Hi all,
The story.
I rode the bike yesterday for several hours, no problems, never got a battery light. Today I go to start the bike for work and it turns over but very weakly. I take off the side covers and take a voltage measurement at the battery terminals- reading 12.45, try to turn the bike over once and the reading drops to 12.06. Jump it with the car and bam!, starts right up. The battery is a sealed gel unit. While it is running at fast idle about 1400 rpm I measure voltage at the battery terminals and I am getting 11.5, 11.3.

Question. Do I need a new alternator? I don't want to buy one unless I have to.
Cheers and Thanks

First, you should open the alternator and check the brushes. If everything looks good, you should check the voltage regulator. The cost of the voltage regulator is about $100 and the BMW reference # is 12311459286
A new alternator cost over $1,000 + the core
Check these 2 sites for the regulator:

RealOEM : $116 + core
BMW Motorcycles of Chattanooga: $96

CF

How do I check the regulator?

Hi there, regulator at http://www.euromotoelectrics.com/category_s/60.htm.
Best regards
JAP

lhead71 wrote:How do I check the regulator?

Check this page. This test is for an Audi alternator, but they use the same type as the K100 at the exception that they are more powerful (110Amp). The K100 alternator has 9 diodes when the more powerful one for the Audi has 14.
This gives you a good idea on how to test it.
Unless the winding or the brushes are shot, if anything else goes wrong, it has to be the regulator.
If you don't have the capability of doing the test on your own, a specialized automotive shop should be able to do it for a fairly cheap price.

CF

i head ...before you do anything .....the voltages you said dont add up

the start should have pulled the voltage of the battery below 12 volts ...
which makes me think that you have a high resistance earth cable .....the additional voltage and current from the car battery boost would be masking this issue.

the high resistance earth might be also an issue during recharging the battery after getting the bike running .... as you say the voltage is low ... after starting it .

its a cheep but neccesary check that you should do .

just undo the bolt holding the earth and see if there is corrosion there ...if so ....file it off ...(possibly replace the bolt and washers) and maybe also the cable itself if there is corrosion evident in the cores of copper as it crimps at the lug ...(check both ends)

hope this helps

Ihead, for interest...measured volts at battery:

pre-start: 12.5V
during cranking: dropped down to 10.5V.
at 2000rpm: charging at 13.9V

I believe these figures are reasonably normal. The battery is about 2 years old, and is a standard wet cell.

I would go with Charlie said, it is an earth problem. The voltages you quote tell me that.

But also check the regulator and brushes because having low voltage at high idle says more than earth to me.

The brushes can be replaced, just unsolder and pull them out as the wire is only crimped in lightly then drill the hole with a 1.6mm drill (1/16in) then you can solder the new ones back in. Make sure the length of brush is right.

Would pay to get an auto electrician to test the armature and machime the sliprings if needed.

I believe it might be an earth problem now as well. I placed a trickle charger on the battery and when I took it off, the voltage read 12.75volts. WHen I started the bike and ran it at 1400 rpm it was charging at 13.8 volts. So that tells me the alternator is working as it should.
Since this bike is new to me, I don't know if any of the grounds or connectors have been cleaned. I got a can of Detoxit and I am going to start with the ground connectors.

thems perfect numbers ric .....cheers mate

I was looking for electrical parts (starter relay) and alternative suppliers (Bosch/BMW are very expensive...)
First, I was reading the following: Bosch no longer manufactures relays; they are all made by Tyco and private labeled for Bosch by Tyco.
A quick search on the Internet shows that Tyco sells these relays to other suppliers. They are not marked Bosch, but their specification are the same.
BMW/Realoem sells the Tyco/Bosch model for 87.18 +core
Euromotoelectric sells the Tyco/Bosch model for $66
Wiringproduct sells the Tyco/Bosch model for $37.74
Tessco.com sells a 75A Tyco relay for $27.40
Pcsconnectors sells a 100A Tyco relay for $29.99

I have not checked alternative suppliers for the alternator parts, but I am sure that we can find after market parts a lot cheaper than the original Bosch.
A little bit of shopping can save you a pile of $$.

charlie99 wrote:thems perfect numbers ric .....cheers mate

Yeah I reckon I would smile at those figures.

So a new item this morning. The battery was left on a charger all night, I took it off this morning and tried to start the bike, no go, weakly turned over. Took a voltage read on the battery and it read 10.5 volts. Put the battery tender back on and re-read at 12.5 volts right away.

I have a voltage leak that is draining the battery. I haven't cleaned the grounds yet but I plan on doing that tomorrow.

Where to start?

eeeeeekik !!!

Looks like your battery is unable to hold a proper charge.

nods head ....that or there is some kind of losses happening rene ....
but im happy to go with the battery being "past it"

in any case ...if there has been an issue with the charge ...and start from the battery there is most likly a reason ....and i guess we should correct that reason before we invest in another battery ... and letting it happen again ....in some cases we really dont know all the history of past owners woes and tribulations .

I remember that a long-time mechanic mate of mine used to scratch his head and mockingly say in a friendly manner to his customers when they brought in their car whenever a problem had arisen. "I dunno what happened, it was working fine yesterday!" And that's the story of mechanical things.

It sounds to me exactly like my battery woes several months back. One day all was well (perhaps a slow crank now and again) then all of a sudden I got no-go. Whacking a voltmetre on the battery told me it had 12.5 volts but when cranking she dropped to well under 10 volts. Charging the battery with a slow, 1.25amp charger brought it back to life, I could start it, and running the alternator would pump out sufficient juice to appear to be doing its job, which it was. In the end it was the battery's sudden inability to be able to hold a charge that was the culprit. The terminals and cables were clean but I cleaned and refreshed them anyway (I also added a larger earth cable). A new battery was popped in and now we're laughing.

I reckon you have a bad connection internaly in the battery which is giving you the same symptoms as a bad earth. Get a high discharge test done to make sure and if the battery is covered by warranty and the bike has not been dropped or pranged have a go at the manufacturer but either way a new one seems to be the order of the day.

I would recommend a new battery. As we all know, starting the bike at low voltage can be very costly and creates other electrical problem.

I replace my battery with a sealed 45 AH and works well ever since.

Cheers.

Bought a new battery and she is running great now. I am trying to return the other to the bike shop.

Hi all,

Had similar faults on my RS, having just rebuilt her - we went to France for the weekend. On the return journey, we noticed the alternator charge light was glowing very dimmly at about 2000 rpm or over, caused a slight missfire at times but still kept running and powered all lights etc. I suspected the rectifier was failing as the battery was quite new, upon further checking when home tho' , we found corrosion in the push connector to the back of the alternator and corrosion at the frame end of the battery earth lead, cleaned up and replaced the lead and all is now well. I see a new alternator can cost a $1000??? crikey I think in the uk we can get new ones for about £150 or recon for less than £100 or secondhand for whatever we can haggle too. Good luck.

"the alternator charge light was glowing very dimmly at about 2000 rpm or over"

This happens to nearly all K 100 bike. This is due to return current. I am sorry I do not remember the exact explanation to this. It happens to my bike too until now, but the bike is running well.

Bert had explained in one of the topics in this forum. He may read this topic an give you the best explanation.

Regards.

Not all K's have a dimly glowing alternator light above 2000rpm...

My alternator light is on quite brightly at all RPM, but the system voltage is correct and the battery charges fine.