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Wiring the M-unit Blue

Discussion in 'The Garage' started by Timothy Kelly, Jul 16, 2018.

  1. Timothy Kelly

    Timothy Kelly check_kelly

    I bought a non-running ‘81 XS400 last year and I’m in the process of getting it running.
    The build started out as a budget job, replacing a few things that needed replacing (battery, fuel line, rear tyre) and getting it on the road. Eventually, I accepted that I was going to be revamping and customizing the whole bike.
    I knew I wanted to use an M-unit Blue in the build, mainly for its ease of useand accessibility. Re-wiring doesn’t bother me too much, but my understanding of electrical theory is limited.

    Has anyone re-wired an XS with an M-unit Blue? It has an Ignition Output wire that I’m not sure how to incorporate into my wiring loom.

    There are two wires coming from the starter solenoid, a blue and white (L/W) and a red and white (R/W). The L/W goes to the Start Output on the M-unit. I assume the R/W is switched power as the wiring diagram shows it being connected to the R/W going from the Igniter to the coils (when I originally had them connected the engine would turn over when I pressed the starter). Since the Ignition Output is supposed to provide the switched power I chose to replace the R/W from the starter with it. I spliced it into the power wire (Red/White) that runs from the igniter unit to the ignition coils. However, when I press the Starter button nothing happens.

    Apart from the wiring, everything else has gone relatively smoothly. Any help or advice is appreciated. I’m posting a photo of the current wiring situation and the XS models wiring diagram.

    E2F9E001-39CD-4D8E-A43C-00949C041D1B.jpeg FFD23E0B-C22F-4465-98D6-3ED12FC9CB51.jpeg

    Attached Files:

  2. Badou77

    Badou77 XS400 New Member

    Hi, did you find the solution. I've got the same issue with my xs750. In my point of view blue/white wire has to be on the ground and red/white has to be connected on output start hole. But nothing happen too. I suppose that red/white has to be on ignition output hole and start output hole. What do you think?
  3. cezzz

    cezzz XS400 Member

    Here is my first attempt at wiring the M unit into my XS, would appreciate comments if anyone sees anything off. Some of the colors if not most are off, but general mapping.
    XS400 Wiring-01.jpg
  4. BBS360

    BBS360 XS400 Guru Top Contributor

    I did draw up a schematic for someone here a while ago, but haven't actually used an M-Unit myself. Forget if it was a Blue or not.

    Seems mostly ok at first glance.
    Make sure the condenser is well grounded to frame.
    The big ground post on the M-Unit needs to go to the frame.
    I feel like something is wrong with the key switch. Doesn't the M-Unit have a key switch input? The regulator shouldn't be switched.
    Also the red from the regulator should go directly (through a fuse) to the battery positive terminal.

    Can't read some of the smaller text in the image.
    I'm assuming you are adding wiring for the handlebar controls? Or is that being done with the
  5. cezzz

    cezzz XS400 Member

    For the Key and the R/R red wire i based it off the diagrams that came with the m-unit. Based on the original wiring diagram I was compelled to do what you recommended but wanted to try this first and would change if something went off. Ill upload a PDF to make it easier to read

    Attached Files:

  6. cezzz

    cezzz XS400 Member

    Have been slow to get back to this project, but the diagram I posted above has not worked yet.

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