Short blowing fuses when I turn on headlamps

[karhawk]

I have a dead short on a 71 124 spider, 4th fuse from the left, 2 yellow w/blk
stripe wires go to L front turn light and R rear tail light and a white w/blk
stripe goes to the head light switch. I have disconnected both lights and still
have a short blowing fuse issue when I turn on the head light switch. Has
anybody
seen these switches short out internally? I had a 75 that caused problems, the
funny thing is it just started happening. There seems to be some shared items
looking at the diagram such as the dash rheostat which I also disconnected with
no
good results. I have the fuse box hanging so there isn't something shorting it
under the dash.
I also have disconnected all trunk and under hood lighting since those are also on the circuit.

Thanks
Chris

[mdrburchette]

You could eliminate the switch by buying a three position toggle switch and trying it in place of your ignition switch. I saw your thread on MiraFiori and it seems you've just about eliminated most circuits on that fuse. Does it happen in both the "on" positions of the switch or just full headlamp mode?

[azruss]

i would be very surprised if the switch is the source of the short. More likely a short downstream from the switch. the switch just connects power to each downstream system. Isolate them one by one to see which one is blowing the fuse. you could save fuses by using a ohm-meter to test for continuity to ground. all light bulbs need to be removed for this test.

[spider2081]

I find shorts in lighting circuits very hard to find using an ohm meter. The reason is light bulbs have a low resistance when they are turned off. When there are more than one light in parrallel as in this case the measured resistance of the circuit is easily mistaken for a short. Bernie mentioned connecting an automotive trouble light to each circuit. A trouble light has one side connected to 12v and the other side connected to each wire that could have the short. This is done one wire at a time. The wire with the short will light the trouble light. The others might but it will be noticably dimmer. You need to disconeect the wires you are checking from the fuse when you thest them withthe light. THe other good thing about this method is you don't keep blowing fuses as you are making the tests.
Not sure about a 1971 but on my 81 the headlight switch is plastic. If you have a plastic switch I don't think its your problem.
I am not sure you mentioned if your headlights are stock sealed beams. Also not sure you mentioned what value fuse keeps blowing. I think it should be aat least 16 amps.

[TX82FIAT]

I think the one wire at a time approach is the way to isolate the problem. Sorry if asking the obvious. Do you have a wiring diagram? I had a similar issue caused by hot wires to ground. At some point in factory or by prior owner a copper wire similar to a clothes hangar was punched through the wiring harness just right of the steering column under the dash making contact with the metal support bar. This was done to hold the wiring bundle in place. Unfortunately, I had to take the dash apart and trace each wire back from the blown fuses to to find microscopic exposure areas on wires within the circuit. In fact, on one circuit i could not find the issue with various meters and simply covered the wire with the smallest wire loom I could find. Not an ideal solution but the short to ground was eliminated without changing the initial wire or color code on the wire along the entire circuit.

I hope you do not have to undue the old wiring harness to find the problem.

Ray

[karhawk]

Thanks for the suggestions, I located the short after much trouble. It was the L front side marker light which I have disconnected until I take it apart. There was also a suspect wire in the rear tail light harness.

Chris
Oakland, Ca.