electrical short - 1976 R90/6 - diode board?

[Darryl.Richman]

I have a 1976 r90/6 that I have been fixing up. I do not have a battery, but I have run it a few times using jumper cables connected to the lawn tractor. Today I went to start it, it has been about a month since the last time I started it. I hooked up the cables to the battery wires on the bike, then made my connections at the tractor. I noticed a big spark when I made the final connection at the tractor. This wasn't right, so I checked connections multiple times and still had the spark each time I made the final connection. Next I pulled out my ohm meter and saw there was 5.6 ohms between the cycles + wire and ground. I tried turning the ignition key off, and then the ohms jumped to zero. I tried hooking up to the tractor again with the key in the off position, and again I had the big spark.

Finally I decided to just hook it up to the battery and see what happened. It took 5 to 10 seconds before smoke started coming from my engine. This is very odd to me because the last time I ran the bike I had no issue, and it has just been sitting untouched since then.

I took apart the top and front covers. I noticed the + battery line goes to the starter solenoid, where attaches a smaller wire that goes to the diode board, where then a larger wire goes to the starter relay. I removed the wire from between the diode board and the starter, and hooked up to the tractor battery once again. I did not get a big spark this time. I assumed at this point the diode board was the culprit The diode board smelled like it was the source of the smoke I made earlier.

So, is this the typical symptom of a bad diode board? What I didn't check before removing the diode board was the wire between it and the starter relay. Could the starter relay have been the true source of the short?

I am leaning in the direction of replacing the diode board to an upgraded unit. I also am wondering if I should also just go ahead and replace the starter relay and also the voltage regulator.

I know diode boards are a bug issue, and I have done a ton of research online about them, but i still don't know if what I have experienced is the result of a diode board failure, so any advice is really appreciated.

Thanks!
David K

[schrader7032]

Wow, there's a lot going on there. As you said, confusing since the last time everything was OK. I don't know much about serios DC electrical problems, but taking a step-by-step process is the way to go. I would also investigate the wiring very closely and find out which portion melted. That will help lead you to the problem.

You say you've done some on-line research. A couple of places that I would go would be:

http://bmwmotorcycletech.info/techindex.htm

Snowbum has several articles on testing the electrical system components.

Also, Robin has a section on testing the diode board:

http://robfrankham.co.uk/bike/testing_t ... _board.htm

You might want to get with Rick Jones at Motorrad Elektrik:

http://www.motoelekt.com/

He sells a nice book on testing various components of the charging system. He might also have good ideas if you called him and discussed the issue.

[CATPart]

Embarrasingly I will admit what the problem turned out to be. I had the jumper cables hooked up with reversed polarity, which I guess will fry things. The tractor I connected the jumper cables to was mismarked (the plastic caps over the battery terminals) and I didn't think to double check them at first. However, after installing a new voltage regulator, a new starter relay, and a new Omega diode board, I decided to put my voltmeter on the jumper cables to check battery voltage before connecting to the bike. To my surprise I saw a negative reading! But, in the end I am pleased, because I have new parts and clean electrical connections. The bike started right up, and will even idle now with no battery connected.

[schrader7032]

There's a 12-step program for that! Admitting the problem is deep into the healing process... That might help someone else down the road.

[schrader7032]

I notice that you have a Valeo starter in the bike. The early versions of these starters were known to have problems when the internally glued magnets became unglued. So be ready for that. The newer versions corrected that problem.

[CATPart]

hmm...any way to tell if it is a newer version?

[schrader7032]

I'm not sure exactly...check this website:

http://www.euromotoelectrics.com/

and/or give them a call. They support both types and could give you some ideas how to check what you have.

Your picture shows the part number as DR6A15 which according to the website is the correct part number although I don't know how to tell if it's "modified". Another page to consider looking at is Anton's. He breaks down the Valeo starter as well:

http://www.largiader.com/tech/valeo/