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Thread: M998 headlight issues

  1. #1
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    Default M998 headlight issues

    Hello all,

    I am hoping someone can help me with the headlight issue I am having on my M998. I recently replaced the 3 level switch and when using that for operation I can get all blinkers, yellow running lights, blackout lamp, etc. to all work fine. I am however unable to get the actual headlights to come on. I have tried all the settings on the 3 lever switch and read the operations manual. I have pressed the dimmer switch on the floor board and it works and the LED on the dash changes to green when on high beams as expected (although with no headlights on).

    Am I missing a setting? What are the odds that both headlights are bad? Could it be a fuse? if so, where would I check? Any and all help very much appreciated.

    Thanks!

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    4 Star General NDT's Avatar
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    There is no fuse, only a circuit breaker inside the 3 lever switch, which protects all the lights, so if some lights work, that is not the issue. It is unlikely that both high and low beams are burned out in both bulbs, but you never know. I would ohm out the bulbs. If good, check the harness end to end from the bulbs back to the dimmer switch and main light switch.

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    Thanks NDT. I thought I had read somewhere the M998 had a fusebox so that is good info. I'll give your recommendations a shot.

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    No update on this problem?

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    Sergeant Major 4x4nutz's Avatar
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    new pict.jpgHaving the same issue. Turned the light on and before I could drive off, both head lights went off. All other lights work, but neither low beam or high beam on the head lights work.

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    4 Star General juanprado's Avatar
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    I suspect your floor mounted dimmer switch is where I would start.
    1984 M923 "Maj. Zippidy" who now is equipped like a M925A1
    1991 M998 HMMWV

    1990 Kasel M101A2
    1968 Case Master M101A1 with fuel tank and gen set up
    1991 Eastern Technologies M105A2
    60's ? m116a1? radar trailer flatbed
    xm1073 flat bed 7 1/2 ton


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    This can be easy with the TM's and a voltmeter or simple probe type test light for 24 volts.

    Light switch in the SERVICE and PANEL BRIGHT position. (there are knock off new light controls. Just know could be good deal or ??)

    At dimmer switch on floor, lead# 16A should have battery voltage when above switch in set above. YES ? NO = bad switch.

    Cycle dimmer switch to HI observe voltage on lead 17A.

    Cycle dimmer switch to LO observe voltage on lead 18A.

    OP stated the HI/LO beam indicator follows the dimmer switch. Based on OP info that hi/lo lamp works. That's a good sign that problem is down stream.

    Check frame to hood connector to be in good condition. Not likely based on your OP info. Other lights work normally.

    Headlight connections: good to remember or mark inside buckets with marker.

    Headlights lead # 91A is GROUND Headlight common. Check these leads to be at ground at headlight/s leads to known good engine frame ground (not in the hood) Lead #91A is main ground to hood and other lights. OP noted other light work correctly. Note: There is a common ground splice in harness to bring all of the grounds together to the 91B lead to connector pin E, then to frame ground. Not likely problem based on OP info.

    Headlight leads #17A is HI beam, 24 volts when selected.

    Headlight leads #18 is LO beam, 24 volts when selected.

    Easier test, check bulbs first. Good chance they are toasted. But it was fun to walk thru this exercise.

    It's part of the hobby / ownership and an exercise in TM's. (at least for me)

    Owning a HUMMWV, big bucks. TM's FREE, CAMO

    No guarantee for my free info... as HUMMWV gremlins are always lurking around.
    Last edited by Milcommoguy; 09-13-2019 at 17:35.

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    Colonel DREDnot's Avatar
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    There is always a great chance that both lights are toast. So test for voltage there first.

    That being said, my headlight malfunction experience with the military headlight switch consisted of no headlights unless the "headlight on" lever was held hard over to the stop. If you let go of the lever and left it in the on position, the lights would go out.

    Took the switch apart and found that a previous short/over current condition had burnt the contact partially.

    20190823_193047.jpg

    20190823_193103.jpg

    20190823_193334.jpg

    I ended up cleaning the fixed contact and swapping the identical contact shoe from the blackout side to headlight side. Headlights now work.
    1998 M1078A0
    1993 M998
    1986 M1009(sold)
    1992 M101CDN(sold)
    "Non Vi Sed, Arte"

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    I've opened up several of these switches to try to make functional units out of partials and found that while they all look similar on the outside their internals and the way the lever shafts seal to the case can be very different.

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