rjtw

I accidentally ran across this fantastic thread about the HVAC relay while searching for something unrelated (how to change the HVAC lightbulb, and if you DO need that, follow the pics in this thread which are great): https://rennlist.com/forums/928/6788...re-w-pics.html

This is a thread from Dwayne involving how to replace the on-board small relay inside the HVAC head unit.

However, several contributors to the later part of the thread (Alan in particular) suggested the idea to preserve a WORKING relay by offloading the load to a new "53" style relay that could be installed in, for example, an unused section of the Central Electric board with some judicious wire juggling on the CE panel (documented, of course, as Dr. Bob suggests in a logbook).

I'm looking for opinions on how likely my still-original HVAC relay is to fail, and thus whether it's worth the time and effort to perform this modification. My car is an '83, the AC works just fine, and although I live in CA I don't really use the AC that much. I also just replaced the main engine wiring harness, and the front-of-engine harness, and the mini-harness going down to the AC compressor from the front-of-engine harness, as well as doing a general cleanup on the CE board itself, so most of the circuit path should be in good (meaning low resistance) condition.

What do you think, should I be proactive to help preserve the poor original relay? Or considering that it's been fine for 37 years and have a lot of new wiring, leave it alone? Would any measurements help inform that decision?

Cheers,
Rick

Adamant1971

I would leave it as-is. Mine just failed last year on my 87, but I replaced it with an original gruner relay out of Germany. It was fairly simple to de-solder and remove the failed relay and to install a new one.

Yours being a few years older and still going strong is a good sign.

rjtw

Thanks for that good feedback. Nice to know that the original relays are still available - I didn't even realize that was an option. And yeah, I tend to overthink these things almost all the time lol!

Adamant1971

Pretty sure I got the last few from the EBay seller in Germany. I will look to see if I can find the seller and post a link.

Edit:
I checked my purchase history and it looks like the seller has 10 on-hand, but the price is over double what I paid in 2018.
https://www.ebay.ca/itm/Gruner-202-R...72.m2749.l2649

Alan

Its quite a bit of fiddly work with special parts required. I'd recommend only if you need to take out the CE for other reasons and can source all the parts you need in time (these are parts you will need to order)... It's a real benefit but its quite a bit of work - only slightly less than the HVAC relay replacement fix. But this fix should make the HVAC relay essentially last forever. You'd need to document the changes to the CE panel (and do all the changes well).

YMMV

Personally I'm OK with changes to the car electrics to fix poor original design work by Porsche (as here) - but not so much with randomly placed & undocumented fuses & relays. An inline fuse at the freeze switch is easy to reach/check and rather obvious to find during evaluation so its low on my dislike scale for such things.

Alan

The Forgotten On

I installed an inline one that is tripped by the clutch power wire in the engine bay. It worked for years before the headgaskets in my 81 went.

It is powered by the hot post in the engine bay and is easily reversible with no cut wires, just spade terminals.

I would do what is recommended above and preserve your original one, but add an inline one like I did to take the load off of it.

It's less invasive than adding one to the relay board. The reason why Porsche removed the relay from the CE panel beginning in 1980 we can only speculate at this point.

j.kenzie@sbcglobal.net

This is exactly what I did maybe 6-8 years ago. It causes very little current to go through the AC on board relay. If the engine bay relay fails, it's easy to replace. It's worthwhile to learn how to use relays for all kinds of wiring needs.
Good luck,
Dave

siscogts

Late hvac control relays are quite difficult to find, here in Italy. I bought 3, in 2017. 2 relays failed at first attempt as they turned on a/c, but then a/c was stuck on. Third apparently did the job..... ,

Schocki

Why would you replace an underrated relay with another underrated? Many things are "fine" after 3 decades plus, or not? The AC clutch relay is a known weak link in the system, why not follow Dwayne's recommendation and it's good for good. No need to wait, it'll fail eventually on the hottest day of the year while you are on your way to a weekend 928 gathering, just Murphy's law. Replace, done. External? Why not? Easy to get and invisible when installed.

