Randy V

Repaired the board trace using silver solder for higher heat loading.

Have decided to go with the triggering relay powered off the hot point with an in-line fuse as the best solution for my situation. I can't allow any high current load to go through that head unit in its current (no pun intended) condition.

The board trace repair:

MainePorsche

Nice repair.

With a robust weatherproof relay, and your quality work, you'll be fine in SD.

FredR

Dwayne,

Excellent write up and many thanks if you happen to spot this post and if do not - I hope one of your pals does and passes on the appreciation - not sure if my problem is the relay but a good chance it is given it seems to be a bit hit and miss and not gas related. I have a spare head unit from my S4 but I suspect it is different- I do not think the S4 has the recirc setting. Getting the old relay out looks a bit tricky but the concept is very good. They design these things to put the relay in but not take them out out! Very neat approach.

One tends to forget just how much current is needed to generate enough force to hold the clutch solenoid open. Maybe it is even a tad better to use the current signal wire as a trigger to open a power relay wired to the hot post- no idea if anyone has done this- apologies but I have not read all the posts. seems that our tired old wiring might benefit from such.

Yet another example of why this list and this fraternity is so precious- without it and specifically the folks on it, I doubt I would have stayed in the game so long! Worth every penny of the subscription and many times more.

Hopefully now that we have a local holiday for a few days, I will start to take a look for the culprit.

Regards

Fred

Ian S4

Many thanks to Dwayne for the write-up!

My own solution was to drill small holes through the upper casing to match the pins on the relay. No external wiring required, and it kinda looks like it could have come from the factory like this...

Alan

You need this relay to work - it is a dual pole relay - a DPST is required - though its equipped with a DPDT wth 2 ways unused.

One pole controls the clutch the other goes to the main mixing unit. Both need to work.

Assuming if either the relay still works - or that you fix it (as above - including with just a small relay of similar to original specs) then you can indeed add a power relay - though I'd add it on the CE panel - there is always at least one relay slot open. you only need easily available big connectors and only need an SPST (or SPDT) relay.

The clutch wiring already goes through the CE panel so its actually easy to change, and you can add a fuse there too (there is always at least one fuse slot open on every model also)

Alan

Ian S4

Hi Alan, I expected someone would notice that I used a single-pole relay!

In fact, I tested the original on-board relay first. One pole still operated, but the one for the clutch did not. Hardly surprising, since the latter is the one that's drawing the most current, and presumably the contacts are no longer making good contact. So my new relay's single-pole is wired across the old relay's dead pole, and the coil is wired in parallel with the original. It works, and I see no reason for it to stop working any time soon.

The Forgotten On

For those of you that have a working relay I recommend adding a secondary relay to the system.

I added mine after the 14 pin connector and ran a weather proof relay and fuse up under the panel that covers the blower motor.

It will save your original relay, mine is going on 33 years now and still works.

It is much easier than disassembling the HVAC controller and adding a bigger relay in there, and allows you to keep all of your HVAC system monitors like the High and low pressure switches intact.

Get the power for it from the jump post and run the power wire to the relay you can hide inside of the firewall (add the fuse to this circuit) then run the other end to the compressor.

These are the thicker wires that attach to the relay holder.

Have the wire that originally went to the compressor run up the main loom to the firewall and through the bung for the washer lines to the relay (this will serve as the signal wire), then ground the other side to the firewall.

These are the thinner wires that go to the relay

I know this is a bit OT but it is just an idea to save effort for other projects and make it a better circuit and make the system reliable without the risk of burning up your A/C head unit.

It's been working like a charm for me, just thought I would share another idea to improve our cars.

GrumpyFinn

Seems like mine has already been "rebuilt" by the PO, and has a slightly different relay than any I've seen so far.



I took the numbers out of it and ordered new ones from Allied, anyway.... Here's what happened and I hope my diagnosis is right and it is the relay and not something bigger.

I don't even know why I accidentally reached in something from under the center console on the passenger side and grabbed the AC connections with my finger tips, and almost burned them... these suckers were hot as hell.



You can almost see the red wire on the smaller connection melted before the the little solder bridge on the board melted and it finally gave out.



You can see the new solder bridge on the 5th tab from the top, and when I tried it, it works but still gets hot with the old relay, will probably be couple of days before the new ones arrive. I'll be adding a fuse or two on the system so next time I won't be having any melted wires

Randy V

The AC saga continues on my GTS.

Recall that earlier in this thread I elected to go with an external relay powered off the 12V hot point under the hood, triggered by the power lead off the HVAC control head.

This week I have lost power from the control head to the underhood relay.

I'm thinking that the HVAC head relay is now toast? When pushing the AC switch, the switch illuminates and the aux fan runs.

So, can I delete or bypass the relay on the HVAC head (since I already have a fused relay downstream) and have direct link to the downstream relay?

Which wire(s) would I tap from the HVAC head unit?

dr bob

Randy--

The relay in my head unit also switches the connections to the fan controller, so deleting that head-unit relay isn't an option.

Before you start tearing into the head unit though, there are a couple other things to check. First is to test for voltage at the wire connections on the freeze switch. That's under the plastic cover at the base of the windscreen. Little aluminum box maybe 2" cube, with the two wires connected. Without disconnecting the wires, and with the AC switch engaged and engine running, you would normally see battery voltage referenced to ground. If you don't see battery voltage or very close to it, you know the head unit relay is toast.

If you do see voltage there, you get to test more stuff like voltage through the low-pressure switch and the connection through the 14-pin connector by the jump post. And your new relay there, and the wiring connections to that relay.


If you do decide to put a relay at the head unit, you can delete the stuff you did before, and eliminate those pieces as possible future failure points.

Alan

Randy see post #123 - you need the head relay - it has 2 parts: one goes to the clutch but the other to the mixing unit. Unless you can verify that the other half works - I think you need to replace this with a DPDT (or even a DPST). Its odd yours should have failed now you have the loading removed from it...

Alan

Alan

I would not do it this way - in fact I already described earlier in the thread how best to do it. Locate the relay on the CE panel with all the other relays - there is always at least one unwired relay slot available. You only need commonly available 1/4" female blade connectors that just push in (ford types work). The wiring for the clutch already enters and exits the panel all you need to do is move some connectors and add some new wires.

On the later S4+ cars the suppressor relay should be on the output to the clutch, the LH connection can be either side. If you move the relay to the engine compartment you have eliminated the CE panel suppressor from performing any useful function.

Folks seem to be concerned about touching the CE panel - its just a bunch of connectors, fuses & relays - utilizing the empty relay sockets here is just logical to do... Document it though!

If my head unit relay failed - I'd replace it with a small SPST relay for the mixing unit and also run the coil connection in parallel to the new CE panel relay - this is what Porsche should have done in the first place, the head unit relay is worse than marginal for the loading it drives.

Alan

Randy V

Thanks, Alan. DPDT/DPST?

Dr. Bob - As mentioned if I apply 12v to the under hood relay the compressor engages and blows cold so it's not an issue with anything else.

dr bob

Randy,

Where, electrically, does your add-on relay sit in the clutch circuit? The power to the 14-pin connector has already passed through the Freeze switch and the low-pressure safety switch by the time it gets to that point. If either of those is 'open', your relay won't close to run the clutch. Jumpering 12V to your relay confirms you have a problem somewhere upstream, but unfortunately doesn't isolate the problem enough to say that adding the control head relay will solve your symptom.

Randy V

Thanks, Bob - one step at a time I guess.

Yes, rather disappointing.