Forum bmwbike.com

So I was cruising to work at about 40 - 50 MPH the other day when the engine started to bog down and accelerator began to be unresponsive until the engine just completely cut off. After coasting to the side of the road, I tried to fire it back up but the battery was completely dead. So I am thinking that alternator/ charging system is in need of some work. My question is how to I go about diagnosing the problem?

I've fully charged the battery and will test it to see if it is holding the charge, but I don't think the battery is the problem since while cruising at 40 - 50 MPH I shouldn't be pulling from the battery, the alternator should be handling it.

So what do I do to diagnose and isolate the cause of the failure?

This type of circuit may be different than you might think of in a car, etc. On these bikes, the battery is critical to the system. The alternator doesn't really "run" the bike, but rather feeds the battery. The battery really "runs" the bike. Dead battery...no fire in the bike.

What type of riding do you do? A lot of stop and go driving will never recharge a battery. It needs sustained RPM for a good amount of time. So, maybe your have just be abusing the battery by not letting it recharge after your rides. Possibly you need a trickle charger to keep the battery topped off.

First test, restart the bike and put a known good voltmeter across the battery terminals. While revving the bike to at least 3K RPM, you should see better than 13.5v...close to 14v would be ideal. If you don't, then you have some kind of charging problem.

A lot of work can be wasted if the battery is no good. It might hold a static charge, but it might not be good enough to start the bike. Another test is to use the same voltmeter across the battery and watch the reading when you hit the start button. If the voltage drops to well below 10v, the battery is no good. Using the starter is essentially a load test on the battery. Or you can take it to a battery or auto parts store and let them test it on their machine.

Beyond that, you'll need to check the regulator, brushes, and the continuity of the rotor. A guy by the name of Snowbum has a very good, albeit lengthy, discussion about troubleshooting the alternator. Here's the link:

http://bmwmotorcycletech.info/trbleshootALT.htm

Hope that helps.

Kurt in S.A.