Brake light not working with headlights on: 1994 (1 Viewer)

[jonnylachinga]

Hello everyone,

I've used the search function and, though I got some good info, haven't been able to find a similar case.

I have a 1994 fzj80. The brake lights work fine. However, when the taillights are switched on (with it without headlights) the right (passenger side) brake light will not work. The taillight also cuts out when the brake is pressed.

I have taken the taillight assembly off and tested the ground on the bulb socket. The ground works fine. However, when the taillights are switched on, the bulb socket loses its ground.

I do have a trailer light converter box I installed last year. I've disconnected each lead going to the box one at a time and that has no effect whatsoever.

Any ideas?

 

[BILT4ME]

Hello everyone,

I've used the search function and, though I got some good info, haven't been able to find a similar case.

I have a 1994 fzj80. The brake lights work fine. However, when the taillights are switched on (with it without headlights) the right (passenger side) brake light will not work. The taillight also cuts out when the brake is pressed.

I have taken the taillight assembly off and tested the ground on the bulb socket. The ground works fine. However, when the taillights are switched on, the bulb socket loses its ground.

I do have a trailer light converter box I installed last year. I've disconnected each lead going to the box one at a time and that has no effect whatsoever.

Any ideas?

First look at your grounding wires.
There's one one the center of the rear cross member. Clean that connection.

Second pull the bulb and check the socket with a multi meter to see if it's the socket.

Third, buy a replacement taillight harness for the corner your working on. It's about $30.

Maybe just skip to #3.

This is actually a pretty common issue.

 

[jonnylachinga]

First look at your grounding wires.
There's one one the center of the rear cross member. Clean that connection.

Second pull the bulb and check the socket with a multi meter to see if it's the socket.

Third, buy a replacement taillight harness for the corner your working on. It's about $30.

Maybe just skip to #3.

This is actually a pretty common issue.

Thanks for the reply.

I would like to find out exactly where the problem is before buying a harness.

It is very strange to be that the bulb socket loses ground when the taillights are turned on.

I did disconnect the trailer lights module and also removed, cleaned, and reinstalled the ground and there was no difference. By the way, that ground on the cross member does not seem to supply ground to the brake light assembly. While I had it off to clean, turned the brake lights on and activated the brake pedal and the left side was still functioning as normal.

 

[BILT4ME]

Thanks for the reply.

I would like to find out exactly where the problem is before buying a harness.

It is very strange to be that the bulb socket loses ground when the taillights are turned on.

I did disconnect the trailer lights module and also removed, cleaned, and reinstalled the ground and there was no difference. By the way, that ground on the cross member does not seem to supply ground to the brake light assembly. While I had it off to clean, turned the brake lights on and activated the brake pedal and the left side was still functioning as normal.

What you are describing is referenced as a "floating ground".

A friend of mine had exactly the same issue and we traced the wires all the way to the firewall and couldn't find the issue.

Finally, out of frustration, he bought a new taillight harness for that side and it entirely fixed it. The two of us spent 3-4 hours trying to find the short or lack of power.

 

[jonnylachinga]

What you are describing is referenced as a "floating ground".

A friend of mine had exactly the same issue and we traced the wires all the way to the firewall and couldn't find the issue.

Finally, out of frustration, he bought a new taillight harness for that side and it entirely fixed it. The two of us spent 3-4 hours trying to find the short or lack of power.

Interesting. I have the FSM. In looking at it, it looks as if the bulb socket ground (for brake light) is shared with the Left-hand side. Similary, the tailight works the same way, except it looks like it has an extra ground wire. I'm not quite sure what this means. Wouldn't either the taillight or STOP light ground serve the same function since they are both grounding the bulb socket?

TAILLIGHT:

STOP Light:

 

[Dave 2000]

May sound odd, but have you replaced the bulbs? Very common (not just the 80), to have a filament failure where it can fall onto an adjacent filament, this messes with the rest of the system.

Regards

Dave

 

[jonnylachinga]

May sound odd, but have you replaced the bulbs? Very common (not just the 80), to have a filament failure where it can fall onto an adjacent filament, this messes with the rest of the system.

Regards

Dave

Hi Dave, yes I replaced the bulb with a brand new one.

I should also mention: when taillights are switched on, they both work. It is only when pressing the brake pedal and actuating the brake lights that the lights go out (from losing ground to the bulb socket).

 

[Malleus]

The answer to your "two circuit ground" question is the tail/brake light bulb has two filaments in it, the socket has two grounds in it. All the 80 electrical connections work by grounding the circuit. If you lose the ground, you can have power and still have no function.

There is no extra ground in the RH tail light circuit. the floor wire harnesses on each side are grounded separately and collectively on their respective sides (you need to look at the complete ground circuit diagram to see this):

Everything south of the seat belt switch warning light connection is grounded on its respective side, through its floor harness:

Ground "BG" above is the center ground @BILT4ME mentioned.

What this means is that if you have a fault in one tail light, but not the other, you have an open circuit on the fault side, upstream of the ground. This is why @BILT4ME suggested replacing the subharness, because that's most likely where the problem is. I understand your desire to "find" the root cause, but the only way to do that is to pull the subharness and perform continuity checks on each separate wire.

 

[jonnylachinga]

The answer to your "two circuit ground" question is the tail/brake light bulb has two filaments in it, the socket has two grounds in it. All the 80 electrical connections work by grounding the circuit. If you lose the ground, you can have power and still have no function.

There is no extra ground in the RH tail light circuit. the floor wire harnesses on each side are grounded separately and collectively on their respective sides (you need to look at the complete ground circuit diagram to see this):
View attachment 3318756
Everything south of the seat belt switch warning light connection is grounded on its respective side, through its floor harness:
View attachment 3318765
View attachment 3318769
Ground "BG" above is the center ground @BILT4ME mentioned.

What this means is that if you have a fault in one tail light, but not the other, you have an open circuit on the fault side, upstream of the ground. This is why @BILT4ME suggested replacing the subharness, because that's most likely where the problem is. I understand your desire to "find" the root cause, but the only way to do that is to pull the subharness and perform continuity checks on each separate wire.

Thanks for the info and suggestion, Malleus.

I have found the root cause of the problem: ground wire that attaches to the bulb socket. For whatever reason, that was causing the strange issue of losing ground only when taillights were on and brake pedal pressed.

I wiggled the wire and noticed a flicker in light while illuminated. Removed and resoldered and it is now working properly.

Thanks everyone for your help.

Pic of repair below:

 

[Malleus]

That's the most common point of failure on the 80's head/tail light subharnesses. I'd recommend checking all four corners for the same damage.