Positive ground wiring question

[skyhighZJ]

So, 1979 Mack R series dump truck. We use this as one of our yard trucks. Safety man came and gigged us because a taillight was out. One of our young shop hands said I got this. Went to cutting 45 years of connectors, electric tape and scotch looks out to R&R and then got lost in his wires cause “things were weird”. I get entered to the situation. What I have found is the truck is positive ground. At the back are 4 wires in a loom.

1 wire for running lights
1 wire for left turn/brake lights
1 wire for right turn/brake lights
1 wire for reverse lights

When I put a meter on the individual wire (ground to chassis and the pos lead to the respective wire) it shows -12.5v.
I wired up a set of universal S/T/T lights with a stand alone ground direct to the chassis and they no worky. The ones I used are Parts Sam LED with a stand alone ground. The ones that came off had the integrated ground where the mounting studs were the ground. I’m reading that LED will not swing both ways but a set of standard bulb type will work but I’m not so sure.
Unfortunately I don’t see the way things were ran before and now I’m trying to recreate the correct wiring pattern. My google is apparently not strong cause I’m coming up empty. Any help would be great.

Edit: at the generator it’s putting out -12.5v.

 

[WARRIORWELDING]

Some led lighting is polarity sensitive. If its positive ground on the unit hook the normally power supply wire on the led to the chassis. Use the led ground wire as the signal wire to the harness. I cant se how it (the fixture) would know the difference.

 

[LT1JEEP]

Switch it to negative ground it not that hard switch the alternator and regulator and you switch a couple wires around. It has been 25 years since i done one but it not hard to do. Sure you can find something on the web telling you how to do.

 

[Mac5005]

LED = light emitting diode.

Diodes are electrical check valves. Allow current one way but not the other.

I’d guess that polarity is reversed to LED light causing no function.

Incandescent and halogen. I should say filament bulbs don’t care about polarity bc the work based on resistance.

My guess is to use incandescent test light, verify operation of the wiring and what does what. Then replace test light with LED and get the polarity correct for the led.

TLDR: neg ground or positive ground the LED just allows the angry pixies to pass in one direction and not the other. Get the check valve in the correct direction.

LED also generate small voltage in direct sunlight, this can throw you off if using digital voltmeter for checking stuff.