diode board check?

[beemer3]

I am trying to get my son back on the road with his R75/6. Problem is the charging system isn't working. He thought the voltage regulator was bad and changed it. (Still not charging) at this point I inherited the job. I tried to check the diode board in the bike by the instructions in my Clymer book and had no luck with this. I have pulled the board now and could sure use some advice on checking one off the bike with an ohmmeter. I.e. what points to check and what values they should have.
john
:confused:

[Cosmoline52]

OK Beemer3, I suggest reinstalling the board and try the simple things first.

With the engine running, does the "gen" light on the instrument cluster light at idle and dim at higher RPM's? The bulb here acts as an exciter in the circuit to trigger the board. If the bulb is dead so is the charging.

Battery connects snug and greased? Electrolyte level up? If all your connections are clean and tight and the battery is OK (check required here also) then we move to the infamous failed rotor...

While it appears that the main failure with a rotor would be with worn contact rings my experience has been with winding dis-continuity... I love my 100S and the general BMW/Bosch design but my bike is at 160,000+ and on it's third rotor. $100 a pop to me is nothing to sneeze at but when I get my bike back on the road I'll be carrying one in the bags.

The rotor is simple in design and if a continuity check proves positive then maybe it is your diode board. One of my more recent issues of MOA has a tech article regarding but of course I can't find it now so maybe another member can point you there. The /5-6-7's are relatively easy to diagnose for problems being the dinosaurs that they are so have at it!

[beemer3]

Thanks for the reply Dean, that's interesting about the gen bulb working being a necessary part of the process as it didn't come on at all but when I pulled it to replace I checked it and it isn't blown. I had thought maybe the diode board was causing the light not to burn somehow. I will reinstall the board and check some more.
John
:)

[James.A]

In my own personal experience, the most frquently failed part on my airheads is the diode board. The rotor comes in a close second.

[kioolt]

The charging light should come on when you turn the key on with the engine off. Two of the possible causes are an open rotor or bad bulb.

[bmwmick]

Originally posted by beemer3
Thanks for the reply Dean, that's interesting about the gen bulb working being a necessary part of the process as it didn't come on at all but when I pulled it to replace I checked it and it isn't blown. I had thought maybe the diode board was causing the light not to burn somehow. I will reinstall the board and check some more.
John
:)

John,
No GEN lamp with the key on engine stopped? Disconnect the battery, pull the front cover, slip a business card between brushes and rotor slip rings. Measure the resistance of the rotor(one probe on each slip ring). You will need a low scale for this and it should be somewhere between 3 and 7 Ohms. If it's open, you have a dead rotor winding.

Mick

[beemer3]

Thanks for the help guys, looks like we have a bad rotor here. I'll order one today and that should get the boy on the road again.
John:clap

[bmwmick]

John,
You do good work! :clap Here is an excellent basic diagram to download or print for future reference.....
http://www.buchanan1.net/charge.shtml

Also, http://www.airheads.org/index.php
and read or save some of the Tech Tips.

Mick

[beemer3]

Thanks for the links Mick, That wiring diagram is much better than any I have seen so far.

John:bliss