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Discussion Starter · #1 · (Edited)
Hello all!!

First post........

Today I fullfilled a dream. I bought a 85 K100RT from a buddy. I have discovered something that my buddy did not know..

It appears that the brake light is stuck in the "on" position. I checked the bulb and it is VERY bright and does not appear to be a double fillament bulb, next I checked the obvious with the switches on the front and rear brake, neither are sticking and appear to be clean...

What should I do??? I have this gorgeous new bike and cant ride it because I have no brake lights!!!!

HELP!!

Thanks, Tim

PS, can anyone advise the bulb # that should be in there? there are two differant ones, one is the licence plate light and the other is the brake light.

Thanks!
 

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Hth

Bowcop said:
… does not appear to be a double fillament bulb
My 87 K100RS had a dual element bulb there. I do know you can get a single element bulb (an 1156) into the socket, which by design will light brightly as the single contact will receive current from BOTH contacts in the OEM socket. AFAIK, 1157 is the bulb. Bulbs HTH.
 

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1157 is teh correct body, but .....

Bowcop said:
.... I bought a 85 K100RT from a buddy. .......It appears that the brake light is stuck in the "on" position. I checked the bulb and it is VERY bright and does not appear to be a double fillament bulb,....

PS, can anyone advise the bulb # that should be in there? .....
All K75, K100 ,K1100 BMW taillights are dual element. I'd second the guess that someone placed an 1156 in there. In addition to Mintaka's info...
  • Correct bulb = P21/5W
  • Will fit (as already posted), but slightly different wattage = 1157
  • Best bulb = 2357
Lots of illumination available here. :sun:

p.s. Don't use an LED unless it has side LED's. The straight on LED bulbs are crap in a "reflector" type of housig design. They look great straight on, but flat out suck from an angle. (However, I do have two NEW 1157 LED's for $5 plus shipping if anyone wants to get hit, or use them in a car.)
 

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Bowcop said:
Hello all!!
It appears that the brake light is stuck in the "on" position. I checked the bulb and it is VERY bright and does not appear to be a double fillament bulb, next I checked the obvious with the switches on the front and rear brake, neither are sticking and appear to be clean...


PS, can anyone advise the bulb # that should be in there? there are two differant ones, one is the licence plate light and the other is the brake light.

Thanks!
Are you SURE that the rear brake switch isn't screwed? I have an '86 K100RT, and got so sick of replacing the switch located by the rear brake peadal (where it always gets soaked), that I made a completely sealed electronic switch to replace it.

If both switches ARE OK, then the problem is almost certainly in the brake monitoring unit located in the electrical box under the back end of the tank. This device turns on the red warning light on in your instrument cluster (the one with the warning triangle and "!" in the center) ever time you start the bike. It only goes out when the electronics sense that you have successfully activated both brakes, and that the light has actually turned on in both cases. It also will activate when you are riding and the light fails to goon when you apply the brakes.

If the monitoring unot is shot, you can temprarily wire around it (you should be able to figure our how by looking at the wiring diagram), and used ones show up on Ebay from time to time.

One more thing. Be sure that you check the switches IN POSITION. These switches have a little plunger sticking out, that must be pushed all the way IN to turn off the brakelight. If things are improperly adjusted, and the plunger is pushed all the way in, the light will stay on.

The rear brake switches were such a problem that BMW later used a different swich located in an area that doesn't get exposed to the elements as much.

One more thing. The bulb most certainly **IS** a double filament type! One for the tail light and one for the brake light. If it isn't, then THAT is the problem! There is only one bulb back there, so it **MUST** be a double filament type, otherwise how could you expect it to have both tail and brake light functions? Maybe some bozo just put in the wrong bulb!

Bob.
 

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CABNFVR said:
Don't use an LED unless it has side LED's. The straight on LED bulbs are crap in a "reflector" type of housig design. They look great straight on, but flat out suck from an angle. (However, I do have two NEW 1157 LED's for $5 plus shipping if anyone wants to get hit, or use them in a car.)[/i][/color]
They probably won't work anyway.

This bike has a bulb monitoring device, and unless the LED brake light has a dummy resistor to allow it to draw as much current as a normal tungsten light, the sensor will probably not sense enough current and will "think" the brake light is burned out.

Bob.
 

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Agree .........

RFWILSON said:
They probably won't work anyway.

This bike has a bulb monitoring device, and unless the LED brake light has a dummy resistor to allow it to draw as much current as a normal tungsten light, the sensor will probably not sense enough current and will "think" the brake light is burned out.

