Isolating fault in shorted circuit

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  #1  
Old 07-22-10, 02:56 PM
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Isolating fault in shorted circuit


I have a shorted line. The way I planned to isolate the cause was to first disconnect from the wiring harness anything suspect, check and then, if that doesn't reveal anything,
disconnect half the branches of the harness of that line (if possible), then divide that half in half again and so on until pin-pointed. (I haven't yet checked into whether it's possible with this particular wiring harness but if it can be disconnected in sections, I'll go that route). I'll probably also check as each item is removed, just in case; I may get lucky. Is there a more logical/faster/more efficient method?

Secondly, (I'm a little embarrassed to ask as this is basic electricity but it's been a while and it's not coming to me for some reason.) rather than testing by blowing boxes of new fuses, how would I test at the fuse box using a meter or test light?

My first thought was to (with battery disconnected) use an ohmeter to check for continuity from fuse terminal(s) to ground but somehow that doesn't seem right. Using a meter or test light, What's the best method to troubleshoot this sort of problem?

Thanks
 
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  #2  
Old 07-22-10, 03:16 PM
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Go to the AP store and pick up a 20 anp Ckt breaker. Plug it in to the affected circuit, and you have a budget short finder.

Your plan is most logical, provided the branches disconnect. I highly doubt it. Also, pay close attention when moving things out of your way, or lifting and pulling on harnesses. Many times, you will inadvertently "FIX" the short by moving a harness, and there is no way to "BACKTRACK"

Common things to watch..The door Jambs, where the window/speaker wiring pass from body to door, Seat tracks rubbing on a harness , and dont forget my personal favorite......THE DIME IN THE CIGARETTE LIGHTER SOCKET...

Year Make model and which CKT?????
 

Last edited by Unclediezel; 07-22-10 at 03:49 PM.
  #3  
Old 07-22-10, 04:01 PM
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With the battery connected and power applied to the circuit (like if you need ignition on for the circuit), you could use a 12 volt DC sound making device in place of the fuse. (Radio, door chime, whatever.)

So long as the short remains, the sound making device would continue to make sound...

However you would also need to disconnect all the electrical devices on that circuit because they would also pass electricity from + to -.

The problem with a light or ohm meter would be you would not be able to see it as you were working on the wiring. With a sound making device, you could instantly hear it stop making noise when you found the problem.

First I would try "giggling" the wiring and see if the sound stops. Wiring rubbing against a sharp metal object can short to ground.

Then if all else fails, then your suggestion is what I would do... That is disconnect each branch of the circuit - find where the trouble is coming from.

As to first disconnecting all electrical devices from the circuit prior to this testing, it may be difficult to determine what all is on a particular fuse/circuit without the factory wiring diagrams.

If you have the wiring all torn apart, then you could of course trace where the wires go (then no need to order the factory service manual set of books). Or if all the wiring is visible like running under a truck to the tail lights.
 
  #4  
Old 07-23-10, 05:18 AM
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Harbor Freight sells a circuit tester. You attach one part to the fuse box then trace the lines until you find the fault. Moss Motors used to sell the same type tester but it was a lot more. One thing you will need is a wiring diagram for your car.
 
  #5  
Old 07-28-10, 09:28 PM
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Thanks for the replies. I liked that idea of substituting a circuit breaker and headed out to the parts store almost immediately. They had several but none the physical size I needed. I realize I could solder some lugs and leads to the larger one but, though that might not be a bad idea, in all honesty, I probably just won't go to the extra trouble.

The car is a 1995 Toyota Celica ST liftback and fortunately, I did get a hold of not only the complete Toyota service manual but a separate electrical manual as well.

mgmine, is the Harbor Freight circuit tester you refer to one of those little neon-light-in-the-screwdriver-handle gadgets or something other?

In an attempt to keep it simple, what I didn't mention is that this circuit was not working when I purchased the car from a kid who (was apparently no electrical engineer but) had tapped into the wiring here and there for alarm system, trailer lights, his big BOOMBOOM Stereo, etc. This could entail a bit more than dealing with a car with original standard wiring.
 
  #6  
Old 07-29-10, 05:34 AM
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Notice that factory wiring is all taped up, then covered with plastic tubing, and then fastened down every few inches so it can't move....

This is because wiring on a car will move and rub against sharp metal objects, then ground out!

Do-it-yourself wiring is not taped, not in plastic tubing, and not fastened down (typically).

And vehicle manufacturers will take care to only load each circuit to no more than 80% if its capacity. Do-it-yourselfers will overload circuits and pay no attention to amperage, fuses, or wire size.

I would start by disconnecting any add-on wiring. Then reconnect things on their own fused circuits - make the wiring look like factory wiring.

Wire sizes chart...
(Scroll down)
Wire Gauge Amps Ratings for 12 volt Automotive Systems
 
  #7  
Old 07-29-10, 07:04 AM
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Maybe I'm missing something here ......
What year/make/model are we talking here?
Have you owned it since new?
Have you had an accident recently? Driven over some debris on the road?
Have you altered the wiring recently?
What fuse blows? What circuit is causing you problems?
 
  #8  
Old 07-29-10, 01:57 PM
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Do-it-yourself wiring is not taped, not in plastic tubing, and not fastened down..
Do-it-yourselfers will overload circuits and pay no attention to amperage, fuses, or wire size.
B I N G O !

I would start by disconnecting any add-on wiring. Then reconnect things on their own fused circuits - make the wiring look like factory wiring.
That's what I'm trying to do. To give you all a better idea of what I'm dealing with, there's a spot where there's a few loose wires and an unmounted, dangling, partially wired relay socket. I have to determine..
.. is that relay stock and needed or something that was added? Can/should it be removed?
..to where do those loose wires lead? Are they part of the original wiring?
..should they be connected together, taped up, etc?

I don't expect anyone reading this to answer these questions. I merely wish to explain that this situation is more than one where only factory wiring is involved, the color code applies, all wiring, connectors, harnesses, etc. match the book 100% and that the cause of the problem is most likely something other than the common "rubbing on a door jamb".

 
  #9  
Old 07-29-10, 04:33 PM
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Oh my!

For this case I would recommend going to a wrecking yard and taking your camera. Find the same vehicle and take plenty of pictures of the wiring. That should help you figure out the way things should be.

If there are short harness runs, you may be able to snatch one off a vehicle in the wrecking yard.
 
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