Alternator Idiot light dim glowing while down shifting and running above 3k RPM

[petermarek]

 :dunno

Folks I have just taken a night ride for the first time since getting my K100RT running. With it being dark, I noticed my alternator light glows dimly (but varies - so it not light ghost light from other instruments) when running in 3rd/4th or 5th above 3k RPM. The lamp is off at idle.

I don't have a voltage gauge, but am guessing I have a bad voltage regulator.

Does anyone have any opinions or advice?

Thanks

[K75RT Keith]

Do you have LED's in the cluster?  If you do, that is reason.

[Motorhobo]

Hey Keith -- I didn't know that...can you give some details about why that is? Is that because an LED requires less power to throw visible illumination? I still have the original lights in the cluster but one of them is dark and I'm thinking about LED replacement.

[Scott_]

The circuitry for the alternator light is designed for an incandescent lamp that will allow 2-way current flow. An led lamp is only designed for 1-way current flow.

[rbm]

Scott, Keith mentioned LEDs in the cluster to illuminate the dials (in place of globes).  He's not referring to a LED replacement for the charging light. 

BTW, BMW service bulletin (number 61 021 89 (2354), issued April 1989) recommends inserting a diode in the charging circuit to eliminate glowing charging light.  And this bulletin was issued to dealers before the popularity of LEDs, so there must be another explanation.  Maybe leakage through the diodes in the rectifier.  In any case, using a LED in place of the globe will accomplish the same end as inserting a diode.

[petermarek]

Well....not really. I have put LEDs in parallel with the Brake light for more brake pop. I have put LEDs in parallel with each turn signal. I also have white LED driving lights in line with the "Parking light". All the original bulbs are in their original sockets and the BMU is not complaining. The cluster is all original incandescent bulbs and all are working... Of nte...only the LED White driving lights are (additionally) on during normal riding. The others are only on when braking or turning.

Thus the "exciter" alternator idiot light is still fine. Having talked to a few friends, the consensus was that the Voltage Regulator/Brushes was a worth while (risk/experiment) replacement. I tended to agree after much research and adding in the fact that at idle I am charging fine.

I pulled the VR unit and manage to find the part (exact number match) below at an acceptable price.

http://store.alternatorparts.com/ib350-359102-voltage-regulator-brush-holder-assembly-for-bosch-alternators.aspx

I'll reply again with results but it'll be about a week I think....

Thanks and other thoughts are still appreciated!

[petermarek]

Just saw RBMs reply...I  bet your right. I'll pull that bulletin and see about the diode if the new VR doesn't resolve the issue. I'm also getting a cheap voltage gauge so I can monitor this better.

Hey...$35 for the new VR and it's replacing the original (1985) part....I'll just go with "paranoid replacement" if it doesn't solve the problem....  :dunno

Thanks.

[The Mighty Gryphon]

I put LEDs in my instrument clusters, but pulled the red alternator and oil pressure LEDs and put back the original bulbs.

The LED in the alternator light didn't pass enough current to excite the alternator until the engine was revved to 4000+rpm.  It also glowed dimly at night which was more annoying than anything else.

The oil pressure LED had a habit of lighting up every time I rode in the slightest amount of rain.

[beemrdon]

The cheapest & easiest thing to do is strategically place a piece of black electrical tape over it.