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MMI intermittent low beams

Discussion in 'Fisher Engineering Discussion' started by Raymond S., Nov 10, 2009.

  1. Raymond S.

    Raymond S. Senior Member
    Messages: 513

    I had this problem last year where the low beams would come and go, hi beams were always fine. In fact, when I disconnect the plow my driver's side low beam hardly works. I have tracked it to the #3 plug at the isolation module. I can wiggle it and make the lights go on/off. I have it sucked up tight w/ a zip tie and can't wiggle it to make the lights go off, however they still go on/off going down the road. Sometimes it's fine for an hour, sometimes it's 5 minutes.
    I thought perhaps it was a bad isolation module last year so I replaced it with another one on ebay. Any chance I got a bad used module? I can run with high beams which I did pretty much all last winter, but I'd like to get this issue resolved if I could.
  2. ALC-GregH

    ALC-GregH PlowSite.com Addict
    from pa
    Messages: 1,143

    sounds like a ground problem.
  3. Raymond S.

    Raymond S. Senior Member
    Messages: 513

    I've always thought it was a ground problem, but why the ability to effect it at the isolation module?
  4. ALC-GregH

    ALC-GregH PlowSite.com Addict
    from pa
    Messages: 1,143

    it back feeds until it finds a place to get power.
  5. bdavis

    bdavis Member
    Messages: 66

    You might want to check the jumper on the number 1 wire next to the two in-line fuses....maybe thats where the loose connection is. That jumper is what controls low level voltage to the low beams. Also what truck do you have.
  6. plowinginma

    plowinginma Senior Member
    from MA
    Messages: 326

    intermittent dim lights = bad ground..
  7. Raymond S.

    Raymond S. Senior Member
    Messages: 513

    The lights are never dim, either on or off. And when they come on/off I can hear it clicking inside the isolation module.
    The truck is a '96 F250
    It feels like it's in the plug end of the harness where it plugs into #3 terminal.
    I'll do some checking of the jumper. I checked the fuses but I can't quite picture exactly where you're talking about a jumper. I'll have a look around and report back if I can't find it.
    I'll also double check the ground. Is there another ground besides the one on the plow frame? I helped w/ the install but all I remember was splicing into the harness for the lights, other than jumping from the battery over to the sylenoid.
  8. Raymond S.

    Raymond S. Senior Member
    Messages: 513

    Ok, well thanks for pointing me in the right direction with the grounding issue as well as the pins at the harness. All is well and here is what I found.

    1. terminals at the harness going into the isolation module on #1 & #2 wire were pushed back. I pushed them in tight w/ a screw driver.

    2. I installed intensifier lights this weekend and the previous owner of the plow put a different plug on from the truckside so it was not just a simple plug/play. I had to use the ends of the harness that came with the lights and adapt them to my existing harness. So I knew in the back of my head the wiring could be an issue if he changed the end for whatever reason. So in lies the problem. Tonight I started shifting the harness around and could get the lights to come on/off, so I'm thinking loose ground. I undo the tape on the harness and find the SOB has every splice halfazzed w/ a butt connector. Long story short I was able to pull the one apart for the low beams. I redid all the connections and bam, I now have low beams consistently.

    3. One thing a bit weird though. Even with the plow unhooked my driver's side lowbeam would be intermittent, I assume from running through the isolation module. It is now fine as I unhooked the plow tonight and tested it out. My only conclusion is that there was in fact an issue w/ the terminals at the isolation module and I actually had two issues, which hopefully they are both now resolved.

    Ahhh, I hate wiring. Dropped the truck off tonight for some preseason work. Got front airbags going on along w/ pillar gauges, speedo cable, coolant flush, and a new starter. (family discount at grandpa's truck shop) I'll be ready for snow shortly.