merchauser

agreed! will install new external relay

not familiar with how to install 3 amp fuse at freeze switch. is there a thread about this? or can you explain?

Mrmerlin

unplug one of the wires at the freeze switch,
and plug the wire onto a 3 amp fuse ,
make a short wire with 2 female spade connectors,
connect one side to the other leg of the fuse and the other to the freeze switch

Christopher Zach

You can add a booster relay over at the freeze switch. A nice little square 5-10 amp relay, wired so it will fire when the head unit relay closes, and let THAT gate the current for the AC compressor clutch. Then the HVAC unit stops going bad because it only has to switch a couple of dozen milliamps to close the bigger relay.

Simple fix, I documented it in some other thread, works well, worth a shot.

worf928

Mrmerlin

hopefully the owner will add a supplement page to the owners manual describing electrical modifications
i know thats a stretch, but they do this when mods are done to aircraft

worf928

Christopher Zach

Oh foo. The original relay was far too small and Porsche should have known it. Hacking a large relay onto the side of the HVAC unit is complicated at best, and would damage the vias and pads at worst. Maybe there is an infinite supply of HVAC heads, not sure.

Putting a booster relay at the freeze switch requires NO wire cutting, no taking apart of the HVAC box, one relay, and a total of three net wires with an inline fuse: One going to a +12 source in the engine compartment, one going to the nearest bolt for a ground and one to connect the freeze switch male to the relay's NO male.

Your mileage, as they say, may vary. But it's a pretty simple and effective fix to a very annoying design flaw.

SeanR

Doing it the proper way isn't hard or complicated to do either. Replace the relay in the head unit, put a 3 amp fuse at the freeze switch and forget about it for 30 years.

merchauser

any tricks I need to know about replacing the freeze switch? how much of a PITA is it to thread the capillary tube?

FredR

If the relay was far too small it would not have worked for 25 years or whatever.

Seems to me it may arguably have been a bit on the marginal side and as the relay performance degrades with age it reaches a point where it can no longer function reliably. No great rocket science here- just common sense.

StratfordShark

worf928

Yup. This is one of the WTF were they thinking design issues.

The on-HVAC-board relay is a 2A relay. A brand new A/C clutch requires ~2.9 A to close. I know you can do math Fred. As to why the original relays don't fail instantly, I can only guess and that guess would be "manufacturing bell curve price-binning." Or, in other words, for the same reason that you can usually 'over-clock' computer processors by 10, 20 or more percent.

What I can tell you is that for 928s that I get that have a full from-new service history I see a lot of in-warranty HVAC head replacements. Of course I don't get many of those kinds of 928s. Maybe 6 to 8 total over 20 years. But, I've seen three documented in-warranty HVAC head replacements. One of those was my '91 GT.

There exists a 5A relay of almost the same size as the OE 2A relay that can be installed inside the HVAC head case.

Theoretically, I agree. It is, however, the typical execution of non-factory wiring that makes me hate it. It's usually thoughtless, or causes problems, or so 'covered-up' (as if the practitioner was so embarrassed that they hoped no one would ever see it) that it has to be reworked.

Unfused random 30 Bus wires in non-factory locations are always fun. The nearest bolt is usually a very poor ground in the long term because there's usually paint and galvanized steel in the way. There are connectors with fused 30-bus in the firewall plenum and actual bolts that are grounds. It *can* be done thoughtfully. It just never see it.

Yup. You just have to have some skill with the soldering iron. While I have a stick of the small 5A relays, I do not have the skill with the soldering iron. One of these days, when I'm bored I'll practice enough to do the relay R&R myself.

Again: 4A fuse. They exist. There's no good reason to put a 3A fuse on a 3A circuit. I expect to see Greg Brown shipping 4A fuses with his rebuilt HVAC heads soon.

Let's also cover why the in-line fuse is such a good idea: The on-HVAC-board relay is not a fuse. If the A/C mechanically seizes and current goes infinite, the relay (any size) might act as a fuse. Or the PCB in the head might be the 'fuse.' The former is repairable. The latter not so much.

Two. One here:

and don't just shove it in like you would a long sword into a scabbard. Some folks have managed to put a hole in the evaporators when doing this. Do it with some delicacy.

merchauser

going slow, one question at a time: in installing the 4 amp fuse for the freeze switch, do I splice this into the wire that goes forward to
the pressure switch, or the wire that goes rearward to the control unit?

worf928

merchauser

^^^^got it. did not mean to use the word "splice"

dave: you are saying you have a 5 amp retrofit relay that fits the footprint of the original? is this something I can find at mouser or other supply house?
also, the dr bob write up suggests a 10 amp external: is it "better" to find a 5 amp external? or better if I could find a 10 amp internal? not comprehending
why its "best" to use a 5 or 10 amp?