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Connection between the alternator and the head light relay

Discussion in 'The Garage' started by LeBouc, Jul 29, 2023.

  1. LeBouc

    LeBouc XS400 Member

    Hi!
    According to the electric diagram, there supposed to be a diode somewhere in the path of the connection between the alternator and the head light relay. Anybody had problem with a blown one? I have checked the connectivity between the plugs on both side of this diode and it doesn't go through! That would be the reason for the headlight to not light at all (unless I plug the + from the battery directly to the blue/black wire that goes to the high/low switch).
    If you ever had this issue, can you tell me where is sitting this diode?
    Thanks
     
  2. LeBouc

    LeBouc XS400 Member

    BTW, the headlight relay has been verified and it works correctly.
     
  3. CaptChrome

    CaptChrome XS400 Addict Top Contributor

    You can check the diode with a multi-meter set to measure continuity, measure the path one direction and then swap the meter leads and test again. There should be near infinite ohms in one test and very low (less than 10) ohms in the other test. Since the diode is only off of one leg of the alternator, you will have to check all three of the white wires this way to make sure you have really tested the diode.

    If you need to replace the diode and have a stock harness, it is under a bit of clear plastic in the main part of the harness where the white alternator wires enter. This is on the left side of the bike just forward of the left air cleaner/battery (80 to 82 SOHC harness).
     
  4. LeBouc

    LeBouc XS400 Member

    Thanks CaptChrome.

    I had already checked the connectivity on the 3 wires and changing the side of my multimeter cables. I understand that a diode will conduct only on one side (that's its main fuction!). I had also checked with the diode tester.

    So I unplugged again everything near the battery to be able to pull (again) the main harness enough to see the clear plastic and could see that the diode is effectively broken (most probably my fault when I pulled the harness the first time and didn't know that there was a diode there).

    So now I have to find a new one. I see that I can have one on EBay but, since it's in the States, I will loose most of my summer before I get it.

    Am I right to think that a 20 Volts 50 amp diode would do the job?
     

    Attached Files:

    CaptChrome likes this.
  5. tstidham

    tstidham XS400 Addict Top Contributor

    If the diode is just triggering a relay, it can't have too much load on it.
     
    CaptChrome likes this.
  6. LeBouc

    LeBouc XS400 Member

    I believe that this diode was placed there to avoid any feed back from the general circuit to the alternator. This could arrive only if there is a short in the headlight relay (only thing connected to this wire outside of the rectifier) and would surely damage the alternator circuit and, possibly, the rectifier as well. Otherwise, a simple wire could have done the trick! But this is only my understanding of the circuits! And that's why I think about a 20V, 50 amps diode. I want to be sure that it would not be overwelmed by a short in the relay.
     
  7. CaptChrome

    CaptChrome XS400 Addict Top Contributor

    The diode is in the circuit because the alternator produces AC. The diode is a simple single-phase rectifier to transform the AC in that white wire to a DC signal for the headlight relay to use as a trigger.

    Yes, any general purpose diode should work. Try looking for something like a 1N4001 or 1N4002 diode which should be pretty common.
     
  8. LeBouc

    LeBouc XS400 Member

    I looked at a video explaining how an alternator works and I understand what you mean. It puzzles me, though, that, since only one third of the alternator circuit is used (one white wire), it means that the relay receives pulses of direct current instead of the continuous current that would come from the rectifier. However, it seems to work since the headlight worked for the last 42 years! lol
    Thanks for your help!
     
  9. CaptChrome

    CaptChrome XS400 Addict Top Contributor

    I thought about this overnight, and I think your previous statement:
    is the real reason the diode is where it is.

    The headlight relay is setup to "latch", which simply means that once it is triggered, the relay feeds some of the switched on headlight current back to the trigger coil to keep the relay on until the key is turned off. If the diode wasn't in the harness, the DC headlight current would be feed back into the alternator's coils, probably causing some performance issues with the charging circuit.

    Anyway, enough geeking out on this - it is good you found the issue!
     
    LeBouc likes this.
  10. tstidham

    tstidham XS400 Addict Top Contributor

    Also thinking further on this, I think the voltage of the diode may be more important then the amperage. I think our alternators can produce 30 to 40 volts AC on each of the three legs depending on rpms, so a higher volt diode may be better, like 50v if available.
     
    CaptChrome likes this.
  11. LeBouc

    LeBouc XS400 Member

    On top, the battery can dump 150 amps to the starter but I think that we are 'covered' as the other circuits are protected by fuses... At the end, though, it might be a better idea to have a smaller diode because, otherwise, if a problem occurs, the wires will burn instead of the diode; easier to replace the diode than a harness! What do you think?
     
  12. tstidham

    tstidham XS400 Addict Top Contributor

    The starter circuit is completely dc and separated from the ac side via the rectifier and regulator, so no worries there. This diode is just the initial trigger of the headlight relay once the engine starts and the alternator starts producing ac voltage. But as I said, that single AC wire can output 30-40 volts.
     
  13. LeBouc

    LeBouc XS400 Member

    Some 35 years ago, I played a little bit with electronics so I still have parts in a box. I checked and found a PTC205 diode which is rated 2.5 amps / 1000 volts. It can withstand 80 amps for 16 milliseconds (that's short!). I don't think that the relay will pull that much juice in normal condition. I have tried to get the specs of the relay but can't find it... I will try with that one. Worst case: it fries!
     
    tstidham likes this.
  14. LeBouc

    LeBouc XS400 Member

    I have checked the power consumption of the command circuit of the headlight relay (the circuit coming from the alternator) and it's 0.135 amp! So the PTC205 diode is largely sufficient for the job!
     
    CaptChrome and tstidham like this.
  15. LeBouc

    LeBouc XS400 Member

    First, the PTC205 diode is doing the job. Andf to make sure I won't break it again, I have put, on top of the clear plastic sleeve in which the diode is 'hidden', a 4 inches piece of flexible tubing, the one that we use to run moving cables like clutch and brake (for the old motorcycles).

    So now, my bike is back on the road again! You won't hear from me until winter because I will be busy riding this little gas sucker! Happy rest of summer, everybody and special thanks to CaptChrome and Tstidham for their great help!
     
    tstidham likes this.
  16. LeBouc

    LeBouc XS400 Member

    One last thing: the white wire that is going to the headlight relay from the alternator is the one that is alone at the plug (two are aligned and one is alone; that's the one). You can also find it in the plug to the rectifier and it's the middle one. Just like in the diagram!
     
    tstidham likes this.

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