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In Bike Testing of Your Diode Board

Bob Stewart –
(Nov 28, 1998)


This note should give you enough information to test the main diodes in your diode board without removing it from your bike.  This is not a complete diode board test, though, as there is another set of diodes on the board that this does not check, along with a “wye” connection from the alternator that is not mentioned in this note.

While I haven’t sat down and traced out the diode board on my bike, it is essentially a three-phase full-wave bridge.  That’s what my drawing represents.

If you have a temperature sensitive diode, this procedure may not find it.  In this case, you might try using a hairdryer to heat up the diode board before checking.

Don’t forget to order a new set of rubber sticks before you remove your diode board for any reason.  You probably want to get some spares, too, as they are kinda fragile.

In the drawing below, the ‘+’ is the B+ that goes to your battery via the starter, the ‘-‘ is the ground strap to your diode board.  The points ‘A’, ‘B’, and ‘C’ are the three heavy wires that connect the diode board to the stator of the alternator.

Get yourself a DVM (Digital VoltMeter) with a diode scale.  You can use the Ohms scale, but a diode scale is much preferred.  The DVM that I prefer is geared to the automotive industry, and has dwell and tach scales.

1. Disconnect the battery, then the voltage regulator.  There is also a small connector (on my R100RT) that must be disconnected to perform step 7 successfully.  On my bike, this is a white connector with two wires, one blue, one black.  If you don’t disconnect this connector, the test at step 7 will not indicate infinite. Instead, it will indicate between .5 and .75 indicating a forward biased diode. I believe that these diodes are the additional small ones I discuss in step 8.

2. Disconnect the three wires going to the alternator.

3. Set your DVM to the diode position.

4. Attach the black lead to the B+ wire.  Touch the red lead in turn to each of the three wires you disconnected from the alternator.  The    reading on your DVM should be somewhere between .5 and .75  .  This is approximate, your DVM’s manual can give you more information.  If any reading is significantly different it probably indicates a bad diode. If any reading is infinite, it does indicate a bad diode.

5. Disconnect the black lead, and attach the red lead to the B+ wire. Touch the black lead in turn to each of the three wires.  All three    readings should be infinite.  Any reading less than infinite indicates a bad diode.

6. Attach the red lead to the ground strap.  Touch the black lead in turn to each of the three wires.  The reading on your DVM should be somewhere between .5 and .75   .  This is approximate, etc., as in #5above.

7. Disconnect the red lead, and attach the black lead to the ground strap. Touch the red lead in turn to each of the three wires.  All three readings should be infinite.   Any reading less than infinite indicates a bad diode.  (See comments in step 1.)

8. There is another set of three diodes on the diode board.  These are small diodes that don't carry much current, and probably don't fail very often.  I'm not sure if you can get to the wire that connects to them without removing the diode board.   They are checked the same way as for steps 4 and 5, above, except use the connector for these diodes, not the B+ line.  (See comments in step 1.)
            +          <--- B+ strap
       |    |    |
       _    _    _
       ^    ^    ^     <--- diodes
       |    |    |
       A    B    C     <--- heavy wires
       |    |    |
       _    _    _
       ^    ^    ^     <--- diodes
       |    |    |
            -          <--- Ground strap

I think you should probably be able to press in a new diode if you find a bad one, but be very careful if you try it.  The polarity of the replacement diode MUST match the polarity of the other diodes on the same strip.   This can be checked with the DVM in the diode position.  You also have to be sure that the fit is Very tight.  Any looseness is a disaster waiting to happen.   I’m not sure if the diodes from an auto alternator will fit mechanically.  If they do, you should be able to use them, as they will definitely handle the current from an Airhead alternator.  If you do try this, be sure to use a press, don’t just try to pound them in with a socket and a hammer.

Disclaimer:  This works for me.  Your skills, equipment, and situation might be different from mine.  I can’t be held responsible for any failure or damage to your bike, property, or person, etc., regardless of the reason.

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