Pulsing headlights and parking lights

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JPMcCormick1

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Hello, this is my first post so please bear with me if I am breaking any protocols or if this is the wrong place to post this question.

I have an M998 year 1995 that we took off-roading the other day and came back at night. All lights worked fine until we got a few miles from home and all lights just quit for no reason. What is interesting is that when we turn the lights on all headlights and parking lights will pulse a faint light very briefly every 5 seconds or so. I did try the horn when the light switch was turned on and nothing. Side lights and turn signal also stopped working. The only ones that work are the Black Out lights. Not sure if this is a bad light switch or something else? Any suggestions would be appreciated.
 

NDT

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That pulse is the self-resetting circuit breaker reacting to a short circuit in the lighting circuit. Find the short circuit. The circuit breaker is inside the light switch.
 

infidel got me

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I found the 3 wire harness chaffed at the high/low switch. Where it turns out of switch there is a bracket/cover/guard with 3 phillips screws holding it to floorboard. My harness did almost a 90' turn exiting bracket. Had headlights -hit a bump- no headlights or headlights when it wanted to..... Took a while to find and just happened to luck up on it. The black mesh coverlay made it hard to see, I actually heard it arcing inside harness when wiggled. Did the normal check every thing pertaining to headlight operation and find the problem last, but she's fixed... Thought I lost my mojo for a while there but I sacrificed a few yuengling beers to the electrical gods and all is good.
 

papakb

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The Master Light Switch is either in Service mode or Blackout mode. After lifting the Lock lever you move the topmost lever to the left for Blackout mode or to the right for Service mode. So if you've moved the top lever 1 detent to the right it's in "Service Stoplight" mode, 2 detents to the right would be "Service Drive" mode.
 

papakb

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I have the pushbutton switch in mine only because I'm too lazy to put the old 3 lever switch back in. It's worked without and problems for years now but the thing I hate about it is that you have to look at it to operate it and that can be difficult if your driving. The 3 lever switch is probably more reliable and can be operated by "braille".
 

JPMcCormick1

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So replacing the 3 lever switch did not solve the problem so this is definitely a short in the wiring somewhere. Will check the harness at the Hi/low switch as Infidel Got Me suggested and see if I find anything.
 

dmetalmiki

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Sometimes being a bit 'medieval' can track down electrical problems.
(Or, Is that another way of 'Lateral thinking')?.
So, Let's start the non tech method.
Disconnect the light switch.
Disconnect the Headlamp connectors.

Now fit up a "SAFE" 'Hot wire'. From the Batter Positive. (A probe that ENDS with a light bulb in it.( A low voltage test light probe would be best)).
Touch the probe to GROUND, It should light up.
Touch the probe to A wire in the headlamp connector.
It should not light up. (As you should have a "Clear route through". I.E. An OPEN circuit to the switch Plug. (Now unconnected).
IF the test light comes on, Then that particular Wire (Route) Is Earthing/Shorting/Grounding somewhere.
You can (Patiently!) Test all the circuits on the truck by this method.
Rear light..Brake light..Turn signal etc.
Just remember to disconnect the circuit you are testing at it's source of SUPPLY. Otherwise the supply (Any switch) will sent your testing route down to somewhere else where something is still connected.
This will test/check the OPEN STATE of the single wires in a circuit.
Find out WHAT still works before doing all this.
I,E. Side light? (Parking light).
Rear lights?/ Brake lights.
Right turn and left turn signal?
What works leave alone.
If you turn on the headlamp LOW BEAM does it work?
If you turn on the headlamp HIGH BEAM does it work?
IF NO to either one, Then the "disconnect 'Test'" is between (The) >SUPPLY< the headlamp switch, The DIP SWITCH and or the headlamp harnesses.
Take you time, use "Logic".. you will eventually find the wire OR COMPONENT 'Murphy' Has his teeth into!.
Remember, The open disconnect "ROUTE/SUPPLY" WILL be including things like, Dash panel lights, Dome lights!, Marker lights.
Is a "Component" (Still) Full of Water?.
Hope this helps you get to the root of the "Nuisance."
 

