Alan

This is bad idea... an easy way to destroy your wiring - don't do it. The fact you have a short means you should stop changing the fuse and do the required detective work to figure out what the cause is.

Alan

MainePorsche

Sounded interesting.
Will keep to the old gum shoe detective work... it never fails.

Rob O

This procedure has been used to find shorts by mechanics for a very long time. Mac tools and many others tool companies make a tool that does exactly what I explained, just costs more.

The one thing I should have mentioned is that the resettable breaker should be rated for the faulty circuits fuse size or less. I keep 10, 20, and 30 amp resettable breakers in my tool box just for this. I believe the tool Mac makes is rated for 10 amps only. Maybe excluding this initially is why Alan doesn't think this is a good idea. The breaker will trip before damage is done to the wiring. Replace the resettable breaker with a normal fuse when your done.


Here's an web page that explains this in more detail.
http://www.inliners.org/tech/tech6.html

I first learned about this years ago in the book, "How to diagnose and repair automotive electrical systems" by Tracy Martin. For me, after reading this book, working on cars electrical systems became much easier. It's something that's been especially useful working on my 928, as it has had more electrical gremlins than any classic car I've owned.

Mongo

First area I want to check out is under the parking brake. The problem started ever since I took that cover off to clean.

Chris Lockhart

That's excellent info!!

Mongo

Found it. Driver side hatch switch is stuck in the release position. It is jammed and now cooked.

Wtb hatch switch 928 613 114 00

MainePorsche

Good hunting it down.
I have to get a new hatch switch too. My son went to reach for something I was handing by standing on the rail - he kicked and snapped the switch head right off on the passenger side.

Mrmerlin

Mongo If your crafty you can fix the switch,
usually what happens is the plastic spacer on the shaft snaps this lets the contact ground onto the shaft and thus,
when the **** is pulled the switch grounds and then the fuse blows.

The solution, for this is to get some carpet thread and wrap it around the shaft to act as a spacer,
this will isolate the pull from the tip ad a drop of super glue to hold the thread in place

Alan

This would not cause this problem on its own - it would just cause the hatch to run nonstop - check to see if the hatch motor is also fried...

Alan

Alan

So I looked at that link... it says: "The reason I recommend a 20 Amp circuit breaker instead of a 10 Amp one is it allows current to flow longer and thus building more magnetism" also it says "Because we have a circuit breaker in there instead of a fuse we're allowing magnetism to build along the wire"

If when you insert a 7.5A fuse it blows immediately - then inserting a 20A auto reset circuit breaker that reacts much more slowly may not seem like the most optimal idea. This will cycle on and off at almost 3x the static rated capacity for the fuse*. The circuit breaker element will cool much faster than the small plastic encased wires in the middle of the wiring looms in the car and it will take you some time to locate the failure point - particularly if you have no idea where to start.

*Actually dynamically it will be much more because it has a finite thermal inertia time before opening - it doesn't really provide any protection during this window - this window width varies depending on the load current but is longest for the first event from cold.

By the time you get there the wire may have melted the loom or just fried itself somewhere new.. now you have an open circuit at some random location in the loom instead of a short circuit - and a potentially damaged loom. It is really a bad idea, you HAVE to overload the wire to make this work, not smart. Porsche uses quite small wires for low load circuits - and this is one of them.

Far smarter to figure out what equipment gets supplied by the fuse and disconnect them one by one to narrow the suspects (as here).

There are also tools that use low power modulation to trace circuits - these are far more reasonable tools to use to trace wires if you get to that point. Usually you will never have to - you can request info on routing & equipment connected here to shorten the search.

Now you also say: the resettable breaker should be rated for the faulty circuits fuse size or less. but that is not at all what the link proposes - it specifically proposes overloading the wire even as a static spec. An equivalently rated auto reset circuit breaker still has a much longer time constant than the fuse element and will still hugely overload the wire dynamically*

Can it possibly work - yes - is there a likely risk of significant damage - Also yes.

Alan

Wisconsin Joe

Well, then he can use the smoke to find the short (yes, I'm kidding).

WallyP

Get:
- A blown fuse that fits the panel - Amp rating not important.
- A small 12-volt light bulb.
- A Couple of feet of speaker twin-lead or lamp cord/zip cord.

Cut a bit of plastic from the face of the fuse (where the numbers are) until you can access the two bits of metal sticking out.
Solder one side of one end of the twin-lead to each of the small metal bits.
Solder the other end to the connections on the small light bulb.

Turn everything off on the problem circuit. Install the new test light into the fuse connection for the circuit, and put the bulb where you can see it. Power up the circuit if necessary.

As long as the short to ground exists, the bulb will glow, but the bulb limits the current flow so the wiring is not overheated. Start checking the circuit - when you remove the short, the light will go out. Remember that everything on the circuit must be turned off or disconnected, otherwise the bulb will glow whether there is a short or not.

Mongo

I am bumping this thread with an update. I FINALLY got the chance to fix this switch 3 nights ago. Removed the old hatch switch that was jammed. Merlin, I think this is the failure you talked about above, but I found nothing externally binding the switch in the upward position. I installed Mark's new (well actually it is a used) switch but have not plugged the fuse in yet. While reassembling, I did see the wires were coming into contact with some of the parking brake hardware. There were no tears in the sheathing though. I'm going to put a 7.5 fuse in it tonight and see if it goes pop again. I'm not holding my breath and assuming I fixed the problem. At least there aren't any wires getting intimate with the e-brake.

However, I am still stumped as to why my battery has not drained as fast as it did before with Fuse 24 plugged in.

Mongo

Odd question: does anyone know if FUSE 24 also powers the gauge cluster gear lights, PRDN32????

Next, does "current pulse tailgate" apply to the hatch release according to fuse chart, or the pulse signal for the speedometer? I know on the digital dash cars it does but isn't it Fuse 10 for pre-1989?

I still have not fixed the damn thing yet. Too much to do lately..

Mongo

Alan, you had mentioned the hatch switch may not have fixed it. I never really fudged around with the lights that are culprits for touching the roof. I did pull the light down during troubleshooting (the light above the driver and passenger, not the hatch) and found no visible fouling. It touched the roof once back in 2002-2003 I remember and that was an easy fix.

If the hatch switch was not the problem, where else should I look then, the rear tailgate and start pulling wires?

I am probably putting too much thought into this before diving into it and plugging in the fuse to see if it is even fixed or not. I want to make sure all bases are covered.