Charging system mysteries - testing diode board

[lukemike]

G'day everyone - looking forward to the day when I can add value on a post here.  I'd appreciate some help troubleshooting a charging system mystery, in particular, what to make of the diode board test I performed.

For background, I recently wired in an Acewell speedo and of course the gen light needed to be accounted for so I wired in a 47 Ohm resistor to mirror the incandescent charge light from the original instrument cluster.

The wiring has been spliced over the 30+yrs so it took quite some work to remap everything and eliminate some of the work done by others...

I managed to get everything wired up clean, and now am not getting charge back to the battery.  I followed some steps in Clymer to try and isolate the Voltage Regulator or the Alternator - and too be honest, both tests seem to align with the Clymer manual for resistance across the stator windings and the rotor slip rings - that left the diode board.

I referenced a great little site: http://jhau.maliwi.de/mot/r-elec.html

The mystery is, it's indicated certain connections cross 2 diodes so one would expect to see 2x the resistance (my interpretation).  When I map the test out, I see variability - not clear cut 2x resistance.  My multimeter is a simple $10 option without a diode test function, I simply used Ohms (2M) for the test.

Does the fact there is variable resistance indicate the diode board should be replaced?

[Tony Smith]

My own recent sad experience is that you cannot trust testing with a multimeter.


I have a diode board that tests perfect with a meter, but fails immediately under real world loads.

Moral of story - test with a lamp.

[D Mann]

Here is a good place to find some good info.  http://bmwmotorcycletech.info/diodebds&grdgwires.htm
Snowbum has many write ups on the electrical system.

[Bob_Roller]

I had the same experience as Tony has stated, my Guzzi, has the same charging system as the airhead BMW's .

Had charging system issues with the Guzzi .

Tested the board with a multimeter tested fine, installed the board from the Guzzi in the '81 R65, charging issues followed the board .

The red GEN charging light is a good troubleshooting guide, if you know what it's telling you .

Nice to have another bike where you can swap parts and confirm if the part is faulty or not .

[Barry]

The diode board may well be faulty but wouldn't it also have been faulty before the Acewell mod ?   If it wasn't then it would seem more likely there is a wiring fault causing the lack of charging.

[Ed Miller]

I see variability pretty often with my multi-meters.  It's generally because I'm not good at holding the leads firmly and making a very good electrical connection with the terminals I'm testing.  It's frustrating.

[montmil]

My '83 R65 has an Acewell. I kinda doubt that your new digi gauge has created a diode board issue. Maybe just a coincidence in timing. Da Bum will walk you through the mysteries of the diode board. The main problem is trying to stay awake.  [smiley=whistling.gif]

[lukemike]

Thanks everyone for the quick feedback as always.

I agree the installation of the Acewell gauge doesn't seem likely to have caused the issues I'm not facing... however, there was one instance I made a bone head oversight and left the battery connected - a hot wire made contact with the frame for a few seconds.

Since that time, I hadn't started the bike or even attempted so when I did and the charge light remained on, I suspected maybe I shorted the diode board when the hot wire made contact.

And then again, given there were numerous wires which needed tending too, it's quite possible in my first attempt at working with electrics on a bike - I may have made a mess of things.

I'll perhaps follow Clymer's instructions on testing output from alternator as perhaps it's the stator or voltage regulator and I failed to test properly - picking up a better quality multimeter while I'm in the US - to continue upon my return mid-Nov.

Definitely have a goal to finish up before end of Nov to enjoy the summer riding.

Maybe someone in the Sydney area would be keen to lend a hand troubleshooting...

[montmil]

<snip> ...there was one instance I made a bone head oversight and left the battery connected - a hot wire made contact with the frame for a few seconds. <snip>

A possible clue to your charging concerns...

Some years ago, I made a similar bonehead mistake and got a hefty spark from the chassis for no more than a nano second. That quick little spot welding moment fried my diode board. Check the back side of your diode board, looking for burned areas. You may test the board all you want, but I think you may be off on a shopping expedition quite soon. Diode boards are relatively inexpensive via eBay, breakers, or perhaps a forum member has one to sell.

My diode board ended up looking just like my breakfast... toast. This is also why new Airhead owners are urged to remove the battery's ground cable before removing the engine's front cover. There's a bare terminal under the cover that is always hot. It's fairly rare to be unlucky enough to make that expensive contact but as the saying goes. What man can make, man can screw up.