FredR

The stock relay is generally considered to be somewhat stressed size wise and thus one of those items that are going to fail sooner or later. The first thing one notices when problems start is that operation becomes somewhat erratic once the unit warms up. Turn it off and the thing will often start to work again until the next "episode" which itself is a crap shoot. Reduce the load on the relay and the lifetime expectancy will increase. Take a fused power supply from the hot post and fit a relay rated for 5 amps or greater that is suitable for installation in the engine bay, preferably somewhere in front of or to the side of the radiator [i.e. where it is a good bit cooler] and you may be good to go for ever and a day.

When my control head started doing this I simply swapped in a spare one I had but now I have no more spares! Personally I liked Dwayne's approach- if the stock item is marginally sized then why replace with something destined to go the same way albeit in however many years time? If you are good at soldering then like for like may well suit.but $100 for a relay that is seemingly underrated at 2.5 amps does not make any sense to me.

jpitman2

I also did an inline relay out in the engine bay. Also powered from the hot post. A little cramped, but much easier than getting into the back of the centre console.
jp 83 Euro S AT 57k

dr bob

My decision to place a relay external to the controller was driven by a desire to have a completely reversible process with no mods to the car's wiring. It would have made great engineering sense to maintain the early car relay in the CE panel, and on my S4 it would be reasonable to wire a relay in the CE panel where the suppressor lives. But again, that messes with the originality of the car. Meanwhile, I added a good used and unmolested control head to the spares bin, so I can go back to complete originality if/when the need demands.

I've looked down on the idea of adding a relay near the jump post. Having extra electrical junk in that area just bothers me, plus it means you get to cut and splice wires in the body harness there to make it all work. Fuses, splices, jumpers, all to band-aid a controller relay problem. It just bothers me, especially with a priority to keeping the car itself as original and unmolested as possible. Any changes made need to be easily and completely reversible, IMO. I've been the victim of too many hacks and shortcuts on otherwise interesting cars in the past. Once you find a few of those in an otherwise original collectible car, you are doomed to chasing down a slew of other possible "improvements" that someone has kluged in the past. I promise myself that I will never be that a$$hole PO, especially when it's so easy to do things right. After 20+ years of fun with this particular car, I'm the most likely victim of shortcuts made in the car's past. Why would I do that to myself?

Christopher Zach

The original relay is just too... small.... Porsche would undersize electrical connections and it's annoying as heck. Put a new OEM relay in and you will have the same problem in another 10-20 years.

In this case that relay is switching a pretty large inductive load (the magnet) for long periods of time. Small wonder the relay fails.

I put a second relay right next to the freeze switch and wired it in to the +12 line from the auxiliary fan with a 5a fuse. That way a nice 30a auto relay is triggered by the little relay and the 30a relay is what fires the AC coil.

Will work as long as what's left of your HVAC relay can switch 100ma. That is usually not a problem and will not wear the HVAC relay any further.

Bonus: This solution requires *no* cutting of existing wiring for the true purist.

Christopher Zach

Fused with a 5a or so fuse, correct?

Otherwise a short in the AC compressor coil or a nick in the wire could cause problems.

Always fuse lines based on the capacity of wires and components to handle load downline. Porsche screwed this up a lot as well, using large value fuses to power too-small wires that melt when shorted (think window mirror heaters, which pull less current than their wire loom but are fused to a big fuse so those wires will melt if the mirror heater ever dead shorts. Sigh)

worf928

1) Put a 4A fuse in a WeatherPak inline holder in the clutch circuit at the anti-freeze switch. This ensures that the HVAC head unit’s board won’t release its smoke if the compressor seizes.

2) Wait for 2A relay in head unit to die. Compressor clutch is a 3A load. (Why hasn’t the 2A relay failed already? Manufacturing tolerances: the bell curve. Porsche replaced a lot of HVAC heads under warranty. 50% chance of getting a 2A relay that fails instantly because it’s actually a 1.5A relay. 50% chance of getting a 2A relay that can function for a while as a 3A relay.)

3) Replace original failed relay with 5A relay of almost-same size. Fits inside the unit.

Done. No wiring hackery. No add-on relays in random locations.