BMW K bikes (Bricks)

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1Back to top Go down   Tail Light Empty Tail Light Fri May 24, 2013 6:49 am


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Tail light failed on the Nerrandera trip. No alarm on the instrument panel. Checked the lamp, didn't work when 12V was applied. Bought replacement lamp. No joy. Removed lamp, measured open circuit volts at the lamp holder. 5½V. Perused Bert's diagram of the bulb monitoring unit. The first item after the light switch is what? Fuse?. Anyway, my understanding is that when the light is switched on, the current drawn by the tail light operates a reed relay which switches an earth to the transistor doo-dad in the dotted box. Being chronically technophobic these days, I don't really understand what happens inside the dotted box.

Any suggestions as to where the other 7 Volts are being lost?

Assumption is the root of all stuff-ups!

2Back to top Go down   Tail Light Empty Re: Tail Light Fri May 24, 2013 7:05 am


It is quite easy to remove and bypass the bulb monitor relay, I have removed mine. You can then eliminate if that is causing the problem. The removal instructions are on the web somewhere.


3Back to top Go down   Tail Light Empty Re: Tail Light Fri May 24, 2013 8:32 am


not all of them are reed relays ulrick ....

i have a couple here with resistance wire ( voltage drop principle )
Tail Light 2013-023
we have seen a few posts that suggest that the pc board solder has broken from vibration etc on the pins that connect to the loom in the bmu ( bulb monitor unit )

Tail Light 2013-024
we have also seen seats and shock absorbers pinch the cables before it gets to the rear globe some cases shortng out and blowing fuses

but also check the connector that sits in the rear cowl that goes from the wiring loom to the globe holders is a spade type multi pin connection ...(undoable connector ) they corrode up

you can see it plainly with the rear lamp remooved from the bike (sits vertically )

get out the deoxit again ?

does the warning light work as normal on startup ....then extinguish with the brakes activation ?

just had another thought

if you turn the key switch on to the park position ...what is the voltage then ?

if it is right it could point to some corrosion in the right hand switch or a connector under the tank no volts is getting to the bmu for it to work properly ?

Last edited by charlie99 on Fri May 24, 2013 6:30 pm; edited 1 time in total

cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O
'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
1987 k100rs ########   "Red"  - (red sports rs TWB style )
1989 K100rt #009637   "Black Betty"  (naked rt ala Nigel , now sporting an rs main fairing )

4Back to top Go down   Tail Light Empty Re: Tail Light Fri May 24, 2013 6:20 pm


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If all else fails, train a mouse to sit in the brake light with a box of matches. Attach the 5 volts you have to the mouse. Good luck.

1983 K100 basic vin 0003960 colour red  GONE
1987 K100RT vin 0094685 colour, orange peel, sorry, pearl..GONE
F800R black

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