Testing the Diode Board                            Bike Index Page    

Description

The diode board is basically an assemblage of 11 separate diodes connected together in one package. The main function of the board is to rectify the alternating current produced by the alternator and turn it into direct current that can be used to charge the battery and run the rest of the bikes electrical system. It also has a secondary function, which is to supply, via a separate set of three diodes, the field coil that provides the magnetic field, which is used to generate the output from the alternator. In order to do this, the diodes are arranged as shown below.

 

 Each Diode is a 'non return valve' for electricity and so should pass current in one direction but not the other .In order to test the diodes, you need some form of continuity tester. This can either be a bought tester or a perfectly satisfactory alternative can be made using a battery, a bulb, a couple of crocodile clips  and a few bits of wire arranged as shown below. Click here for a note on analogue multimeters.

 

 First remove all connections from the Diode Board. It is actually easier if you remove the board from the bike. Now, referring to the diagram below:-

Connections:

  1. B+ Positive line to battery - Thick Red or Black wire (T)

  2. Not normally used

  3. Thick Wire from alternator to diode board (1 of 3) (U)

  4. Thick Wire from alternator to diode board (1 of 3) (V)

  5. Thick Wire from alternator to diode board (1 of 3) (W)

  6. Wire from D+  on voltage regulator (S)

  7. Thin Wire from alternator to diode board (Y)

Procedure

1) Connect the negative clip (marked -) on the continuity tester to terminal number 1 then connect the Positive clip (marked +) to each of  terminals 3,4 & 5 in turn. In each case, the bulb should light (or the continuity tester should indicate a circuit).

 2) Swop the clips so that the positive clip is attached to terminal 1 and the negative terminal connects to 3,4 & 5 in turn. Now the bulb should NOT light. (The Continuity tester should not indicate a circuit).

3) If you examine terminals 1 & 2 you should find that they are firmly fixed to one of the two metal frames of the diode board. Got to the other metal frame and attach the positive clip to it. Now again attach the negative clip to each of terminals 3,4 & 5 in turn. In each case the bulb should light (I'm going to ignore the bit about the continuity tester from now on, I'm sure you get the picture!)

 4) Now attach the Negative clip to the same metal frame and attach the positive clip to terminals 3,4 & 5 in turn. Now the bulb should NOT light.

 The preceding four stages test the main rectifier diodes. If the results are not as shown, this indicates that one of these diodes is dead. There's no point in going any further, it's not practical to replace them so you might just as well bite the bullet and order a new diode board. If you've got this far without hitting the problem, then move on to the field diodes. 

5) Attach the negative clip to terminal 6 on the diagram and once again connect the positive clip to terminals 3,4 & 5 in turn. The bulb should light. 

6) Now put the positive clip onto terminal 6 and the negative clip to 3,4 & 5 in turn. The bulb should NOT light. 

That deals with the three field diodes. This just leaves the last two diodes which connect the centre of the 'star wound' alternator into the circuit. 

7) Finally connect the positive clip to terminal 7 and the negative to terminal 1. The bulb should light. Now move the negative clip to the same metal frame as mentioned above.  The bulb should not light. 

8) connect the negative clip to terminal 7 and the positive clip to terminal 1. The bulb should not light. Move the positive clip to the metal frame and the bulb should light.

 This completes the diode tests.

  It is possible to replace one of the small diodes if you're handy with a soldering iron, and it is certainly a lot cheaper than replacing the diode board. I'd suggest a diode rated at least 4 amps at 100 volts for the job. I have done this. It lasted for at least 20,000 miles and was still going strong when I got rid of the bike. The only thing you have to do is make sure that you put it in the right way round.

 

R. Frankham                                                       Bike Index Page   

 

25/11/2000 (Modified to HTML 15/1/03)