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Diode Board Checking

Brian Curry   –
(Feb 28, 1998)


Someone asked about diode board checking. Here is a writeup based on the the BMW manual. As is my usual wont, it is general, you have to have some electrical knowledge, and you can do it based on what should be easily available to you.

From the manual, a check between the D+ (the two little terminals off to their own and forming a T looking at the back of the board.) (From the front, they are the two bumps in the center in their own little conduction space on the left hand side fo the board.) and the B+ (Two big terminals on the side.) for voltage differences with the bike running. [B+ is the diode board output to the battery, D+ is a seperate set of diodes that feeds the regulator, and rotor field.]

They say that a >0.0 -0.5 volt difference indicates a regulator problem. A voltage difference of 1.5 to 4 volts indicates a diode board fault. They have you check the diode board diodes as follows: Each of the three terminals that the leads from the rotors go to, goes to three diodes. Two of them, one positive, one negative, are the main power diodes. The other one is the little regulator feed. The regulator is fed by a three phase half wave rectification set up. The big diodes provide three phase full wave rectification. They say to use not more than 24 volts DC for a test voltage. So, the bike battery will work fine.

Positive diode test (reverse direction):
With the negative test lead on the diode board input from the rotor terminals, and the battery going through a light bulb, the light SHOULD NOT LIGHT up when positive test probe is put on the large aluminum heat sink with the B+ terminals on it, on the bottom. (Note that this heat sink, has the insulated/isolated mounting points.) (This is a reverse bias test.) (If they short, the test light lights, the battery should not hold charge since it discharges through them.) If they open, you don’t charge.

Negative diode test (forward direction):
With the negative lead on the diode board input from the rotor terminals, and the battery still through a light bulb the light SHOULD LIGHT up when the positive test probe is put on the large aluminum heat sink on the top. (This is a forward bias test.) If the polarities are reversed, the light should not light up, and you know the diodes are not shorted.) If these diodes short, you are way down on output. Also check the regulator feed diodes. They are the same as the positive diodes, except that the test point, is the “T” terminal arrangement. Also check the two little diodes from each heat sink to the terminal that comes out on the left side. This is the grounding connection for the three phase alternator, since it is floating. (It is a delta connection, not wye.) They have cathode markings on them so you can figure which way they should and should not conduct. Watch the size of the indicator light you use. It should be small. The regulator feed diodes are not too large, and don’t have high current ratings. And for test purposes, the current passed is not critical.

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