Thread: Voltage drop when brakes applied

Super oddness. As per previous posts , had issues with the parker lights coming on when the switch was in the off position (and battery ground disconnected!) investigating this I found a loose ground wire behind the instrument pod. Connected that back up and then the parker lights started coming on with the brake pedal!

The odd part is that these issues only started when I had removed the tail light assembly to clean out the mud in the rear cross member.

After cleaning out the rear light assembly (worth doing - contacts were all loose and corroded, fixing those had made the rear lights much, much brighter) and re-installing, the parker lights stopped coming on with the brake pedal.

Very strange, but it seems that the wiring loom does not like have the rear light assembly removed.
Something to keep in mind if you have issues at the front of the car - the cause might be in the back!


Now that I've cleaned every contact in the rear assembly, and every connection I could find behind the dash, the voltmeter still drops a full volt when the brakes are applied. I'm going to assume it's just a thing with my truck because I cannot find any source of a problem.

maybe i'm missing something obvious, and maybe my knowledge of electrical circuits is worse than i thought, but i just don't get how the lights can be on with the earth disconnected from the battery; how is the circuit being completed? can someone fill me in?

either way, i'm glad to hear things are sorted!

Originally Posted by The Beast

maybe i'm missing something obvious, and maybe my knowledge of electrical circuits is worse than i thought, but i just don't get how the lights can be on with the earth disconnected from the battery; how is the circuit being completed? can someone fill me in?

either way, i'm glad to hear things are sorted!

I'm not too sure.
What I am thinking is that the disconnect battery cable was sitting on the floor of the battery box and then the loose ground cable behind the dash was just touching the metal frame enough to make a circuit.

I don't actually think that could happen as the battery box is freshly painted with 10 layers of paint total. Even though they were quite thin layers (as any from an aerosol can will be) I would have thought that that would be enough to insulate the box.

Barring that, I have no idea what happened.
The issue cropped up when I removed the rear light assembly, and went away when I put it back in........

Chris, Having a very long shot here.

Is yours an FFR with the batteries still in?
Just wondering if so, not sure if the power can bleed back to earth from those

Originally Posted by Trigger82

Chris, Having a very long shot here.

Is yours an FFR with the batteries still in?
Just wondering if so, not sure if the power can bleed back to earth from those

This thread is over a year old.

Aaand the 24v system should be completely separate from the 12v.

should be but makes a common earth at some points.

if the earth lead is off the battery then in theory its not possible to operate the 12V system form the earth of the 24v system or vice versa

however...

if you drop the earth by unbolting the earth strap and dont insulate it you can still get a feed. enough to make low current stuff work but not enough to start.

you could also have a canabilised and center tapped 12v feed from the 24v system.

I know its an old thread. Just had a thought if the theory was possible it may help someone else who has a similar issue.
The 24v system should be isolated but then again it seems old wires rubbing through the insulation are a pretty common thing on these vehicles