My Bosch Rotor Needs to Magnitized?

[Semper Gumby]

Hmmm Monte circuit uses a 330 ohm resistor....

[georgesgiralt]

Hello !
There is a mistake on the various fiche I've looked and they corrected it after a while. 1.2 W is correct wattage for indicators like neutral or high beam or even the oil pressure. But for the gen lamp you need 3 W or more.
So often when one dismantle the tacho for cleaning the wrong bulb get in the Gen position... And problems or funny things start to appear. I bet you may have a 3W bulb in your tacho; somewhere ..
These 5W bulb are often used for parking light on Japanese motorcycle and some BMW too. so you may find one at a Jap dealer... Try Honda. You can get any part of a Honda bike by them. (they even helped me fix my Triumph brake problem, once).

[Barry]

Hmmm Monte circuit uses a 330 ohm resistor....  

That's because the LED bypass resistor doesn't set out to replicate the current draw of a 3W lamp, if it did a high wattage resistor would be needed and it would get too hot. In fact it doesn't even replicate a 1.2W lamp. So the LED/bypass resistor combo passes a lower current to the rotor which by all accounts seems to be enough even though it ought not to be.  

All of which is not promising for a 3 W lamp resulting in a cure for your problem.  You still got to try it though. When I suggested earlier to substitute a 47 ohm resistor it really is only good for a temporary test because of the excessive heat.

[Semper Gumby]

So a good test in my case would be to put a bulb holder out side of the tacho case and run a 3 or 5 watt bulb in it to see if the alternator lights off earlier...that I can do.

[Barry]

Georges,

The different gen lamp ratings of 1.2W vs 3W for early/later bikes came from the respective riders handbooks which should be correct but who knows.

[Semper Gumby]

You are not going to believe this.

I read the Snowbum's article on this subject

http://www.bmwmotorcycletech.info/genlampresistor.htm

Not having the right resistor at hand but having a spare bulb holder and wires I rigged up my holder using Method #2 between the hot side of the coil and the DF+ wire into the regulator (Blue Wire),
 I've got both lights off at 1250 rpms using a GE90 bulb.

Now I've got to figure out how to make this permanent.

The BMW 1.2 watt bulb doesn't provide enough current to light off the Omega 400 watt alternator!!!  Holy Freekin Sh....

So what I really need is a 1981 or later Tachometer so I can put the higher wattage bulb in it.  Anybody got one laying about?

So George My Humble apologies - you were right.  But a higher wattage bulb for my 1980 Tacho doesn't exist until I can get a later model Tacho.  Right now the extra GEN bulb is zip-tied to the air cleaner on the RH side.

[marcmax]

GEN lights on the air cleaner. Next thing you know you'll have neon tubes under the oil pan.  [smiley=bmw_smiley.gif] [smiley=laugh.gif]

[Semper Gumby]

Actually I was thinking about what would be a nice 5-7 watt load while stopped at a traffic light.  It's got me thinkin' - At least until I find a later model Tachometer.  

Seven watts of Neon?  More fun than a stuffy old resistor..  That could be a fist full of LED's coming on at idle.

Hmmm  

[Bob_W]

I will preface this saying I fixed, designed, built and installed electrical stuff for a living for many years. Also I am only on my second cup of coffee and will thus eliminate the math.
Electricity is generated by a wire cutting a magnetic flux line. In our alternators the magnet is in the rotor and the wire is the stator. Bigger magnet and more wires means more electricity generated. Cutting more lines of flux faster (higher RPM) means more electricity.
Euro Moto Electrik sells a permanent magnet alternator. Think refrigerator magnets. They do not need remagnetizing. The
EME system has no brushes.
The Bosh system uses brushes to connect electrical current to the electromagnet in the rotor. The generator light allows a small current flow to the rotor when the switch is turned on causing the alternator to start generating power.
The light going out means there is power being generated, but a digital meter on the battery will indicate the voltage. Until the voltage being generated exceeds the battery voltage, the system is not charging the battery. A 100 watt bulb in the circuit will not make the alternator put out 13.9 volts at 900 RPM.
Snowbum as usual, has many pages of detailed explanation of the BMW charging system. For me I will magnetize my rotor every time I turn on the ignition switch through the generator light just like it was designed.
Bob

