The other day, the bike stopped and didn't run fire more, dead battery. No red light on the dash while running but curiously enough, red light constantly on without keys inserted!

I had the fire the big bike Le Mans style, then once home I measured the tension on the battery while engine was running and gave about 9V.

Last week I have unmounted the starter and cleaned by blowing all the carbon powder but I don't see how this can influence.

Now trying to unmount the alternator but the screw on the bottom is very tight and can't lose it which is a real bummer. I wanted to test the brushes and if there is carbon powder generating funny gound around the generator.

Any ideas?

Bike is a 1986 K100RS, 35000km, 19Ah battery (a year old)

No need to take the alternator off just unplug it and if the light goes out the problem is in the alternator if it doesn't go out start unpluging fuses one at a time to find which circuit is causing the problem.

Thank you Rick, I've unplugged the alternator and the light went out.
I also checked the brushes and they seem in good shape to me, about 2cm tall.
Might be diodes now, I'll have to take the bike to a BOSCH service tomorrow (Monday)

With the ignition sw off I think the only way the charge light would come on is if the alternatator's diode board was faulty.

What's the voltage with the engine off after an hour or two rest (static voltage)? If its 12.45V or below your battery can't hold a charge or it hasn't been charged properly from the alternator.

What's the voltage with the engine running at >2k rpm (charge voltage)? If its below 13.6V your alternator is not charging the battery.

19Ah battery is a bit light imo.

Thank you cpt, I've done your test and I'm more and more puzzled now

12,66V with engine at idle rpm
13,55V with engine at 3000 rpm



Who can be the responsible then for that damn red light when key is off the dashboard?

What's the AC volts measurement across the battery terminals with the engine at idle and at 3000 RPM?

As you surmised earlier, it might be the diode board.  From a description of the Bosch Charging System

What happens when the engine is off and the key is in the off position? What happens when the key is in the on position and the engine is not turning? When a part fails, how does the idiot light sense this?  ...  A bad diode board can cause similar problem. If either the big diodes or the little diodes fail to produce the expected output, there will be a different voltage across the lamp. It will light, warning of the problem.

With the key off, if one of the big diodes is shorted, current can flow from the battery +ve, through the failed big diode, through the small diodes, through the charge light to ground through the wiring harness.

Thank you Rob, I hope tomorrow the BOSCH service will be at least a tenth as helpful and spot on like you are!
And also that won't be expensive to fix, I know a diode price is nothing but you never can tell.

sobonatovada wrote:Thank you cpt, I've done your test and I'm more and more puzzled now

12,66V with engine at idle rpm
13,55V with engine at 3000 rpm


Who can be the responsible then for that damn red light when key is off the dashboard?

Measuring the battery voltage at idle won't tell you anything useful but 12.66V seems more like its static voltage and is on the lower limit of OK.

13.55V at 3k rpm is not enough to charge the battery (battery starts charging at 13.6V) so your alt is not working properly. Red light when ignition is off indicates faulty diode board.

Apart from not charging your battery, a shorted diode will cause a parasitic drain discharging your battery even further

I changed to a lithium battery with resistor to replace lamp but my battery drains fast. Is it possible I have a diode issue in my alt also?
Also is there any way to increase the charging as 13.8 is the max I have seen but idd rather it was up over 14? Lithium and K100 may not have been a good idea but I wanted to hide the battery.

13.8 is not great but it should still be charging. There are a few things you could look at.

• A new voltage regulator. EME - BMW K Internal Voltage Regulator
  
• Check the size of your battery. Should be minimum 25Ah. If its smaller than this then a new voltage regulator would help.
  
• You might have excessive load from accessories or a parasitic drain.
  
• Poor earth connection
  
• Your B+ cable is probably fairly long with the bat in the tail so I'd check the resistance on that. Looks hefty enough tho.
  
• If the resistor you replaced the charging light with is too big or its gone 'open circuit' it might prevent the alternator from energising at startup and you may get little to no output from the alt. One of the little BMW quirks. It requires a small current to get the alt to output at startup. You could test for continuity and impedance on that circuit. Someone else may have a better idea about that.
  
• Test the alt for bad diodes but I doubt that's the problem. I've not done this test but here are instructions
  

If you're having to top the charge up, be careful how you do that. Li-iron batteries are a bit finicky when it comes to abttery chargers.

Kaptain Holister wrote:13.8 is not great but it should still be charging. There are a few things you could look at.

• A new voltage regulator. EME - BMW K Internal Voltage Regulator
  
• Check the size of your battery. Should be minimum 25Ah. If its smaller than this then a new voltage regulator would help.
  
• You might have excessive load from accessories or a parasitic drain.
  
• Poor earth connection
  
• Your B+ cable is probably fairly long with the bat in the tail so I'd check the resistance on that. Looks hefty enough tho.
  
• If the resistor you replaced the charging light with is too big or its gone 'open circuit' it might prevent the alternator from energising at startup and you may get little to no output from the alt. One of the little BMW quirks. It requires a small current to get the alt to output at startup. You could test for continuity and impedance on that circuit. Someone else may have a better idea about that.
  
• Test the alt for bad diodes but I doubt that's the problem. I've not done this test but here are instructions
  

If you're having to top the charge up, be careful how you do that. Li-iron batteries are a bit finicky when it comes to abttery chargers.

Most of your list checks out.
Battery is only 18 though and Lithium batteries are even lower rated for small draw.
Standing draw is 7 milliamps .
Charger is a Lithium battery minder.