Porsche Enthusiasts Club Forum

Hi Guys,

The above is not working, I have done the following:
Checked the fuses
Changed the fuse tester relay
Renewed wiring/ connectors into the rear hatch glass

What else to do- help!!!!!

Is none of it working or are only a couple of bars working? I had a partially working one in my old car and noticed a break in one of the bars. Fixed with some connection fluid from halfords and hey presto a fully working demister. The previous owner must have put something in the boot which rubbed through the bar.

Hi, if you have checked and swapped components then it is time to break out the multimeter and take a few measurements... .

Check for +12V on the supply side when the switch is operated. Firstly, where the supply is connected to the strut, then where the strut connects to the heater element. If nothing at either you need to get back to the relay and check that it is actually clicking when the switch is operated.

If you have +12V at the supply to the strut, but none at the heater, then investigate the strut for bad/loose connections and perhaps continuity.

If you have +12V, then switch it off and check the continuity across the heater element.

If the heater is OK, check the ground continuity from the heater, through the other strut and then to the chassis. Clean all these connections and ensure they are all tight.

Hope this helps a bit...

Just a quick update, I noted that the wing mirrors are not de-misting, changed the relay which I believe is also the fuse tester, no change, checked the fuses, all fine.

Where to start..

As Rhett says, you must start with checking voltages.
Fuses can often look fine but not making contact at one end.

Ah, that piece of info changes things a bit. Don't bother checking voltages at the rear hatch element, as the two mirror elements are on the same circuit, so if they are not working the relay is not providing the +12V required.

The Fuse (13) is common for all three elements and if OK provides the +12V supply to the relay contact. When the relay operates it closes the contact and +12V should be available to the heaters.

So, is the little tally light coming on when you use the dashboard switch? This will confirm if the switch drive to the relay is present. If not then you might have some faulty wiring to the switch, poor contacts in the switch, dirty switch connections, and so on..

Does the relay click when you operate the switch?

It will be something simple...that's for sure!

Hi,

Thanks for your ongoing help.
If I Understand correctly, yes the small light on the switch appears upon being switched on, I don't hear a click from the relay.
I presume the correct relay, is the one with the fuse tester built into it??
I have switched this relay over with a second hand one, with no success.

Regards
Adrian

Yes, relay G3 is the one with the fuse tester built in. If the relay isn't clicking when the switch is operated but the light is coming on...then I would suspect the wiring from the switch to the relay itself. Could be that the pin (pin 1) contacts are simply oxidised and need a clean. Might be an idea to pop the switch out and check. Clean with electrical contact cleaner or gentle use of fine emery cloth or wet n dry paper. The wire is red with a black tracer, follow this to the fuse/relay box and check for +12V on this when the switch is operated - it should be on pin 86 of the relay.

Hope this helps...

Update - just had a deeper look at the circuit, there appears to be another fuse involved (I think..these wiring diagrams are pretty confusing sometimes! ). Are the electric mirrors and reversing light working and what year is your car? Check fuse 28....

Hi,

1989 944 Lux.
Wing Mirrors / Reversing Light work.

Right, that narrows it down a bit more...so it is not fuse 28. I think it must be around the switch to relay wiring. Pop the relay out and operate the switch, check the voltage on the contact I mentioned. If not +12V then you will need to investigate the switch and wiring..

Hi,

Attempting to remove the trim that holds the rear demist /headlight switch, can anyone confirm, is there a screw holding in place behind the dashboard, before I remove the instrument panel etc.
I have removed all visible screws!!!

Thanks.

Hi, I guess you are not getting +12V at the relay contact then?

It has been a while since I removed the trim on the dash, but I recall that removing the steering wheel always helped access, etc. Then unscrew and remove the column switch shroud (two pieces). You should then be able to see the trim securing screws that are a bit deeper into the dash. You shouldn't have to remove the instruments. You will have to pop off the heater fascia to get to the screws that hold the heater control unit in place, as they go through this trim panel too.

** Always disconnect the battery before doing any of this work as the dash has a lot of metal parts/brackets etc. and the switch contacts can easily short-out when you start moving stuff! **

Don't force anything as old plastic breaks real easy..I know all too well

BINGO.

But what a job, in getting to the switch.
I undertook open heart surgery on the switch, cleaning a dirty inner contact, popped it back together, not hoping for the best & what a surprise, both wing mirrors heated up, I await the results on the rear window, when I have a it more time.

Thanks

Adrian

Brilliant - hope the rear screen plays ball now too...

I did say it would be something simple....however, I didn't say it would be easy! The dash trim pieces and switches are quite a nightmare at times. There never seems to be enough slack in the cable harness to move 'stuff' easily. Obviously cable was expensive in the 70's and 80's