Author Topic: Brake light not working on K100  (Read 364 times)

Offline jande9

  • Curious
  • Posts: 7
Brake light not working on K100
« on: September 01, 2019, 01:02:25 PM »
Hi Everybody,
I have a very early K100, sn4196.  I've owned the bike for 6 years and it is completely stock.
The brake light has stopped working.  It is a separate bulb, single filament.
The bulb is good, tested on a battery.
There is about 6v at the socket constantly, which goes to 12v when the brake lever is squeezed.
The foot brake switch doesn't do anything.
The console isn't original and the bulb monitoring light is not on ever, even when starting.  The neutral light and gear indicator has never worked.  The console has the pinstripes which appeared about 1986.

I have thoroughly cleaned the contacts at the socket with sandpaper.

So, I don't get how there can be voltage but the bulb doesn't light.

Thanks for any insight because I'm stumped.

Jan

Offline bocutter Ed

  • ^ SuperNatural Motobricker
  • Posts: 661
Re: Brake light not working on K100
« Reply #1 on: September 01, 2019, 01:07:30 PM »
There's a good chance the wire from the foot brake switch is pinched. I'd check that first.
  • Toronto, Canada
  • '61 Puch DS60 - '66 Puch 250 SGS - '87 BMW K75s

Offline jande9

  • Curious
  • Posts: 7
Re: Brake light not working on K100
« Reply #2 on: September 03, 2019, 10:07:26 PM »
I traced the wire to the brake light switch by the pedal and it seems clear.

I'm thinking it may be a dirty connector because it does show 12 volts but not enough current will flow to light the lamp.

The contacts in the socket are aluminum which can be an issue but I scraped and cleaned them and that didn't help.

Jan

Offline daveson

  • ^ SuperNatural Motobricker
  • Posts: 480
Re: Brake light not working on K100
« Reply #3 on: September 04, 2019, 07:52:33 AM »
Yep, it maybe a dirty connector.

You got 12V but maybe not enough Amps.  Check the fuses are clean and tight.  Use a test light both sides of connectors and switches.  Or maybe better, just go ahead and clean them all anyway.

You might have better luck using an early model instrument cluster on your early model brick,  for your instrument lights
  • Victoria, Australia
  • Current;'86 K100RT Past; '97 Yamaha V Star 650 Yamaha V Star

Offline Laitch

  • ^ Quintessential Motobricker
  • Posts: 6932
Re: Brake light not working on K100
« Reply #4 on: September 04, 2019, 09:18:46 AM »
 :thisplacewhack
  • Along the Ridley in Vermont.
  • 1995 K75 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline jande9

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  • Posts: 7
Re: Brake light not working on K100
« Reply #5 on: September 04, 2019, 12:50:44 PM »
greetings...

id getts a 6v bulb...

j o

Hmmm.  My manual says its 12v.

Jan

Offline Scott

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  • Posts: 39
Re: Brake light not working on K100
« Reply #6 on: September 04, 2019, 01:06:20 PM »
Maybe a poor ground from the socket?  Have you tried a temporary (clip lead) ground to the socket to see it the bulb lights with the extra ground?  Might rule a ground issue, in or out.
  • SoCal
  • bmw R100r, bmw k75, hondapotamus G/W, XT 500 enduro

Offline jande9

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  • Posts: 7
Re: Brake light not working on K100
« Reply #7 on: September 04, 2019, 08:25:15 PM »
Maybe a poor ground from the socket?  Have you tried a temporary (clip lead) ground to the socket to see it the bulb lights with the extra ground?  Might rule a ground issue, in or out.

I tried that too.  The brale light shares a ground with the tail light, and the tail light works.   I think I will have to get at the bulb monitoring relay.

Jan

Offline Laitch

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  • Posts: 6932
Re: Brake light not working on K100
« Reply #8 on: September 04, 2019, 09:11:51 PM »
I think I will have to get at the bulb monitoring relay.
You might be interested in bypassing that relay for purposes of diagnosis, or permanently.
http://www.motobrick.com/index.php?topic=906.0
  • Along the Ridley in Vermont.
  • 1995 K75 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline beemuker

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  • Posts: 548
Re: Brake light not working on K100
« Reply #9 on: September 05, 2019, 10:26:04 AM »
I tried that too.  The brale light shares a ground with the tail light, and the tail light works.   I think I will have to get at the bulb monitoring relay.

