Randy V

It's looking like the relay that controls the AC function within the climate control unit has failed on my car ('94 GTS).

Is this a common failure?

I don't recall ever seeing it discussed here over the years.

MainePorsche

Yes, quite common so that Wally wrote a plan for a secondary relay, and Dwayne wrote up one of his pictorials for an on the board relay replacement.

SeanR

Very common, I end up replacing many a year.

Randy V

Huh - goes to show if it's something that doesn't directly impact my situation I seem to not notice.

Nice. Any links handy?

Mrmerlin

FWIW Greg Brown also repairs these with plug and play units ,
otherwise some searching around should have you with more info ,

Randy your car needs a newer style relay IIRC either Bill B or Dr Bob has a writeup for the newer style relay and connections to use,
Maybe Alan will also chime in, since he also has a GTS

dr bob

There's a Dwayne write-up that's pretty good. Uses a readily-available Radio Shack relay. My now-ancient original instruction used a more industrial relay, so you'd have to adapt for the different relay. Dwayne's writeup has more detail on disassembly of the controller housing.

WYAIT list includes bulb and fiber-optyic updates if you are so inclined.

dprantl

The relay used by Porsche to power the A/C clutch is very underpowered for the job and will eventually fail all on its own. However, for cars that have a refrigerant leak that is continually topped up, what can happen is too much refrigerant oil leaks out and is never put back in during recharges, eventually causing the compressor to begin grinding its pistons into its case. This causes the clutch to drawn more current and the first thing to blow is the head unit relay. If you upgrade to a bigger relay, the next thing to go is the rubber portion of the A/C clutch. This recharging scenario is becoming more common as more cars begin to get the dreaded front evaporator leak - another bad design where the evaporator pipes are sealed using an epoxy that eventually degrades and leaks.

Dan
'91 928GT S/C 475hp/460lb.ft

Tony

Isnt there also a fuse inline to the freeze switch that is added?

Mrmerlin

^^^^ Tony yes there is so it wont burn the relay if the compressor fails

jeff spahn

At first I thought you were joking.

MainePorsche

Search is your friend.
Now I know I've heard that somewhere before...

GregBBRD

The internal relay in the HVAC unit is rated at 3 amps, which is apparently a bit small for the task, as they fail very often. I "upgrade" the relay to one with a higher rating......however I believe you need to be really careful about how big a relay you use. It's way better for the relay to fail, internally, than for the circuit board to burn....something that is very common with "home repaired" units.

I believe that the common relay recommended for use in "home rebuilding" is way too big.

Any short circuit in the wiring or the magnetic coil will "kill" the relay.....or as mentioned burn up the circuit board. I've provided an additional fuse to protect the HVAC unit, virtually since the very first unit I rebuilt (for use in my own shop).....which had a bad magnetic coil and fried the brand new relay.....instantly. Being pretty quick to learn, I added a fuse to this one (after a replacement magnetic coil and a second relay) and all others after this one.

I do see units with failed relays without short circuits. I believe that as the magnetic coil ages, the resistance goes up, which "draws" more current through the relay.

I also believe that there can be absolutely nothing wrong with the system....and the relays just finally wear out.

GregBBRD

Hmmm....

I don't see how the magnetic coil can be an "interactive" unit that draws more current as the compressor wears....

James Bailey

Perhaps more likely is the copper coil winding of the electro magnet of the "clutch" starts to short out over time and becomes to some degree a lump of copper which flows way more current than the long thin wire winding...... and that blows the relay.../circuit board etc...

MainePorsche

I like Wally's idea of having this small 'internal' relay simply switch a more robust external relay to run current to the compressor clutch rather than have this small existing relay continuously running current as part of the compressor power path.