Vintage BMW Motorcycle Owners

New to this group. While attempting to get my R80RT back on the road, I accidently began to connect the new battery backwards, causing some sparks to fly. Even though I stopped immediately and realized my error, it now seems something is wrong. After taking the gas tank off and the top engine cover to find the starter, a meter shows that the starter solenoid is ok, but the wire to the diode board shows a short. Looking at youtube, I learned the diode board is inside the front engine cover, which is described as having 5 bolts/screws, the top 2 of which are nearly impossible to remove while the engine is in the frame (which it is). Not to mention the lower part of the fairing needs to be removed to get to the engine cover.

I am guessing I fried the diode board - is there a way to test it without taking the front engine cover off and removing the board? My mechanical skills are fairly good - electrical, not so much, but my brother is nearby who is better at electric.

Hoping for some guidance, and thank you!

Dee in Maryland

Welcome to the forum! I might be inclined to hook things up normally and start the bike...the diode board is part of the charging circuit not the starter circuit. If it starts, read the battery voltage...should be upwards of 14v with engine RPM around 3K and above.

There are tests that can be done of the charging system without taking the diode board out but you will need to remove the front engine cover. On the RT, that does mean the front fairing cover.

There's a lot to take in here, but Snowbum has a page on his site for troubleshooting the charging system:

https://bmwmotorcycletech.info/trbleshootALT.htm

The Classic Boxer Charging System is a good book to have:

http://www.motoelekt.com/goodstuff.htm

Thank you for the welcome and the response. I will try reconnecting the battery (properly this time) to see if it will start. I didn't know the diode board isn't necessary for starting.

Thank you again, I'll post back.

Dee

And when/if you need to go inside the front cover, remove the negative ground to battery before doing so. There is a possibility of shorting across power as the cover slides off. Then reconnect the battery and go about your troubleshooting.

Making progress (I think). Took the fairing grill off and removed the front engine cover - not as difficult as I expected, and yes, I disconnected the negative lead on the battery first. Removed the diode board which was clearly damaged/burnt. Have ordered a new one, but the holiday will delay the shipment. Hope to get the new board at the end of the week. I made lots of notes as I took things apart since I expected it would be awhile before I could receive a new part.

Before reconnecting the battery, I removed the tank again and disconnected the wire on the starter solenoid that goes to the diode board since it seemed to be causing the short. In an effort to confirm that was the problem, I connected the battery and just hit the start button to make sure the starter worked and it did. I didn't attempt to actually start the bike since the gas tank is off, but at least I knew the starter was working.

I've read lots of articles suggested by you and also here in the forums. A lot of them are over my head but occasionally I pick up a tidbit that is helpful. I appreciate the guidance.

Dee

Thanks for the update, Dee. Sorry to hear about the crispy board. I'm not sure when updates were made but early on the diode board was held by rubber mounts to the front of the engine. Over time, the rubber decays and fails and the board sags or falls and shorts out on the cover. Possibly by the time of your bike, BMW decided to use solid mounts. On my /7, I had the rubber mounts and I bought a set of solid mounts from Motorrad Elektrik. I also bought a grounding harness that was supposed to help with grounding.