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Robert Staton:
Hello everyone,

I hope that someone can lead me in the right direction.  I have a 66 Mustang that was pretty much completed and then I took it to a paint shop.  Evidentally they left the switch on and ran the battery down.  They tried to jump it, hooked the cables up backwards and burned out the wiring harness.  I have since replaced the wiring every where except to the tail lights as it showed no sign of being shorted but I have an issue none the less.  Independently all light systems work, tail lights, turn signals, brake lights, but when I put the left turn signal on and mash on the brake pedal, the left turn signal quits flashing and the right brake light go out and my electric fuel pump which is only hot when the switch is in the on position will pulse like it is getting juice through the turn signal.  I am at a loss and do not know where else to look.  I have replaced the entire wiring harness except the tail light wiring hareness which tested okay, including the head light switch and turn signal switch.  If anyone has any ideas I would appreciate hearing them because I am at a loss as to what to look for anymore, my mind is fried. Thanks.

ouch1:
I am not the most knowledgable about wiring, but to me it sounds like a bad ground. I would check all of the ground connections to see if one is bad or just flakey.

-ouch1

Robert Staton:
Thank you, I will check that out.

Jeff73Mach1:
How is your fuel pump wired into the car's original wiring (since it came with a mechanical fuel pump)

I suspect your tail light or turn signal specifically has a bad ground and is back feeding through the  the other wiring, but since the fuel pump is involved you should be able to narrow it down to which wires in the three systems are common.

Just a WAG but since fuel pumps are in the rear-start by checking where you grounded your fuel pump.

Sometimes you can pinch a wire that looks good and it will be soft where the internal copper wire has burned away, but that is time consuming and requires unwrapping too many wires.

Robert Staton:
The fuel pump is wired into the cold side of the ignition, it gets power after the ignition switch is in the on position.  I have already checked the ground on the fuel pump (4 times) and it seems like it is a good ground.  I have just bought the oem wiring manual and I am in the process of finding exactly where all grounding points are located.  Since everything works independently and develops the issue after using the turn signal and brake lights at the same time (in my mind) it has to be a grounding issue as you and a previous response indicated as well.  Thank you for your help.

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