanted to close out this thread and thank everyone for the input. After evaluating the condition of the entire AC system Jim recommended against just swapping out the clutch coil and we decided to go for a complete overhaul. The following parts were sourced and installed by 928 Classics

Remanufactured Denso Compressor 80 to 89
A/C Receiver Dryer to 87 to 92
Front & Rear Expansion Valve 86.5-95
A/C Pressure Hose 87 to 89 (aftermarket kevlar)
A/C Suction Hose 87 to 89 (aftermarket kevlar)
A/C O Ring Set 87 to 88 Front

Thanks to Jim for showing some love to my non OB shark completing the repair on time and budget. I am enjoying my Black on Black S4 without sweating for the first time in months and am looking forward to my trip to Frenzy latter this month.

GregBBRD

I supplied/continue to supply a fuse holder and instructions for installation with my rebuilt HVAC heads. My prices for rebuilding were too high and 928 International returned to their original rebuilder for their units last year, so this fuse is no longer included.

SeanR

Ahh, that would explain a couple of things. Is Roger still selling your rebuilds?

You should be getting one in the post that was a rebuild in 2009 which had failed, not one of yours, but I'm curious as to what the problem is.

Jadz928

Adam,
It was a pleasure working on your 928... gorgeous S4!