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GMC- No Low Beams on Plow

Discussion in 'Fisher Engineering Discussion' started by gerardk, Nov 26, 2010.

  1. gerardk

    gerardk Junior Member
    Messages: 4

    Recently installed straight blade plow from an 05 GMC to an 08 GMC. Have a 4 Port Isolation Module 27781. Called Fisher and obtained 29458 kit to convert HB4 to H11 bulbs on the 08 and the in-line adapter 28027-2 . The vehicle harness is 26357 and the lighting harnesses are 26351 and 26352. When the plow is mounted on the truck all the lights work but the low beams. The low beams work on the truck when the plow is not mounted.
    What gives with this, I have checked and rechecked all the connections and all is good there ? Any help would be greatly appreciated.
     
  2. 06Sierra

    06Sierra PlowSite.com Addict
    from Maine
    Messages: 1,329

    Where the plow harness plugs into the vehicle light harness, plug it in backwards. If the low beam comes one, take the plug apart and flip the wire around. I had the exact same problem with my 06.
     
  3. gerardk

    gerardk Junior Member
    Messages: 4

    Thanks, which plug-in, the high or low beams?, I tried reversing the wires on the low beam light on one side with no change . Could it be possible that the lighting harness will not work with the H11 bulb adapters as that is the only thing I changed in the harness, it worked fine on the other truck. I also tried another 27781 Isolation module with no change and another vehicle harness with no change.
     
  4. 06Sierra

    06Sierra PlowSite.com Addict
    from Maine
    Messages: 1,329

    I think I actually had to swap all four plugs, high and low on each side.
     
  5. gerardk

    gerardk Junior Member
    Messages: 4

    Thanks for your response. Is it possible that the 08 truck has positive ground for the lights rather than the negative ground on the 05. It is weird why the low beams will not work when all I really changed was the adapters for the HB 4 bulbs to H11 bulbs ??
     
  6. Burkartsplow

    Burkartsplow PlowSite Veteran
    Messages: 3,245

    you have to switch the low beam plug upside down to the common side and it will work perfectly.
     
  7. weshermanllc

    weshermanllc Junior Member
    Messages: 12

    I am having this same issue on a 2012 GMC.
    Same as the OP low beams work on truck not on plow. The plugs in the connector are slotted so then can only be inserted one way.
    Disassembling a plug and reversing a common would render the truck lights inop, would it not.

    I have Veh Light Harness 29861 (11pin) with Relay assem 29859. [3 plug, 4 port iso] moved from an '07 Chevy to a '12 GMC
     
  8. fullahead

    fullahead Member
    from Maine
    Messages: 51

    +1 had the exact same issue with the drivers side low beam. The people at fisher said they made them reversible for ease of installation, but on GM trucks the polarity matters. Just make sure to only flip one connector at a time (they should be marked as the low beam, either a green or blue wire) to avoid causing more trouble.

    The connectors I'm referring to are where the fisher wiring joins the vehicle headlight wiring. You will have to pop out your headlights to find those connectors, super easy on our trucks.
     
    Last edited: Oct 21, 2012
  9. brad96z28

    brad96z28 PlowSite.com Addict
    Messages: 1,526

    Luckily I have used the correct 3 port module and correct headlight harness. Ive never used the adaptors. I know you said they are slotted but try flipping the connector on the low beams and force it in the plug It should still go. You need to do that on the older models and it works.
     
    Last edited: Oct 21, 2012
  10. fullahead

    fullahead Member
    from Maine
    Messages: 51

    Another method, call Fisher, that's how I figured mine out. The guy said he gets this call for headlight problems on a GM about 6 times a week. They'll talk you through it.
     
  11. brad96z28

    brad96z28 PlowSite.com Addict
    Messages: 1,526

    So do I..:laughing:
     
  12. weshermanllc

    weshermanllc Junior Member
    Messages: 12

    Well I have to say that you guys now your stuff and that is why I came here for the answer.

    As much as I told myself there was not way reversing the polarity was the answer, I used a set of jumpers and criss crossed on one side and guess what, low beams on the plow.
    So I have taken the connectors out of the plugs and reversed them, The molded slot was too large to allow just flipping the plug.

    plow done, speader next...

    Thanks again.

    :redbounce