JPMcCormick1

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So had time yesterday to check all the wiring and could not find anything that looked chafed or damaged. Disconnected the headlights from the from main connector that ties into the base of the radiator and still shorting out in Service Drive. I could not find a similar main connector for the rear of the vehicle so could not test with rear disconnected. I did find that the stop light mode does work with no shorting so the only part that shorts out is when she is in Service Drive (headlights and side lights). Will try dmetalmiki's approach but that will only tell me which wire is shorting out it wont tell me where in the circuit it is shorting out. My suspicion is that it might be one of the side lights as those come on at the same time as the headlights. It I cannot find the short I am thinking of hooking the headlights to the black out lights connector since those work fine and disconnect the black out lights since we never use them. This would at least give me headlights but no side lights. We never drive the Humvee on the streets and only use it for off-roading so it would not be a street legal issue not having parking or side lights. Does anyone have any reason to think this might not be a good idea electrically wise.
 
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papakb

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If the problem is in the lighting system all of those lights can be unplugged for testing. Unplug the side marker lights because they're the easiest to get to and see if your problem is there. If the problem still exists I'd disconnect the taillights. There is no rear harness connector in the HMMWV. Keep in mind that the trailer receptacle is also in the lighting circuitry.
 

JPMcCormick1

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If the problem is in the lighting system all of those lights can be unplugged for testing. Unplug the side marker lights because they're the easiest to get to and see if your problem is there. If the problem still exists I'd disconnect the taillights. There is no rear harness connector in the HMMWV. Keep in mind that the trailer receptacle is also in the lighting circuitry.
That is one area I was suspecting being the trailer receptacle however it would seem to me that the trailer receptacle would work with the stop light being on so if it did not short out with the stop light setting then it probably would not be the trailer receptacle when service lights were on. Re disconnecting all the lights, I tried that but still get the short which means that even with the light plugs disconnected from all the lights the short is somewhere in the wiring.
 

papakb

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The service lights run thru the trailer connector too. Everything on the trailer has to work with everything on the truck. With the hood connector undone and the connector on the turn signal switch disconnected you can read the resistance of the rear lamps thru the trailer receptacle. As long as you know the resistance of the lamps themselves (just a few ohms) a direct wiring short should still be apparent.
 

Coug

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I believe the horn is also controlled by the headlight switch, as it's only functional in the service settings, the button in the horn completes the circuit to ground, so there is a wire going to the horn that is always live in service mode. There is always the chance that something in that circuit is the failure point, but it would be before the horn in the wiring unless you're hearing the horn beep as well, and even with anemic as the horn is you'd probably have noticed that and mentioned it.
 

dmetalmiki

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Will try dmetalmiki's approach but that will only tell me which wire is shorting out it wont tell me where in the circuit it is shorting out.

OHHHHHHH yes it will!,

Let us suppose the right rear Side Light is the culprit.

The TEST HOT LEAD lamp will light up.
PERO! (But), With the rear lights and side lights disconnected, You GO with a 'HOT' wire from the battery and "POKE" the right rear disconnected side light.
Lights Up?.....OK.
Go to the left rear Side light.
Lights Up?...OK
Dun't LIGHT UP? (But) (smokes, burns, howls, screams, catches fire!! ) THAT'S a "Faulty Light unit".
Do the same with EACH front sidelight.
I Repeat. If your running hot down the wire and It shorts out, With the LIGHTS disconnected, then YES! it is the cabling.
RUN HOT Tests as described above. It will not be too long before you twig which supply wire OR component is the trouble.
(Email me the truck! I will sort it and email it back to you.).
 
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Coug

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So thinking about this a little more, if it's doing this when it's just the service and not service headlights, the short would need to be in:
the horn circuit before the horn (or possibly the horn itself is shorted out)
the wiring from the switch to the turn signal box (or is it possible the turn signal box is shorted somehow?
the brake signal circuit before the brake switch (or maybe somehow the brake switch has failed)

as those are the circuits that are active in the first service mode.


Can someone else chime in with any other circuits or wires that would be live in this service stoplight mode? Just trying to narrow it down a little bit.

(if it was in one of the turn signal paths it should only happen when that signal is on, not when no signals are being used, and the horn isn't working either, so not there. Same with the brake lights)
 
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