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Regulator/rectifier on 1980 XS400

Discussion in 'The Garage' started by K3ith, Mar 29, 2021.

  1. K3ith

    K3ith XS400 Enthusiast


    Im building a 1980 XS400 SOHC and completely redoing the wiring from ground up because It was real mess before.
    I also need a new reg/rectifier. The bike had a regulator/rectifier where the field coil was going to ground. But because of fully rewiring it, I think can go with the newer one where field coil goes to positive. (The alternator should work with both)
    Is the newer one somehow better or more ''fool proof''? Or is there no difference?
  2. K3ith

    K3ith XS400 Enthusiast

  3. Bear Hands

    Bear Hands XS400 Member

    I've got an XS400 SOHC build ongoing, like you I want to loose the original separate regulator and rectifier.
    I've chosen an XS750 combined unit, mostly because the wiring colours are the same and I could get a brand new one cheap on eBay.

    XS400 reg/rect wiring:
    XS400 reg rectifier.PNG

    XS750 reg/rect wiring:
    XS750 reg rectifier.PNG
    Bolted in situ:
    reg rect.jpg

    Much neater. I cant comment on its function yet as the bike is still in pieces.
  4. amc49

    amc49 XS400 Junkie

    Newer type includes diodes inside instead of a rectifier (same thing). It is electronic instead of mechanical, making it much more reliable.

    Confused about field coil too. ALL fields originate with a positive going in to end up going back out on a negative, can't work any other way using DC power. There is more than one way of doing that though.
  5. K3ith

    K3ith XS400 Enthusiast

    Okay. By going to positive I was thinking connecting it by the same wiring diagram that Bear posted.
    By going straight to negative I was thinking about this diagram: Screenshot_20210402-082253__01.jpg
  6. amc49

    amc49 XS400 Junkie


    The field MUST have a positive AND a negative and it doesn't matter which way power flows through it as it magnetizes going either way.
  7. K3ith

    K3ith XS400 Enthusiast

    Okay, I understand that. But how do it connects to negative on the diagram that Bear posted?
  8. amc49

    amc49 XS400 Junkie

    'There is more than one way of doing that though.'

    Remember what I said.

    You can't simply put the full 12 volts through the field coil, it has to be regulated up and down. That can be done on either end, the plus or the minus, the reg does it and may even read both sides to cut one side as needed. Doesn't matter which side and can be either, some have clean ground that doesn't go through the reg and some systems have clean power and ground then goes through reg. The reg can regulate the field from either end.
  9. Jasiel Roman

    Jasiel Roman XS400 Member

    Hi amc sorry to bother you on this thread but I'm going through a similar issue. I have an 81 sohc. The field and stator are both rewired. Ik that the field coil does work because when energized independently it magnetizes. The stator has continuity. I recently wired in a r292 regulator and with the ignition on my field coil doesn't magnetize as well as the bike doesn't charge while running. You seem to perfectly understand what must get powered and where to ground. I'm desperate to get this bike running for my wife

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