Bob.
True, it's just that they aren't worth concidering in the first place. I did a side by side with the Ducati ST2 and the RS in the basement, and straight on the LED looks great. At an angle it's just awful. They make LED's with the resistor, but I'll stick to, and recommend, the 2357 bulbs.
 

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Discussion Starter · #8 ·
ARGGGG!!!!

I went and bought a 1157 and it does not fit, tried it both ways, the contact point at the bottom of the socket does not make contact with both points on the bulb.....

Any other ideas???

This is going to cost a small fortune!

Tim
 

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Discussion Starter · #9 ·
RFWILSON said:
If both switches ARE OK, then the problem is almost certainly in the brake monitoring unit located in the electrical box under the back end of the tank. This device turns on the red warning light on in your instrument cluster (the one with the warning triangle and "!" in the center) ever time you start the bike. It only goes out when the electronics sense that you have successfully activated both brakes, and that the light has actually turned on in both cases. It also will activate when you are riding and the light fails to goon when you apply the brakes.


Bob.
My light does come on and goes out as soon as I hit the back brake..... :confused:
 

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CABNFVR said:
True, it's just that they aren't worth concidering in the first place. I did a side by side with the Ducati ST2 and the RS in the basement, and straight on the LED looks great. At an angle it's just awful. They make LED's with the resistor, but I'll stick to, and recommend, the 2357 bulbs.
I agree totally. Some people get carried away with the gimmick of having LED brakelights, but inreality there is no significant advantage. And, no, the 50 to 100 milliseconds that LEDs come on faster than an incandescent results no significant decrease in stopping distance. At 30 MPH, in 100 milliseconds you will have travelled only 4 feet; a mere half a bike-length.

Bob.
 

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Bowcop said:
My light does come on and goes out as soon as I hit the back brake..... :confused:
OK, the electronic black box has determined that the switches are OK. It sounds as though the wtrong light is installed. Make sure it is a dual filament lamp like an 1157, or others that another poster mentioned.

Also check the lamp socket to ensure it is not damaged.

Bob.
 

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Discussion Starter · #12 ·
Ok, I tried the 1157 and it does not fit. Is there another bulb I should be trying? Does anyone know what the origianl equipment calls for?

The 1156 does fit, but is a single element.
 

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Bowcop said:
Ok, I tried the 1157 and it does not fit. Is there another bulb I should be trying? Does anyone know what the origianl equipment calls for?

The 1156 does fit, but is a single element.
Now THAT is really strange! THe tail lamp of a K100 is DEFINTIELY a dual filament lamp. I have a K100, and the wiring diagram from the factory clearly shows this. It won't work with a single filament. You need one for the tail light and a separate one for the brake.

Further, an 1157 should fit as I recall. I am at work now but when I get home I will look to be sure that what is in there is indeed an 1157 or equivalent.

Look in the socket to see if there are 2 contact points at the bottom that wouild make contact to a 2-filament lamp, or if there is only a single contact point.

If there is only a single contact point in the recess of teh socket, then someone has taken out the factory-installed socket and put in the wrong one. You MUST have a 2-filament lamp and socket to fir one in there.

I'll get back to you as soon as I can verify that there is in fact an 1157 in my bike.

Bob.
 

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Bowcop said:
Ok, I tried the 1157 and it does not fit. Is there another bulb I should be trying? Does anyone know what the origianl equipment calls for?

The 1156 does fit, but is a single element.
Go back to square one.
I had a '85 K100RT and there are 2 bulbs in the taillight housing. One is a 21 watt stop light and the other is a 10 watt tail light lamp.
Try taking out the turn signal lamp and using it in the stop light socket. It(turn signal) should be 21 watt also.
Remember when you turn on the key 1 lamp (tail) should be on all the time. If it is very bright, possibly the bulbs are in a reversed position( I don't remember if both the sockets were alike or not).
You need to find an old-fashioned auto parts store that has a Wagner lamp catalog that shows the lamp base and wattages of 12 volt turn signal, dome, tail light bulbs.
Bruce C
 

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Discussion Starter · #15 ·
Bruce,

There is only one contact point in the bottom of the socket, I can assure you that it is the original....

As for having them swithed or reversed, it can't happen, the small light won't fit in the upper hole.

I appreciate everyone's help.

Tim
 

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Bowcop said:
Bruce,

There is only one contact point in the bottom of the socket, I can assure you that it is the original....

