Searching for a Short

[Hoogah]

My engine cut out suddenly today, just as I was sitting, idling the motor to warm it up before a drive. No history of electrical problems. The fuse did its job (thank goodness), so obviously I have a short somewhere.

I could find no symptoms in my cursory look around (e.g. no smoke to be replaced in wires! ), so now I need to systematically trace the problem.

The only area that I might have had a suspicion about is the ignition switch, which sometimes wants to rotate with the key when I turn it on.

I do own a Multi Meter, as yet unused. That's about to change. Is there a commonly agreed order of things to check in this situation? Thanks in advance for any advice offered.

[Mitch//pa]

If the short is present all the time you can find it easier. I use a circuit breaker in place of the fuse. Many shorts can be intermittent by moving wiring around. A common spot is inside the junction box and behind the dash, but if you also have accessories wired into the stock system check that also by removing the main feeds out of the circuit. Also try moving the light switch / horn wires out of the circuit off the generator and see if it stops shorting another common area. Work your horn, lights and brake light to see if any of those bring on the short then attack those circuits. You can prove out a circuit with an ohm meter
Take your time and use common sense, for example say you find the problem area with the lighting circuit dsconnect the headlight feed conduits from the buckets to start eliminating areas.
I had the fuse blow once and couldn't find anything, it never happened again..
I am sure others can add other helpful ideas

[Tom Wesenberg]

To check for shorts, I'd use a light bulb and flasher in place of the fuse. The circuit breaker Mitch mentioned is also a good idea. Then I'd remove the horn/lights terminal from the cutout and see if you still have the short. This will let you know if the short is in the lights/horn circuits, or somewhere else in the car.

I've seen brake light switches short to ground. Ignition switches, if repro, may short to the gas tank. I had to bend my switch terminal to the side, so it wouldn't touch the tank. I've seen loose repro contacts at the headlamp connections touch each other or touch ground. I've heard repro terminal boxes have had problems with the stud screw heads rubbing the firewall.

[Tomy Turbos]

To expand on Mitch's remarks above, fuses CAN fail on their own. Like anything else, they fatigue as they age.

I also concur with using breakers during trouble-shooting. I went to the Truck Parts supplier and got a number of the resettable ones in varying ratings for that very purpose. Simply make up a set of connectors by cutting a clip-on jumper wire of an appropriate gauge in half and put female spade disconnects on the cut ends.

I chose the resettable ones purely as a matter of convenience but regardless of which type, they give an audible report when they trip. Fuses don't tell you when they blow, nor are they reusable. And lights on work if you are in line-of-sight.

[Tom Endy]

One method to isolate the short:

Refer to the diagram and disconnect the three wires attached to the cut-out (or alternator) and spread them so they don't touch anything.

One comes from the firewall supplying battery power:
One goes to the ahooguh horn:
One goes to the light bail:

With the wires disconnected replace the fuse. If the fuse blows the short is not in the cut out\generator, horn, or light bail. It is in the ignition circuit, in the terminal box on the firewall, or behind the dash panel, or in the dome light circuit if you have one.

If the fuse does not blow the short is in one of the three wires you disconnected. Replace them one at a time until the fuse blows. Start with the wire supplying battery power from the firewall terminal. If the fuse blows the short is in the cut out\generator circuit. If it does not blow, connect the horn wire, if it does not blow, connect the bail light wire.

The most common places for a short to occur is:


In the light bail circuit:
In the terminal box on the firewall:
Behind the dash panel:

Tom Endy

[Hoogah]

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Thanks, all the Toms and Mitch. I now have an action plan based on your advice!

[Tinbasher]

Also check the ignition cable. I just spoke with a member in our club and had the same problem. the wire inside of the shielded cable was shorting out on the outer spring cover.

Move the wire and it would short. The engine would die.

Another Idea.

John Poole

[imacrazy]

Like John said, I had the same problem some years back and it was the wire inside the ignition cable. I was able to take the cable apart and replace with a new wire. No problem since.

Bill

[cw253748]

I would put a light in place of the fuse. If the short is still there the light will be on. start disconnecting wires or shake wires until the light goes out and you have found your short

[Chuck Sea/Tac]

First thing I would do is check the rotating ignition switch (behind dash) to see if you've damaged wire insulation or it's touching where it shouldn't.

[larrys40]

My recommendation is to use your meter... you can pinpoint the trouble quickly with it. The trick is knowing how to use it.... look for low voltage first. Check your battery voltage at the starter switch, you should have the same at both lugs of the terminal box. If one is lower you have a bad connection in the terminal box or ammeter. Next check both sides of the coil, they should both be hot and same voltage. If that's good insulate the points or hold them open and with ignition on you should have the same voltage at the moveable point arm ( not grounded). If it's low voltage you have a short or bad connection.


It can be as said above by the ignition switch itself, inside the distributor from the lower plate connection to upper pigtail to the upper plate. Many times this is a source of trouble when the spark is advanced or retarded due to connection and wire issues.

Also check the dash light pigtail for issues... they often get neglected and forgotten.
As Tom Endy said you can unhook your light wire harness from the cutout and take it out of the circuit. Put it back in if you cannot locate it in the engine/dash/ ignition circuit harness side
Use the ohm function to check for continuity and resistance. If you're not sure how to do this do a web search for how to use a volt/ohm meter... experience will come with practice.

Best of luck!
Larry Shepard

[katy]

This may help?

[henry's 31]

Check inside your headlight connection. That is a common place for a short. Thai is why a lot of folks connect them with the bullet nose connector.

[Hoogah]

Thanks again to the next batch of helpers - lots of good stuff here. I think I'll pass on the 1482 amp engine bolt fuse if that's OK! But if I was interested, does the pitch of the thread matter?