UPDATED: Tail light help

[smwmustang]

I am reviving this thread again. We have the same issue with no tail lights and just need additional advice. Here is what we have done this time.
1. disconnected all rear tail lights and checked bulbs and sockets - tail light fuse still blew
2. disconnected all front lights - fuse still blew
3. trailer plug is still disconnected so should not be an issue - unless I have missed something here
4. we loosened the fuse panel on driver footwell and got it out enough to pull the tail light relay and the fog light relay as fog lights out too. New Denso relays coming in Tuesday.
5. Also disconnected the dash light dimmer that was the issue before. Strange thing is that the dash light still work and it says they are on that fuse.

If it is not the relays all we have left to do is pull the aftermarket radio and check the wiring there. It has been in for a couple of years now, but could have a wire that came loose I guess.

Anything I am missing here. Running out of ideas from the forum.

TIA

 

[Dirt J]

Which fuse is blowing? And its blowing immediately when you insert it into the fuse holder with the key off and no foot on brake pedal?

 

[smwmustang]

Which fuse is blowing? And its blowing immediately when you insert it into the fuse holder with the key off and no foot on brake pedal?

The tail light fuse - a 10 amp fuse (could be a 15 amp as I am not at home. It is blue in color) and it blows immediately.

 

[Dirt J]

34 here?

The FSM and owner's manual for the 2000 MY shows 15 amp:

 

[Dirt J]

Regardless, once you get which fuse is in question, check both terminals of the fuse holder for 12v. Only one should have 12v.

Once confirmed, remove the tail light relay, and then insert the fuse with the relay still removed. Report back.

It sounds like a dead short somewhere down the line.

 

[smwmustang]

Regardless, once you get which fuse is in question, check both terminals of the fuse holder for 12v. Only one should have 12v.

Once confirmed, remove the tail light relay, and then insert the fuse with the relay still removed. Report back.

It sounds like a dead short somewhere down the line.

Your are correct, it may be a 15 amp as I am not at home. If I plug in an LED on a two wire home made tester it stays lit and we searched for a short everywhere. Obviously due to where the relay is, it makes it almost impossible to test with it plugged and unplugged as you have to disconnect so much from the fuse panel mount to get to the relays. I am willing to try anything at this point. Two tickets for my son in a week. Both warnings, but now out of time and it is just dangerous anyway.

 

[jaewastooshort]

once you find the 12V pin for the fuse, do a continuity check for the other pin to ground. that will at least tell you if you have a chaffed wire somewhere that's making contact... past that you'd probably have to just start locating any connectors along the wiring path and test for ground at each side of the connection to try to isolate where the short is.

 

[smwmustang]

once you find the 12V pin for the fuse, do a continuity check for the other pin to ground. that will at least tell you if you have a chaffed wire somewhere that's making contact... past that you'd probably have to just start locating any connectors along the wiring path and test for ground at each side of the connection to try to isolate where the short is.

Thanks, it is a pain for sure as wiring is not my expertise...but I am getting better.

 

[smwmustang]

Update: relays did not come in so I went ahead and put the old one's back in (taillight and fog light) and then found the trailer wiring module behind the driver rear light and took it out. I put a new fuse in with the trailer wire completely disconnected and connected back without the module and the fuse still blew. New relays now coming today and I will see if those help at all. Running out of options.

 

[Dirt J]

Regardless, once you get which fuse is in question, check both terminals of the fuse holder for 12v. Only one should have 12v.

Once confirmed, remove the tail light relay, and then insert the fuse with the relay still removed. Report back.

Did you try either of these?

 

[smwmustang]

Did you try either of these?

Not yet but reading again now I understand what you are saying and why it matters. Sorry, I am learning the wiring as I go but now understand why what you put matters. I will try that as well as it is easy to pull the relay out with it all still taken apart.

I assume if without the relay there should not be a 12v showing or if there is then it is not the relay? I understand why to test but what will I be looking for when doing so.

 

[smwmustang]

Regardless, once you get which fuse is in question, check both terminals of the fuse holder for 12v. Only one should have 12v.

Once confirmed, remove the tail light relay, and then insert the fuse with the relay still removed. Report back.

It sounds like a dead short somewhere down the line.

Only one port has 12v. With relay removed it still has power to that port. With an ohm reader it reads .98 -1.05 with and without the relay in. I have unplugged every wiring bundle from the fuse panel plate/bracket with all readings staying around d .98 - 1.05. Fuse blows immediately and I have connected a test light that stays lit as we disconnect each light at the connecter and not bulb socket. Only time light ever flickered was messi g with the headlamp bundle of wires in front passenger side near fender and behind the headlamp. It only did it once and cannot be recreated.

Any suggestions welcomed. I have ordered a short tested that puts out a tone to see if that helps at all.

 

[Dirt J]

Remove the tail light relay, and then insert the fuse with the relay still removed. Report back.

 

[smwmustang]

Remove the tail light relay, and then insert the fuse with the relay still removed. Report back.

With relay out the fuse does not blow

 

[Dirt J]

Ok that's good. The relay is the next thing in the circuit after the fuse.

If it were me, I'd remove all 10 bulbs that are on this circuit and test each socket. Either by installing the bulbs one at a time (and then removing each one), or check for continuity at each socket. The former will cost you some bulbs and fuses.

Looks like 4 rear bulbs, two license plate bulbs, and 4 front side marker bulbs. My hunch is this will lead you to a corroded or otherwise messed up socket or a pinched wire. This will narrow it down to which wire.

Make sure the towing wire harness is still unhooked.

 

[smwmustang]

Ok that's good. The relay is the next thing in the circuit after the fuse.

If it were me, I'd remove all 10 bulbs that are on this circuit and test each socket. Either by installing the bulbs one at a time (and then removing each one), or check for continuity at each socket. The former will cost you some bulbs and fuses.

Looks like 4 rear bulbs, two license plate bulbs, and 4 front side marker bulbs. My hunch is this will lead you to a corroded or otherwise messed up socket or a pinched wire. This will narrow it down to which wire.

Make sure the towing wire harness is still unhooked.

So basically check each socket just like you are checking the fuse location to see if it has a reading? Sorry still learning and figuring out the meter. Appreciate the responses.

 

[Dirt J]

Yes. Remove all the bulbs first, then put the relay in. Hopefully the fuse won't blow. But you have to remove all 10 bulbs for first. If it blows with all the bulbs out, when the relay is inserted, then there is an issue with the wiring somewhere. Can still track that down by checking continuity.

 

[smwmustang]

Yes. Remove all the bulbs first, then put the relay in. Hopefully the fuse won't blow. But you have to remove all 10 bulbs for first. If it blows with all the bulbs out, when the relay is inserted, then there is an issue with the wiring somewhere. Can still track that down by checking continuity.

Ok narrowed it down to this connector that plugs in at top of fuse panel next to a yellow connector. I have taken readings on all wires and appears maybe its a black wire with blue dots on it. Disconnected everything i can at main plugs in rear with no change in resistance. May have to pull driver cargo trim to see the full harness. With this unplugged the fuse did not blow and brake lights did not work. I left unplugged by accident and it blows fuse when plugged back in

 

[Dirt J]

The only wire in that connector that is on the taillight circuit is the green wire with black stripe. It is the ground path for the taillight relay through the combination headlight switch.

When that wire is grounded and and the fuse is in, it tells the relay to turn on.

 

[smwmustang]

The only wire in that connector that is on the taillight circuit is the green wire with black stripe. It is the ground path for the taillight relay through the combination headlight switch.

When that wire is grounded and and the fuse is in, it tells the relay to turn on.

So could the headlight combo switch be bad?