K100RT Electrical Problem

[ramblin]

My 1985 K100 RT was running fine today before I changed the coolant. During this process I disconnected wires from the negative terminal of the battery, removed the necessary body panels, moved the fuel tank over without disconnecting the fuel hose. After connecting the battery wires, I had no power to the main switch. Further diagnosis showed that the problem was no ground connection as a jumper from the battery negative terminal to the frame restored power. The problem is that my Clymer manual electrical diagrem shows the battery negative connected directly to ground whereas my K100 has two wires (1 red, 1 brown) from the battery negative terminal disappearing into a wiring harness on the frame. Anyone have a suggestion as to where I should look for the problem?

 

[cjack]

My 1985 K100 RT was running fine today before I changed the coolant. During this process I disconnected wires from the negative terminal of the battery, removed the necessary body panels, moved the fuel tank over without disconnecting the fuel hose. After connecting the battery wires, I had no power to the main switch. Further diagnosis showed that the problem was no ground connection as a jumper from the battery negative terminal to the frame restored power. The problem is that my Clymer manual electrical diagrem shows the battery negative connected directly to ground whereas my K100 has two wires (1 red, 1 brown) from the battery negative terminal disappearing into a wiring harness on the frame. Anyone have a suggestion as to where I should look for the problem?

The K100 had a single ground wire from the battery neg to a place on the left side of the bike just below the battery area, I think screwed into a threaded hole on the side of the transmission. There were no other wires factory connected to the negative terminal.

 

[ramblin]

Thanks for the reply but my 1985 K100RT, serial number WBI051403F0052990, has two wires (1 red, 1 brown) from the battery negative terminal disappearing into a wiring harness running fore & aft under the upper right frame tube. The harness runs rear to the tail light and forward to a group of relays mounted inside the right side fairing. The harness appears to be original with no evidence of repair or modification. Also, it is hard to believe that someone would change it from a simple ground wire to something more complicated.

I can make it run by adding a wire directly from the battery negative terminal to the frame but was hoping someone could explain the wiring.

 

[cjack]

Thanks for the reply but my 1985 K100RT, serial number WBI051403F0052990, has two wires (1 red, 1 brown) from the battery negative terminal disappearing into a wiring harness running fore & aft under the upper right frame tube. The harness runs rear to the tail light and forward to a group of relays mounted inside the right side fairing. The harness appears to be original with no evidence of repair or modification. Also, it is hard to believe that someone would change it from a simple ground wire to something more complicated.

I can make it run by adding a wire directly from the battery negative terminal to the frame but was hoping someone could explain the wiring.

My guess is that this was an authorities bike or perhaps wired as such. Maybe even a demo by BMWNA. In the stock '85 K100RT, all of the relays were inside the relay box under the gas tank. Looking at the parts fiche, there is an authorities wiring harness which runs back to the tail area for extra police lighting back there and up to the front for the two tone horn sequencing and perhaps the blue and red police lighting there. I would guess that the horn seq unit and light relay would be somewhere under the fairing there. Do you also have the two police two tone round horns or the stock fiamm type horns?
In any case, if you don't have the heavier battery ground wire going to the left side of the transmission area, then you need one. This wire was replaced in later years with a yet heavier one. Apparently the original, about awg 10 wire, was deemed too light for the starter duty, etc. Our two '86 K100RSes had the lighter wire until we sold them a few years ago however. No problems.

 

[ramblin]

My guess is that this was an authorities bike or perhaps wired as such. Maybe even a demo by BMWNA. In the stock '85 K100RT, all of the relays were inside the relay box under the gas tank. Looking at the parts fiche, there is an authorities wiring harness which runs back to the tail area for extra police lighting back there and up to the front for the two tone horn sequencing and perhaps the blue and red police lighting there. I would guess that the horn seq unit and light relay would be somewhere under the fairing there. Do you also have the two police two tone round horns or the stock fiamm type horns?
In any case, if you don't have the heavier battery ground wire going to the left side of the transmission area, then you need one. This wire was replaced in later years with a yet heavier one. Apparently the original, about awg 10 wire, was deemed too light for the starter duty, etc. Our two '86 K100RSes had the lighter wire until we sold them a few years ago however. No problems.

Jack,

Looks like I may have the "authorities wiring harness". There are extra 2-pin and 4-pin electrical connectors at the rear under the seat and I always wondered what they were for. Also, there are two extra 6-pin connectors near the head light. The horns appear stock and there is no evidence of extra lights or attachments to the body. It was more likely a demo than a police bike as it had only 32,000 km on the odometer when I bought it in 1998. I'll poke around a bit more to see why it worked before and not now, but will add a heavier ground strap anyway.

Thanks for your help.

Jim