Jan
If you are going to take the tank off, and the 100 has the same grounds under there that the 75 has, wouldn't hurt to clean the grounds that bolt to the frame under the tank. That was causing my turn signal whack.I missed it as there are a lot of wires covering it up.
  • Panama City, FL
  • 94 K75s , Dakar yellow ABS

Offline jande9

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  • Posts: 7
Re: Brake light not working on K100
« Reply #10 on: September 06, 2019, 06:31:20 PM »
So I popped the tank and had a look at the bulb monitoring unit.  It looks spotless and the diodes all seemed to work the way they are supposed to.  I tried jumpering the connectors in the socket as per that great link (Thanks!) and it didn't help as the brake light shows 12 volts when the handle is pulled but the light doesn't illuminate.

Hmmm.  I really dislike chasing electrical gremlins.

Offline Scott

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Re: Brake light not working on K100
« Reply #11 on: September 06, 2019, 09:41:08 PM »
So I popped the tank and had a look at the bulb monitoring unit.  It looks spotless and the diodes all seemed to work the way they are supposed to.  I tried jumpering the connectors in the socket as per that great link (Thanks!) and it didn't help as the brake light shows 12 volts when the handle is pulled but the light doesn't illuminate.

Hmmm.  I really dislike chasing electrical gremlins.
Have you tried a new bulb, to rule something else out?  Seems like it has to work.  Keep at it.
  • SoCal
  • bmw R100r, bmw k75, hondapotamus G/W, XT 500 enduro

Offline alabrew

  • ^ Proficient Motobricker
  • Posts: 133
Re: Brake light not working on K100
« Reply #12 on: September 07, 2019, 01:00:32 PM »
Going to have to have a look at mine too, it is flashing on and off on my bumpy road. Thought it was the bulbs, but changed both of them. Looks like I'll be doing some contact cleaning this weekend.
  • Alabama
  • 1985 K100, 1991 K100RS
Also:
2005 K1200LT
1979 R65
200,000 miles on BMW motorcycles

Offline jande9

  • Curious
  • Posts: 7
Re: Brake light not working on K100
« Reply #13 on: September 07, 2019, 01:07:23 PM »
Have you tried a new bulb, to rule something else out?  Seems like it has to work.  Keep at it.

It was the first thing I tried even though the old bulb worked when connected straight to a battery.

I know, it does have to work.  I'm perplexed.

Offline Laitch

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  • Posts: 6932
Re: Brake light not working on K100
« Reply #14 on: September 07, 2019, 01:33:08 PM »
If there is 12V on the meter but the lamp won't light, something is limiting current under demand. Try changing the fuse dedicated to that circuit and clean the fuse's pins and receiver sockets with emery cloth and contact cleaner. You haven't mentioned you have done that. Fuse defects can be hidden.
  • Along the Ridley in Vermont.
  • 1995 K75 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Online Martin

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  • Posts: 3872
Re: Brake light not working on K100
« Reply #15 on: September 07, 2019, 04:26:05 PM »
Alabrew a while ago a mate noticed that my brake light was flashing while leaving his property. It ended up being a non OEM brake spring on the rear master cylinder. The one fitted was too light so I just replaced it with a heavier one. The entry to his property is full of lumps and bumps along with exposed tree roots.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline jande9

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  • Posts: 7
Re: Brake light not working on K100
« Reply #16 on: September 07, 2019, 09:59:37 PM »
I was going over the wiring to the foot switch to check yet again that nothing was pinched, and I pulled the connector to see if that helped, and it did.  Without the footswitch in the circuit the lamp lights when squeezing the front brakes so at least I feel OK about riding it now.  The wire is clear and not damaged so I don't understand why that happened.
Thanks for everyone who pitched in their thoughts.

Jan
1983 K100.

Offline Stumpy

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  • Posts: 5
Re: Brake light not working on K100
« Reply #17 on: September 08, 2019, 05:57:02 AM »
Have you switched the wires, try plugging the brake light socket to the taillight if the bulb works then the holder is working . Same for the taillight to brake light, then you will know if it is the switch wires or the condition of the connectors.
  • Croydon
  • K100RT

Offline alabrew

  • ^ Proficient Motobricker
  • Posts: 133
Re: Brake light not working on K100
« Reply #18 on: September 10, 2019, 06:27:51 PM »
Thanks Martin, Iíll look it over. It was flashing on smooth roads before I changed the bulbs.
  • Alabama
  • 1985 K100, 1991 K100RS
Also:
2005 K1200LT
1979 R65
200,000 miles on BMW motorcycles

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