M103A3 (M105) Brake Light Issue

[mkcoen]

Read the TM and it's advising replacement of the wiring harness!

The issue is all lights working except the brake lights but when brakes applied the tail lights go out (still no brake lights).

I checked the bulbs first then I checked the continuity - fine. Power to the socket, plug, etc. If I take the tail light out and then make the connection with a multimeter the brake lights work. Next thought is ground but if everything else is working and brake light works making the tail light connection how could it be the ground?

I HATE electrical issues!

 

[emmado22]

Bad grounds... The tail lights ground thru the bolts that hold them on.

Take off the tail lights, clean the metal on the back of the tail lights where the bolts are, and the holes on the frame (both sides), and clean the bolts/star washers. Also clean the ground wire where the harness is on the front of the trailer. That should fix it. Bad grounds cause all sorts of problems.

 

[WillWagner]

High resistance grounds cause all kinds of goofy problems.

 

[jasonjc]

Read the TM and it's advising replacement of the wiring harness!

The issue is all lights working except the brake lights but when brakes applied the tail lights go out (still no brake lights).

I checked the bulbs first then I checked the continuity - fine. Power to the socket, plug, etc. If I take the tail light out and then make the connection with a multimeter the brake lights work. Next thought is ground but if everything else is working and brake light works making the tail light connection how could it be the ground?

I HATE electrical issues!

Bad ground. The taillights could be pulling a ground thought the brake lights. So when the brakes should come on there is no ground now so the taillights go out.
Yes elec stuff will drive you nuttssssss

 

[Barrman]

Yep, as written above. Grounds. Do you have the star washers on both bolts of both lights? They really do make a difference.

You want to freak yourself out, try this. Loosen the mounting bolts, have only a single wire out of the 4 hooked up. Then, turn on whatever that wire is. There is about a 20% chance another bulb besides the one hooked up will come on real dim. I don't know why, just that bad grounds cause weird problems.

 

[mkcoen]

RESOLVED!!!

Thanks for all the quick responses. I guess it would have helped knowing in advance that the tail lights grounded through the housing.

Both tail light housings on this trailer are plastic. There were NO star washers on any of the bolts and the bolts were throughly rusted (lock washers were fused to the bolt head with rust). Four new bolts, four new star washers, a little scrap and sand action and I now have brake lights.

I hate electrical issues but I do love this site and the knowledge that's available here. Hopefully the title is enough to help someone else with this problem if they do a "search" first.

 

[emmado22]

The plastic ones ground thru the bolts also. Except for the all LED Grote/Trucklight versions. They have their own seperate ground wire.


Clean up the ground wire that comes off the harness that bolts to the trailer frame also!

 

[runk]

For future searchers, don't forget the bulb bases and sockets ! The mil lights are a lot better sealed then civ tail lights, but a little corrosion in the socket connections causes the same symptoms (this is a classic British car problem).

 

[DAP]

Did you build up your own wire harness? Just curious.