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I have an '87 K100 RS. I bought a BMW factory gauge and temperature sensor, but decided to build my own wiring harness.

Under the fuel tank, i found the optional gauge wiring connection -- its five wires, Grey/ blue, Green/ black (2X), Brown, and the yellow fuel level sender wire (not applicable for a coolant temp gauge). I removed the stock plastic connector and spliced one of the green/ black (power) wires, the brown (ground) and the grey/ black (gauge's light) into a new 4 pin connector that links to my temperature gauge. I installed my temp. sensor where my coolant drain plug had been, and ran its wire (black) as the 4th pin on my connector.

Brown went to brown, green went to green, grey went to grey, and black went to black. I double checked. My connections were insulated with high-temp electrical tape at every junction. As far as i could tell, all these connections were made good -- but something was wrong.

I turned the key, moved the ignition switch to on, saw the needle on the temp gauge jump to about 1/4 and stay there. tried starter:

nothing. no starter whatsoever. I think this is a clue, but i don't know why it happened.

then smoke -- the wiring i had introduced was melting.

I turned everything off and took it all apart (except the temp. sensor and wire to coolant drain plug... thats still on the bike). With the temp gauge uninstalled, the starter turned over and everything was back to normal.

Unfortunately (and perhaps very obviously) i know very little about automotive electrical systems. Maybe someone out there can guess what happened with the information i've already given, but i've taken some photos of the mess i've made, if not.


Of note is that the brown and green wires got the hottest: of the two, i would say that brown (ground) took the brunt of a lot of misplaced heat -- both at my fabricated 4 pin connector and at every splice i had made on the wires near it.

Why? There must be an obvious cause.

Also, the existing wiring harness under the tank has a "mysterious 'other' green wire" (noted in the photos) -- what is this? could i have confused the two?

Anyone recognize my mistake?
 

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