So last year my rear wiper switch melted. For all the gory pics of the melted switch see here:

Melted switch post

I was once again forced to scavenge off the 87 as I was in need of a new sunroof switch. I grabbed the rear wiper switch while I was at it, the 87 has had the rear wiper delete done so it didn't need it. I also had to steal the wiper switch harness, I'll let the pics tell you why...





That wire that has all the insulation burned off? That's the ground wire. I cut the wrap back to where there was some insulation left, about 5-6 inches were bare as the pictures show. The other wires were not exposed, but the insulation was fused into the wires and was in very bad shape. It wasn't in great condition even father back, stripping them was hard as the rubber had melted INTO the strands some. But I went back about as far as I could without risking damaging the other switches wires.

This all melted and burned WITHOUT BLOWING THE FUSE!!! The only thing that stopped this from becoming much worse was that the switch melted and stopped making contact. See, these switches even run power with the switch "off". I'm guessing it's a duel pole relay, needs power to be "off" and to be "on". So the switch originally melted while the wiper was OFF!!! I'm not sure why Porsche chooses to wire this way, but it's kinda scary! If I had been designing the system I've have used relays that rest in "off" and only throw when turned "on". In other words, power is only applied when the wiper is on. But in our case it's powered any time the key is on.

Anyway, a project this winter is to take ALL my switches apart and clean them. I've done this in the past with the 87, as all my window and door switches were corroded to the point of not passing current anymore. I'd just buy new switches, but they are kinda pricey. But looking at the pics from both threads, it's just too scary to keep chancing it with these 30 year old switches.

General consensus last year when it melted was carbon buildup. Looking at the switch there was no way it could have shorted, and testing with a multi-meter showed me there was no shorts in the wiring anywhere else. It pretty much HAD to be the switch.

Still can't believe this didn't blow the fuse. Porsche BBQ anyone?

Just beware and be forewarned. This is one reason these things like to catch fire!

 

Did you check to see if the fuse is the correct fuse for that switch? Too many times I've seen where it's supposed to be a 7.5A fuse but people replace it with a 10A or 15A because the correct 7.5A blows the second they put it in.

 

No.

But just because I can't FIND it. I posted about it in the other thread, but I can find no mention in the manual, fuse box itself, or on the internet on what fuse the wiper uses!!! So instead, I just checked each and every fuse! All my fuses are the correct ones for the slots they are in.

Good thinking! But I had already suspected the same thing. Maybe this revival will bring someone in who can finally tell me what fuse it is?!? My only other idea is it's in the motor itself. I have the 87's laying around, but always forget to look at it when I'm out in the garage.

 

I'll give it a go at what I think goes on,
https://www.manualslib.com/manual/39...age=152#manual
the motor has 2 positive feeds, one permanent and the other comes from the switch,
When you turn on the switch it starts the motor,, but shortly after, a set of contacts mechanically linked to the motor interrupts this "switched positive" and switches to the permanent feed, when the motor comes near to it's "stop" position, these contacts switch from the permanent feed back to the one from the switch, this then powers the motor until the contacts open, engaging the permanent positive feed, and so on and so on,
When the switch is turned off, the the motor still runs as it's using the permanent positive feed, UNTIL it gets near to its "stop" position, this time the motor recieves a negative supply from the switch, this means that BOTH sides of the motor get a negative supply,, putting an electrical short across the motor (which in fact at this stage is a generator) causing it to stop abruptly so as not to trigger the contacts connected to the motor (as this would mean that the motor would go through another cycle)
My opinion is that the burnt wiring is due to the shorting of the motor when coming to a stop, how to stoip this from happening, give me a while, got to pick the dog up
Steve.
EDIT(1) Try fuse 28 (30A)
EDIT(2) If the diode in the motor circuit fails, you will get a short between the permenent positive feed, through the switch, to ground (while motor is not at the stop point and the swich is off), and as the wire supplying the switch with ground is only 0.5mm CSA, (compared to 1mm+ for the other wires) this is the one which would melt,
to test the diode, you need to get the wiper in mid stroke, then using a diode tester, test between terminal 1 &2 of thew motor, current should flow with postive probe on terminal 2 and negative probe on terminal1, and no current flow when you reverse the probes

 

i once found melted rear window shock/holder end where the window heater connects. but i bet old fuel lines are the first reason they burn like fireworks.

 

Back of the headlamp switch

Headlamp switch melted

 

The issue with the headlamp switch is that it passes all the current the headlights use through it. All 110+ watts of it...

It's the reason things like the Rennbay headlight relay harness exist. To take that load off of something and put it onto a relay.

I would add relays to the windows for that reason alone.

 

When you consider how many times the switches get used, headlight for example, average of 4 times a week, 200 times a year, 6000 times in 30+ years, I think it's done what it was designed for (and more) I would suspect that a few relays might fail as well, probably a certainty if you buy the cheap (Chinese made) relays
I'm not saying relays are not a good idea, but there are possible pitfalls for using a switch which is designed for high(ish) 10Amps and just to energise a relay coil (mA's)
https://www.findernet.com/en/uniteds...does-it-matter
Check out the section, "Reliable switching at low power levels"
P.s that article was written by Norman Carnt, my Dad.

 

Awesome sauce!!

I had wondered about the internal workings myself, as I had inspected and tested the wiring at the switch side and had been trying to figure out how it kept wiping. I had figured it was a done with relays, but your explanation explains it perfectly.

And your thinking is sound to me as well. But if the diode was bad, wouldn't the new switch be shorting as well? They are not normally flaky like that. They either work, or don't.

I still feel a fuse should have blown somewhere, but we've seen many examples on here over the years that tell us the wires and switches often melt/burn before the fuses blow.

And while I'd agree most "running" fires are the fuel lines, I'm thinking this crazy wiring Porsche did on these is the cause of most "it caught fire parked in the garage" and the even more famous "I came home and found it in my living room" stories.

These still are the only cars I've seen multiple stores of that fun event happening with!!!

Still love em tho.... Just keep full coverage on mine .

 

This was electrical on a very poorly executed 6 to 12 conversion. Had to make a new harness and drop in an old single port.