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Electrical Gremlins and More Vintage Mustang Tech Questions and Answers
Beyond the Basics
I have an electrical problem with my interior and exterior lights staying on and I’ve now drained two batteries because of it. I thought it was my turn signal switch because it had a bare wire exposed inside the switch. I replaced it but still the problem is there. The interior lights stay on and my marker lights as well. I can’t say if the headlights are on or not because I have them removed at the moment—I am restoring the car.
I hooked up the harness from the new Painless Performance chassis wiring kit to the new turn signal switch harness and I think I found the problem. The solid green wire coming from the dash harness, when plugged in to the turn signal switch plug the lights come on and the headlight switch is not on. Oh yes, I replaced the light switch also. I pull out just the solid green wire out of the harness plug and the inside lights and marker lights go off. I can send you pictures of the plugs and wires if you wish. Does the solid green go to the solid green like the manual says? Please help! Thanking you in advance!
Via the Internet
I am going to assume you are using the Painless Performance chassis harness for the 1969-1970 Mustang, part number 20122. My answer is contingent on this harness. I think you have several issues here, and I think most of them revolve around the reusing of the original turn signal switch and headlight switch connectors.
First, the green wire: The solid green wire going up to the turn signal switch is the power to the brake lights. When you press on the brakes, the switch closes and sends power to both turn signal filaments in the rear taillights. Power from the brake switch is only on when you depress the brake pedal, and it appears yours is on all the time. Now that you have the green wire pulled, use your meter and check for power on this line to ground. If you have power all the time, something is activating the switch. If you have changed the brake pushrod with a brake upgrade, this might be the problem. You may have to pull the switch on the brake pedal and test it.
Second, to add additional fuses to the system the Painless harness is wired a little differently than the factory harness. For example, the “A” post on the headlight switch was originally used to power the brake light switch, and in the Painless circuit it gets its power directly from the fuse block. D1 and D2 are used to power the interior courtesy lights, but Painless simplifies the setup and uses one line for power and one line to feed the lights.
It looks like you may have switched some of the wires going to the headlight switch and the turn signal switch. It is easy to do. The power on the brake line switch is somehow going through the taillight circuit in the turn signal switch and ending up on the “R” post of the headlight switch, which feeds the taillights and the interior light circuits on the original circuit.
Before you start changing wires, take a digital picture of how the wires are placed now. Painless is good about keeping wire colors consistent with the originals, but before you start document where they are now. Look over the pins carefully—the “R” pin and the “D1” pin are opposite of each other on the headlight switch, and it is easy to get them backwards. Where Painless is good about keeping wire colors, some of the turn signal switches are not. Some of these switches are generic and have an additional pin for multi-car use, and some require you to use the original connector over again. Make sure you have the pins in the right order on both sides.
Summary: the green wire going to the switch is brake light power and should only have power on it when the brake is engaged, and it looks like you are getting external power to the dome light and taillights, which makes me think that they are swapped around on the connectors. If you don’t find it there, then you may have to buzz out all the wires going to the headlight switch.