• Welcome, Guest! We hope you enjoy the excellent technical knowledge, event information and discussions that the BMW MOA forum provides. Some forum content will be hidden from you if you remain logged out. If you want to view all content, please click the 'Log in' button above and enter your BMW MOA username and password.

    If you are not an MOA member, why not take the time to join the club, so you can enjoy posting on the forum, the BMW Owners News magazine, and all of the discounts and benefits the BMW MOA offers?

How to test the starter relay (also, is this even the right relay?)

Thank you Brook, that article was very helpful! I've gone through the system and found a couple of issues, and I realized I had completely missed an important clue (the battery charge light doesn't turn on when I turn on the ignition).

One of the small diodes on the diode board is broken (fully open in both directions). It also looks like a couple of the copper leads on the printed board have overheated and burnt off the coating.

Inspecting the alternator the brushes look fine but the rotor is open. Connecting ground to the blue wire on the stator lights up the charging light.

I'm guessing the broken rotor is the cause of the charging issues, although the broken diode seems like is could cause it on it's own too. Is it possible that the issues are related?

I'll go ahead and order replacement parts. Does anyone know where best to buy the rotor? Euromotoelectrics has one they claim fit (https://www.euromotoelectrics.com/product-p/boalt-rotor642.htm) but it's 2.9 Ohms and I believe I need one that's in the 6-7 Ohm range.

Thanks again everyone!

jakobw,

I don't think the two problems are related, as in one caused the other.

EME products work well and I installed the 105 mm stator version alternator in my /5. It works well. They will answer any questions you have, so don't be bashful using the phone or email.

The small diodes generate DC voltage that feeds the voltage regulator.

You sound like you are on the right path to success.

Best.
Brook Reams.
 
I'm guessing the broken rotor is the cause of the charging issues, although the broken diode seems like is could cause it on it's own too. Is it possible that the issues are related?

With one out of three diodes gone, the feedback voltage to the voltage regulator would always be lower than the voltage produced by the alternator. Hence the voltage regulator would always over shoot the target voltage of ~14 Volts to compensate for the too low input. Which would cause a higher current through the diode board, a higher current through the rotor and a higher charging voltage to the battery. All would work for a while until the rotor or the diode board burnt out.

I think the rotor failed first. Is there a chance that you connected GND to the D+ spade on the relay without having disconnected the blue wire to the diode board first? If so the starting motor would have created enough plus voltage from the alternator through the diode board to the spade (now grounded) to cause a short.

Diodes don't fail all by themselves! They typically fail when the current through the diode becomes too high which causes a burn-out.

Anyway, good findings jacobw!

/Guenther
 
With one out of three diodes gone, the feedback voltage to the voltage regulator would always be lower than the voltage produced by the alternator. Hence the voltage regulator would always over shoot the target voltage of ~14 Volts to compensate for the too low input. Which would cause a higher current through the diode board, a higher current through the rotor and a higher charging voltage to the battery. All would work for a while until the rotor or the diode board burnt out.

I think the rotor failed first. Is there a chance that you connected GND to the D+ spade on the relay without having disconnected the blue wire to the diode board first? If so the starting motor would have created enough plus voltage from the alternator through the diode board to the spade (now grounded) to cause a short.

Diodes don't fail all by themselves! They typically fail when the current through the diode becomes too high which causes a burn-out.

Anyway, good findings jacobw!

/Guenther

That makes sense. It's possible that I connected GND to the D+ spade at some point when I first started troubleshooting and before I had really taught myself the details of the system, but the starter failed before the charging system (although only by a couple hundred miles). I did ride a couple hundred miles after the charger failed, occasionally recharging and not always with MC friendly chargers, so it's also possible that I burnt out the diode board during that ride.

Either way, thanks for all the help! Hopefully with these new parts the bike will be charging and starting again.
 
Back
Top