Looking for a short

[Pete]

Hey y'all,

I'm a long-time FJ40 owner and I've successfully chased and remedied electrical problems on my Cruisers as well as on a couple of older Harleys ('59 XLCH and '76 FXE) but I'm stymied and could use some help on my current baby, a '97 911 Carrera. It has an alarm-immobilizer-central locking system that went on the fritz and I found a blown fuse on the electronic brain of the system. I replaced that but the new fuse immediately blew. I recently had it in the dealer shop for something else and asked them to check out the locking-alarm-immobilizer and they found that the electronic brain was OK (I had thought this might have been the problem) and that there was a short in the circuit. I had already checked for frayed insulation at obvious locations such as where the wiring goes from the body into the doors but hadn't found anything. Any suggestions as to how to track down a short, save examining every inch of every wire in the car? What I'd like to be able to do is to at least narrow down the possibilities rather than to have to chase every wire. In the meantime, I manually lock the car and so I'm able to drive her which never fails to bring a smile to my face.

No, I don't waste my time with the Porsche boards when it comes to technical issues. About anyone there who owns a late model wouldn't want to get their hands dirty with mundane problems like wiring shorts.

Thanks,
Pete

 

[WES1977]

um I would go about this by connecting a multimeter to ground and measure the ohms to both sides of the fuse holder(one at a time). I would then keep the meter on the bad side with the beeper on to let you know when the ground is gone, and start unpluging things down the line.

do you have wiring drawings for the car?

 

[Pete]

um I would go about this by connecting a multimeter to ground and measure the ohms to both sides of the fuse holder(one at a time). I would then keep the meter on the bad side with the beeper on to let you know when the ground is gone, and start unpluging things down the line.

do you have wiring drawings for the car?

That's what I was looking for Wes. When you say connect the multi-meter to ground and then to the fuse holder, do you mean on the fuse that blew or to each fuse in the main box? I do have parts of the wiring diagram that includes the alarm-immobilizer-locking but it's not the easiest to read. Along with the alarm-immobilizer-locking, the passenger compartment lights are also out and I think are on the same circuit. Maybe this is a hint on which circuits to check using the multi-meter?
Thanks,
Pete

 

[WES1977]

That's what I was looking for Wes. When you say connect the multi-meter to ground and then to the fuse holder, do you mean on the fuse that blew or to each fuse in the main box? I do have parts of the wiring diagram that includes the alarm-immobilizer-locking but it's not the easiest to read. Along with the alarm-immobilizer-locking, the passenger compartment lights are also out and I think are on the same circuit. Maybe this is a hint on which circuits to check using the multi-meter?
Thanks,
Pete

What i'm refering to is pull the blown fuse out and meauser the ohms to ground to the contact points of the fuse holder. One side is going to have 12 volts dc to ground the other side will be the one shorted. A short is like 1.2-.2 ohms not megohms. Touch the 2 leads of the meter in ohms and should read about .2 ohms(to see what a direct short is)

If you don't know the ohms symbol looks like this

 

[Pete]

What i'm refering to is pull the blown fuse out and meauser the ohms to ground to the contact points of the fuse holder. One side is going to have 12 volts dc to ground the other side will be the one shorted. A short is like 1.2-.2 ohms not megohms. Touch the 2 leads of the meter in ohms and should read about .2 ohms(to see what a direct short is)

If you don't know the ohms symbol looks like this

Got it, thanks! Why wouldn't the short read closer to 0.2 ohms? Is that because there's enough resistance between the fuse holder and the point on the wire that's shorted to give more like the 1.2 -2 ohms? I always wondered about this and it will help me a lt in the future. Thanks again,
Pete

 

[ToyotaDon]

Another way to track down a short is to use a circuit breaker in place of the fuse. Tool distributors sell these with a buzzer that lets you know when the breaker is tripped. You can also get them from Napa or other parts stores, be sure to get one with at most the amperage of the fuse you are substituting for. Once the circuit breaker is in place, start splitting the circuit by disconnecting whatever connectors are in that circuit one at a time. Here is where it helps to have a good wiring diagram. When you disconnect the section of harness that has the short, your breaker will stay on and not be tripped again until you reconnect that section.

Also, be aware that if you disconnect some connectors, you may also be taking away the power supply to the fuse.

Edit: The problem with using an ohmeter is that you will always have continuity to ground as long as your loads are connected. You will be measuring the resistance of the load, just like hooking the leads of your ohmeter to the terminals of a light bulb.