Intermittent short: How do you find those?

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Victor H
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Joined: Tue Apr 03, 2001 3:01 am

Intermittent short: How do you find those?

Post by Victor H » Sun Sep 03, 2006 4:32 pm

I have an intermittent short that I just can't seem to find. Happens randomly while driving, I can hear the arc or snap and then the fuse goes. No relation to turning, turn signal use, brakes, stopping, starting etc. Sometimes when sitting at a light, "click" and fuse pops, othertimes driving down the road. Can't go more than 15 miles without using a fuse. Also, I never use the wipers or headlights.

Here's the set up. '61 bug, converted to 12 volt. No radio. It's always the first fuse in the box (the one that's hot when ignition is on), the left hand one nearest the steering column. That circuit has my oil pressure gauge, tachometer, fuel pump and in the engine compartment, that wire goes to a relay with direct power from the voltage regulator to run my electronic ignition & coil. All the gauges otherwise work normally.

What approach would you suggest? Should I disconnect one thing at a time and see if that does it? Is there any way to wire in an amp meter and see if something is spiking the voltage? Any ideas appreciated.

The Noof
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Joined: Sun Jun 19, 2005 7:13 pm

Post by The Noof » Sun Sep 03, 2006 5:07 pm

Easiest way is process of elimination:disconnect evrything related to that fuse, one circuit at a time untill the fuse failure stops.Leave only whats neccessary to keep the ignition/charging system active.

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Marc
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Post by Marc » Mon Sep 04, 2006 10:17 am

Yes...and that's way more stuff than I'd ever put on the same fuse.
In this case I'd first put the ignition/gauges, fuel pump, and wipers/t'signals/brakelamps on three separate circuits and if the problem persists start isolation from there.
Note that if the park switch in the wiper motor is broken it can cause mysterious intermittent shorts - there's power going there when the wiper switch is "off".

LarryInSanDiego
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Joined: Fri Jan 18, 2013 2:23 pm

Re: Intermittent short: How do you find those?

Post by LarryInSanDiego » Fri Jan 18, 2013 2:45 pm

Hi folks. Here's a method I've used that works great for those intermittent shorts:

Grab an old sealed beam headlamp (one that won't get so hot that it'll burn whatever it resting on) and wire it across the offending fuse terminals so it is essentially replacing that fuse. Now operate that circuit. If the headlamp glows dimly, ignore it. Now do the wiggle test, gently flexing the wiring, etc., until the headlamp glows brightly. When it does, you just pinpointed the short.

A headlamp works great because as long as it's pointing in such a way that it catches your eye (upward for example), you'll definitely notice it and know precisely under what exact condition the short occurs. Plus, it has the right amount of resistance for circuits of at least 5 amps so you won't start an electrical fire.

Just remember to restore the circuit protection device (fuse, etc.) immediately after you have located and corrected the short. This is for diagnostic purposes only!

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