924Board.org

[MrMoMo]

OK folks, I think I got it figured...

For those of you who are not familliar with the story, check here:http://www.924board.org/viewtopic.php?t=24023

So... Tearing into the wiring harness and using a process of elimination, I determined that the wire that burned up was green. It goes from the "p" terminal on the ignition switch to the turn signal assembly. -The nice part about that, is it is a very short wire that does not really get into the main harness and should be very easy to replace - but the repair isn't the real issue, the cause is - I don't want this to happen again!!

Using the wiring diagrams & a good ignition switch, I found that the "p" terminal is live with the ignition off. - that was step one in confirming that this problem could have happened with the ignition off. (key point #1) Then to trace down where it goes, and find out why it was not fused.

Entering the turn signal assembly, the pins turn to copper strips, which run up to the switch contact wiper assembly on the right hand side of the switch/column. 2 of these strips are exposed to the back side of the steering wheel hub, to allow clearance for the signal canceling tab on the wheel. (KP #2 & #3)

Now on to the aftermarket hub I use for my Momo wheel. The Momo hub has the signal canceling tab cast in to the steel portion of the hub. (KP #4) The tab on the Momo hub is about 4mm longer than the OEM one (KP#5), and the OEM one is plastic (KP#6). The small plastic tab which cancels the signals is on the right hand side of the switch assembly, when the steering is straight, the canceling tab is on the left. (KP#7)

So, the chain of events happend as follows.

Pulled into the gas station, parked at the first pump. The pumps in this station are on isles which are at 90° to the entranceway. As I straightened out at the pump, I did not return the steering to center, I actually ended up about 1/2 turn to the left (#3). So... (key point #7) this lines up the tab with the exposed circuitry (#2). Because the tab is longer (#4), it is able to make contact with the exposed live circuit (with ignition off) (#1). Because it is cast into the metal portion of the hub, not plastic (#6) it is now in contact with the hub, which is in contact with the steering column, which is ground.

I now have a circuit from Battery Positive, through the ignition switch (even when off) to the turn signal assembly, into the steering wheel hub, back down the steering column and back to the battery negative. Because the wire from the ignition switch to the turn signal switch is so short, and normally does not carry a large amount of current, it is able to be a fairly small wire. Smaller than the wires feeding the ignition. Because it is the smallest wire in the circuit, it acts as the fuse when enough current flows through it. (yes, it did actually burn right through) Since the short occurred before the fuse box, it was not protected by any of the fuses. Odd that Porsche would have overlooked that.

So, PLEASE... IF YOU HAVE AN AFTERMARKET HUB... Check yours out!!! It is obvious looking at the old signal assembly that the hub was rubbing, however - it was not obvious when I was driving - did not feel drag or hear drag. I can only assume that my steering column shaft was slowly sliding down into the engine compartment - as there doesn't seem to be anything to stop that from happening??? (or it was never pulled up all the way)

As pictures are worth a thousand words, here are my next 6000 words...

Turn signal assembly as pulled from car after fire;



Turn signal assembly used to replace!



Momo hub vs OEM hub:


Cut off this much:


Clearance from the hub to the circuitry


So there should be about 2.2mm of air there now!


Please, if you have an aftermarket hub, check it out. It doesn't take long to pull the wheel & hub off and measure your clearance, and check the material that the tab is made of.
_________________
Bruce

-------------------------
'87 924S (project 937)
'87 924S parts car
'87 924S parts car
-Strewn amongst the other vehicular mass engulfing my property!

[bcblase]

Good job, I had a similar problem with that tab switching a 951 steering wheel over to my 924S, but there was no fire issue. I think the longer tab on the Momo is meant to be used on a 951. I switched the tab from the original 924S wheel onto the 951 wheel, and had no problem.

Thanks for the sleuthing report - a good thing to keep in mind.

BC
_________________
1987 Porsche 924S - track toy
1986 Mercedes 190E 2.3-16v - autocross
2007 F-150 5.4L Lariat Supercrew - tow beast
1994 Volvo 850 Race Wagon - 24 Hours of Lemons Car
2001 BMW 325xi - daily driver

[jrrhdmust]

Great Info thanks!
_________________
67 Mercury Cougar XR7 under Restoration
71 VW Superbeetle Completed Resto 11/07
73 Mustang Mach 1 10 Year old restoration

[MrMoMo]

Apparently I'm not the only one this has happened to... the harness that I got a chunk of to repair with, had the exact same wire burned. According to the guy I got it from, it is not all that uncommon.

Anyway... up and running again - WOO HOO!!
_________________
Bruce

-------------------------
'87 924S (project 937)
'87 924S parts car
'87 924S parts car
-Strewn amongst the other vehicular mass engulfing my property!