Randy V

I got a replacement relay from 928 International a few years ago and installed it in the HVAC head. A better solution than the Radio Shack relay hack.

It turned out that a failed relay is often caused by a bad AC compressor.

Alan

Yes but a failed compressor should blow a fuse - not blow the relay. 2 problems:
1) The relay specs are too low - hardly above the clutch operating range, and this is a highly inductive load - without the suppressor extremely problematic.
2) The only fuse in this circuit is the HVAC Blower fuse and at 30A this can't protect the relay with a much lower rating. The AC clutch deserves its own fuse

Agreed entirely.
I'd not suggest this to folks without any idea.

But IMO adding a relay (and fuse) to the CE panel in a spare slot (and documenting it) makes more sense than adding an unseen relay (/fuse?) behind the HVAC.

You largely need the same skills to implement either one. Implementing this on the CE panel is more obvious to any future owner and provides for easier access and replacement & somewhat easier documentation. Using a standard relay & fuse makes it inter-changeable with all the others and keeps the fuses & relays consolidated. It also allows a widely available cheap & capable relay to be used.

I'll admit changing only the HVAC head means a new module is always a possible replacement AND the wiring stays completely stock (important to some/many) - but Porsche did not initially design this well - I think the relay/fuse for this should always have been on the CE panel. Interestingly it was on the earliest cars!

Alan

dr bob

FWIW, I was faced with a non-functioning compressor clutch this spring. I ended up going through the troubleshooting process to find that the freeze switch has actually failed in open position. In that process, I pulled the control head to make sure everything in the original modification is intact. It is. The replacement relay stuck to the outside of the controller is still stuck to the outside of the controller, and it maintains full functionality. Advantages: Very durable, out-of-site and out-of-mind, works, reversible modification, "modular" in that a future un-modification is as simple as plugging a replacement controller into the console.

For the freeze switch patch until I get a replacement, a 5A fuse is doing the jumper duty. When I get a new switch, the fuse will stay is series there for circuit protection. More notes in the logbook then with a picture showing where it's located. Again, an easily reversible modification with changes to any factory-supplied parts.

There are certainly a number of options for how to solve a failed control-head relay. Any of the mentioned 'fixes' will get your compressor running. I happen to like the mod I did because there are no changes to factory wiring or connectors, and the car can be very easily returned to original condition in a matter of minutes. I've learned after being the victim of too many aftermarket wiring hacks that having an original or easily restored to original car pays big dividends as the cars move from "used car" to "collectible car" status in the market. The "do no harm!" mantra is rapidly gaining importance with the cars these days. Think about it as you plan your work.

Speedtoys

Common 12v relay under the hood fed with 12v from the head unit, and you wont need to do any of this.

I did this, and I use that relay to feed the AC clutch & dual 12" SPAL fans on the Hans fan mount.

Speedtoys

Interrupt the 12v clutch feed at the 14-pin engine harness block (plenty of slack there to cut it, strip it, and solder with it).

The HVAC feed from the cabin with a 1a fuse to pin 86 on the relay.
Pin 85 on the relay to ground.
Pin 30 receives a 5a fused (higher if feeding fans too) feed from the jump post.
Pin 87 feeds a 5a feed to the AC clutch wire back up where you cut that ac clutch black wire at the 14 pin connector.
If also feeding fans from pin 87, fuse those appropriately as well.