[Semper Gumby]

<snip> A 100 watt bulb in the circuit will not make the alternator put out 13.9 volts at 900 RPM.
Snowbum as usual, has many pages of detailed explanation of the BMW charging system. For me I will magnetize my rotor every time I turn on the ignition switch through the generator light just like it was designed.
Bob

Hi Bob,

True.  But my tiny 1.2W bulb wasn't enough to energize the system.  The added GE 90 is overkill I suspect.  So now that I have a properly working system I will find a way to put the right bulb in a different Tachometer and then start reducing the wattage of the bulb until the system starts to not function properly.

And you are right again.  This doesn't affect the top end.  The system, before and after this farkle, produces about 13.6 volts.  But it sure is nice to have that GEN light out at 1250-1500 rpms rather than the 3000+ I was getting before.

BTW - I sent a flurry of emails to Rick at Motoelectrk and he says there are different bulbs that fit (I could not find them only the one that BMW sells) and that as long as the bulb is about 3-4 ohms of resistance the system should work properly.  So I'm going to disassemble my tacho and put an ohm meter on that bulb and check it.  Then it's off to BMW of Atlanta ohmmeter in hand.  Perhaps I don't have to replace the Tach after all.

So the gist of all this is that you don't need to permanently magnetize your Bosch rotor.  But you do have to have the right Gen Bulb in the circuit to light off the alternator at a nice low RPM.    If you have a system that was performing badly at low rpm as mine was then a Bigger Gen Bulb is may be better!  

[Bob_Roller]

Bill, any idea what bulb you had in there before .

I got my bulbs at a chain type auto parts store, O'Reilly's, NAPA and I think Walmart has them as well .

I think the 168 and 194 bulbs are the ones you can use, I know the 168 bulbs are the higher wattage bulbs, if it makes any difference or not .

Did a little searching, I found 3 bulbs that have the same T-3 1/4 inch glass base wedge type bulbs .

161 has 2.66 watts .
168 has 4.9 watts .
194 has 3.78 watts .

[Barry]

BTW - I sent a flurry of emails to Rick at Motoelectrk and he says there are different bulbs that fit (I could not find them only the one that BMW sells) and that as long as the bulb is about 3-4 ohms of resistance the system should work properly

I hope he missed a zero out unless he's talking cold resistance. I guess he must have as the cold resistance of an incandescent lamp is at least a factor of 10 lower than operating resistance

[georgesgiralt]

"A 100 watt bulb in the circuit will not make the alternator put out 13.9 volts at 900 RPM. "
Entirely right, but this 100W bulb will ensure that the alternator gives all it can give at 900 RPM. Even if it is only 1W... If you reduce the energizing current (i.e. place a too low wattage bulb) the same 1W will be obtained at a higher RPM (faster cutting flux lines)... which was the initial problem (the bulb going off at 3000 RPM instead of 1200 ~ 2000 RPM...
So we are right in suggesting to stick with what MM BOSCH and BMW have designed (a stock system with a 3W GEN bulb )

Edit :
I suggest you remove the actual GEN bulb from your bike and check a European car dealer (Porsche, VW, Mercedes, Peugeot, whatever)  and ask for the same bulb with 3 or 5 W rating. These bulbs where common on every car of the era as indicator or dashboard lighting so they may be found new somewhere were you are !

[Semper Gumby]

The bulb that is in my bike is different than the bulb you are talking about.  It is used in the illumination of Ashtrays in Audis.  It is about 1/2 the diameter of the 168 (t3?) bulb you guys are talking about.

I will post a picture.

[georgesgiralt]

Funny, On what kind of bike ? Because all the R45~65 I kow use the wedge 12V 3W bulb.
This may explain the difficulties to find a suitable one.