Proper Procedure for Finding a Short?

[Dayzedandkonfuzed]

I like to think of myself as pretty electrical savvy, but this is something I've never been able to wrap my mind around.

How do I find a short circuit in my car? Its somewhere in the accessory circuit, horn, interior lights, radio and cigarette lighter. The worst part about this is it seems to be a broken wire, in that sometimes I can put the fuse in and it pops the second I put it in, other times it will work for a day or two before blowing. How do I trace this without a $300 fault finder? Should I be checking continuity from the fuse box to every accessory? I've heard of checking the hot wires for ground, but won't I see ground regardless being that there are more loads on the same circuit? I'm thinking I should disconnect everything (pull light bulbs, remove plug from cig lighter, horn,) then check the hot wires for ground? Can anyone tell me what will all be on this accessory circuit on my 80 Malibu? I have wiring diagrams, but they don't help me distinguish what is all on the same fuse.

 

[pontiacgp]

I would check the cigarette lighter first, horn next and the lights last, the radio is on another fuse. I would check the continuity since it's the easiest way to check for a broken wire

 

[Dayzedandkonfuzed]

Sorry I should have said 'clock' instead of 'radio'. being that I have an aftermarket deck the clock wire is used for the constant, so my radio doesn't work when this fuse is blown.

I though there was a 'trick' or procedure for finding shorts, but if it's simply checking continuity then I guess that's what I will do...

 

[ssn696]

Tracing intermittent shorts is a huge PITA. If you put a fuse in and it pops immediately, your short is touching ground, so this is the best time to run it down. Since you mention courtesy lights in your circuit analysis, the dome light might be to blame. The hot wire goes directly to the light, the ground side goes to the bayonet switch on the door jamb near the hinges. With the door close, the ground is disconnected. With the door open, a spring pushes the switch into contact with the body. Squeeze and remove the dome light cover, pull the bulb. Disconnect the hot lead at the battery. Use a multimeter, connect the ground wire to the body. With the door open if both bulb terminals read zero resistance then it's in that line.

 

[Dayzedandkonfuzed]

Use a multimeter, connect the ground wire to the body. With the door open if both bulb terminals read zero resistance then it's in that line.

This should go the same for all the circuits I suppose. For some reason I never thought of checking resistance rather than continuity, I'll get started on it tonight!

Just another quick question, the dome light feature on the headlight switch must also close the ground side, right?

 

[Dayzedandkonfuzed]

So I'm not sure which scale to use on my multimeter, but with the door open or closed, I get 30 when set to the smallest setting (200) and obviously .3 on 20k, on both terminals. Continuity through both, also with the door open or closed. I've even held the door jamb switch in with my thumb, no difference.

In assuming I've found my short, but it looks like when I find and fix it, my dome light will be stuck on? Or it won't work? Not sure what acceptable resistance is I guess...

 

[ssn696]

Before tearing down your headliner, take off the A-pillar trim and look for an orange and white pair of wires. I think that's where they route. Follow them down all the way to the fuse panel if you can. There may be a plug you can pull out. Then put in a fuse and see if the fuse blows any more. The wires are either taped to the headliner or held onto a roof bow with clips, so it will be a bit of work to replace the hot lead if that is in fact your short.

 

[xitabl442]

I thought the BEST way to find a short was to use a larger fuse and watch for smoke!
I had fixed a short in a Tahoe once where the wire to the license plate light in the rear gate was burned all the way to the fusebox from rear gate hinge.

 

[Dayzedandkonfuzed]

Well luckily my headliner has been out for months. There's no breaks from the dome all the way down the a pillar. I'll look for that plug

 

[ssn696]

I hope this was not a goose chase...