As for having them swithed or reversed, it can't happen, the small light won't fit in the upper hole.

I appreciate everyone's help.

Tim
Damn! You guys are right, now that I think of it. There are 2 filaments, all right, but there ARE 2 separate bulbs!

Here's somthing to try......

- Switch on the ignition. The red warning light on in the speedo assembly should come on.

- Now, activate the front brake. The warning light should stay on.

- Activate the back brake. The light should now be off.

OK. So far so good. Now, reverse the procedure.

- Turn on the ignition. The warning light will be on again.

- Activate the BACK brake (reverse order as above). The light should still stay on.

- Activate the front brake. The light should now turn off.

So here's the thing. If, in EITHER of the above procedures, the warning light goes off by activating only ONE of the brakes, then the switch on the OTHER brake is stuck ON.

For example, if you hit the front brake first, and without activating the rear brake, the warning light goes out, then the REAR BRAKE SWITCH is stuck on.

Conversely, if you hit the rear brake first, and the warning light goes out before activating the front brake, the FRONT BRAKE SWITCH is stuck on.

Once I get home, I'll verify what lights are installed on my own K100.

Bob.
 

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Bowcop said:
Bruce,

There is only one contact point in the bottom of the socket, I can assure you that it is the original....

As for having them swithed or reversed, it can't happen, the small light won't fit in the upper hole.

I appreciate everyone's help.

Tim
OK. That definitely takes the dual-filament bulb out of the picture. A dual filament socket would have two contacts and the side pins on the lamp would be at different heights so that it couldn't be installed wrong.
Try swapping the brake lamp with a known working turn signal lamp. They should be the same wattage.
Does the tail-light bulb work?
Be absolutely sure that all lamps ,sockets and points of connection are good and corrosion-free.
When you first turn on the key does the red bulb monitoring lamp in the dash come on and stay on? It is supposed too until you operate BOTH the front brake lever and the rear foot pedal. This lamp monitoring device checks the stop light, brake light and BOTH brake light switches.
I really think that you have a stuck rear brake light switch. They were notorious for sticking.
Bruce C
 

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Discussion Starter · #19 ·
brucecha said:
OK. That definitely takes the dual-filament bulb out of the picture. A dual filament socket would have two contacts and the side pins on the lamp would be at different heights so that it couldn't be installed wrong.
Try swapping the brake lamp with a known working turn signal lamp. They should be the same wattage.
Does the tail-light bulb work?
Be absolutely sure that all lamps ,sockets and points of connection are good and corrosion-free.
When you first turn on the key does the red bulb monitoring lamp in the dash come on and stay on? It is supposed too until you operate BOTH the front brake lever and the rear foot pedal. This lamp monitoring device checks the stop light, brake light and BOTH brake light switches.
I really think that you have a stuck rear brake light switch. They were notorious for sticking.
Bruce C

Bruce,

The red warning light goes off after I hit the back brake, stays on if I hit the front brake first...

Just to let you all know, I am heading out of town for 10 days tomorrow morning, so I will check back here often and I will fiddle with it more when I get home.

I appreciate everyone's willingness to help a rookie out!

Tim
 

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Bowcop said:
Bruce,

The red warning light goes off after I hit the back brake, stays on if I hit the front brake first...

Just to let you all know, I am heading out of town for 10 days tomorrow morning, so I will check back here often and I will fiddle with it more when I get home.

I appreciate everyone's willingness to help a rookie out!

Tim
AHA!!! If you hit the back brake first, and even before you hit the front brake, the warning light already goes out, then your front brake switch is defective, or maybe it has crud in its plunger and the plunger is stuck in the "in" position.

The reason for this is as follows. If the front brake switch is stuck ON, then the system already thinks you have hit the front brake, since its switch is already indicating that you have the brake ON. You don't need to hit the front brake to make the warning light go out, because the system already sees the switch is on (even though you haven't activated the front brake). So all that the system needs to make the warning light go out is for you to hit the rear brake only.

It is also possible (but really unlikely) that the pair of wires leading to the switch in your wiring harness, are shorted together somewhere. But I doubt it.

I have had these switches apart, and the internal construction is such that an internal short (that simulates the switch sticking ON) is nearly impossible. When they fail, they almost always fail "open", which means that a failed switch will never switch on the brake light. Your problem is the opposite... the light won't go OFF!

I think that the most likely situation is that the plunger is stuck in the "in" position as a result of crud buildup. With your desctiotion of when the warning light goes out, it appears the problem is with the front switch.

Bob.
 
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