Tail Light not working after short

[Brick_hack]

1997 K1100LT in Ohio.


I am in the midst of rewiring the sidecar for my rig. I used a 5 pin trailer wiring harness found at the auto parts store. The tail, 2 indicator lights and ground wire are the ones used for the sidecar. The fifth wire I ran to the red terminal on the battery via a 10amp fuse. The tail light is used as the tail light, the brake light is going through the left turn signal wire on the trailer harness (not used for a turn signal in the sidecar), the right turn wire is right turn on the hack, and the ground is a ground. Now, the PO installed a sidemarker light on the hack and apparently one of the wires shorted somewhere unbeknownst to me until I hooked this up. To wire in the side marker light, I spliced the ground into the ground on the bike via the trailer harness ground, and wired the + to the tail light via the trailer harness. When I turned the key, fuses 1 and 2 popped.


I removed the bulb, replaced the fuses with 15 amp fuses, and everything lit up. Then I fiddled around with the light again trying to find the short. I put the bulb back in and only fuse 1 popped. I removed the bulb, replaced the fuse, turned the key and everything worked fine. So after checking for any shorts, I put a bulb back in the light and the tail light would not turn on. None of the fuses are blown, but the tail light will not come on.


I checked the voltage for the tail light at the socket, at the plug before the socket, and at the fuse block (spot #1). There is a reading of 2.44volts everywhere I check it. Does anyone have any idea what happened and how to fix it? I am really frustrated and don't know how to proceed from here.

[Scott_]

Fuse 2 is fed from the Ignition switch,
Fuse 1 is fed thru the ignition relay located in the fuse box, and, the ignition relay is fed from fuse #5(direct battery).

So you either burned the contact in the relay, or you also popped fuse #5.

[Brick_hack]

Fuse 2 is fed from the Ignition switch,
Fuse 1 is fed thru the ignition relay located in the fuse box, and, the ignition relay is fed from fuse #5(direct battery).

So you either burned the contact in the relay, or you also popped fuse #5.

Thank you for the concise response. Wiring diagrams are not my forte, and I'm color blind so I'm really disadvantaged here. Is there an image of which relays are which? I have the tank off and the fuse box open and am looking around. I am just not certain what I'm looking for. All of the fuses are good (I changed them all and checked them) so I suspect I killed a relay. Where would I find a new one, or how do I check the relay? Thanks again for the help.


Alex

[Scott_]

From the Tech Library here.
http://www.motobrick.com/index.php/topic,536.0.html

ABS2 bike, it would be the middle relay closest to the middle of the box.

[Brick_hack]

I just checked, all of the fuses are intact. Thank you very much for the advice and for the resource. Is the relay a special BMW part, or is the relay a generic part?

[Brick_hack]

Also, I don't know if it has any bearing or not, but the motronic "brain" was not hooked up when this happened.

[Laitch]

. . . apparently one of the wires shorted somewhere unbeknownst to me until I hooked this up.
Then I fiddled around with the light again trying to find the short. So after checking for any shorts, . . .

Exactly which methods were you using to check for or find shorts?

[Brick_hack]

Exactly which methods were you using to check for or find shorts?

Good morning, I traced the wires from where I soldered them into the harness back to where they connected to the light bulb. There were a couple of places where the insulation had been rubbed away against the fiberglass of the hack. The hot wire was connected to the trailer harness wire that goes to the tail light wire on the bike. The ground wire was connected to the ground wire on the trailer harness that connects to the ground circuit feeding the rear section of the bike. I am planning on removing that ground path and grounding the trailer harness to the bike rather than the electrical circuit on the bike though.


I also checked the continuity between the +/- of the offending marker light, and then of -/+ with the bulb disconnected. There was continuity between them indicating there is a short somewhere.


Chris Harris suggested the bulb monitoring device (BMD) may have been fried, but admitted that wiring was not his strong suit. I ordered a used BMD on eBay.


If I missed something else, please let me know.

[Laitch]

Chris Harris suggested the bulb monitoring device (BMD) may have been fried, but admitted that wiring was not his strong suit. I ordered a used BMD on eBay.
If I missed something else, please let me know.

Maybe that's it but diagnosing by part replacement can get costly. The upside is that it creates a spare parts inventory. You could bypass the BMU to determine what its role is. Here's a little info.

[Brick_hack]

I agree it could potentially get costly, but I suppose on the other hand it's not a terrible thing to have a couple of spares. The other thing is that the PO had done an utterly reprehensible job of the wiring, so who knows if they did something to weaken the system before I came along...

[Brick_hack]

The problem was the BMU. One of the internal fused connections had blown. I disconnected the ground on the hack harness that was spliced into the stock wiring harness and routed it up to the ground wires under the tank. Now only the LED tail light for the hack is being run off the bike. The LED light bar and the amber marker light are on a separate, isolated circuit. I'm going to try and repair the other BMU and keep it for a spare.

[Laitch]

The problem was the BMU.

Thanks for the followup.

[Brick_hack]

Here's the BMU, you can probably easily identify the problem!

[Laitch]

Peanut M&Ms shouldn't contain white chocolate!

[Brick_hack]

Peanut M&Ms shouldn't contain white chocolate!

You're not wrong!