R51 Rotor commutator measurements

[Vegtamr]

I'm diagnosing weak charging system output on the new-to-me R51/3 and have a question about measuring resistance on the rotor.

The Barrington manual states that resistance between any two adjacent segments on the commutator should be .1 Ohm all the way around.

On mine I am getting in the vicinity of 1.5-1.6 Ohms all the way around using two different test meters. Is this a case of "well there's your problem"? .1 seems awfully low compared to what I'm used to from the later 12v bikes.

I have not tested generator output with the engine running yet; will do so once reassembled. I do know that my father was having trouble keeping a charge on the battery before he handed the bike over to me, and the lights were non-op unless the battery was fresh off the trickle charger.

Thanks

-J

[wa1nca]

Good you confirmed your readings with 2 different ohm meters
1.5-1.6 ohms is not good

I bought a new one 2 years ago

https://vintagebeemerparts.com/vintage- ... r-for-r27/

[caker]

Hi,

I'm just a meathead, but from my understanding what's important is that the resistance is consistent across each adjacent pair, and also consistent when testing contacts 180 from one-another. The actual ohm reading is less important than consistency?

But before chasing anything down on suspicion, run the engine and report what charging voltage you see at the battery. It should be about 7.2V when revved above idle.

-Chris

[wa1nca]

Good advise Chris
He did say he would try it after he reassembled the engine

[Vegtamr]

The update here is that there is zero output from the generator (measured between the F terminal on the voltage regulator and the ground terminal on the generator face).

I'm thinking the rotor is gone...

[wa1nca]

Free copy off internet
Doug Rinckes Electrical systems

bmw electrical system (4).pdf

(4.58 MiB) Downloaded 34 times

Make sure your field coils are ok before replacing the rotor

With Both sides of the field coils disconnected measure the resistance from one end to the other
Should read 2.5-4 Ohms
Also check the both leads don't read 0 ohms to ground

[schrader7032]

Does Rinckes book cover the plunger series electronics? From what I can find on specs:

- R51 has Bosch RD 50/6 2800RS 15 for a generator

- R69S has Bosch LJ/CGE 60/6/1700 R for a generator

I don't have a clue what the numbers all mean!!

[wa1nca]

The electrical and ignition theory for R1/3 are the same for r50 r69s
Only real difference crankshaft size for rotor 17mm to 20mm and output power

His paid edition (I thing no longer available) which I also have, does have R26,R27 and circuit diagrams in color
Tommy

[Rodolfo850]

Hello,

In order to make a correct diagnostic, you need to check resistance between commutator slides and ground, if any point gives continuity its grounded, and will not work.

Check varnish color, it has to be yellow or brown, black means burned varnish.

Carbons must have free movement and slide easy, and commutator must be clean, flat and shiny, otherwise if has wear, carbons will not make good contact.

If all its OK, need to check stator coils, ceramic resistance and connecting screws as well, also need to check and clean regulator contacts.

Last step is to Polarize again rotor, with direct voltage, this step is full described on Doug Rinckles book.

Good luck