• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

GMT400 HEADLIGHT SWITCH?

FellowTraveler

Well-Known Member
Messages
2,795
Reaction score
2,310
Location
Nature Coast
I'm trying to chase down a dead short when I turn on the headlights. I did find some squirrel wiring damage shorting the license plate light sockets repaired that problem.

I disconnected the front head & running light harness and now suspect the headlight switch as the culprit any of you have info on known headlight switch issues with the GMT400 series?
 

FellowTraveler

Well-Known Member
Messages
2,795
Reaction score
2,310
Location
Nature Coast
Another issue is that the emergency flasher indicators are always on but dim if I engage the flashers they flash along with the security light.
 

ak diesel driver

6.5 driver
Messages
14,082
Reaction score
5,070
Location
alaska
Only issue I've ever had was from running higher wattage bulbs. There must be a thermal switch in it as it would cycle on and off and eventually not cycle on anymore. I suppose it's possible that it could melt something and short out
 

Will L.

Well-Known Member
Messages
9,227
Reaction score
9,817
Location
Boulder City Nv
I remember we had to change out a lot of there units in the steering column for the flasher and it would mess with the whole system. Sorry it is been way too many years to remember what years had the problem, but I do remember it was the component the flasher switch itself was on.

I hate getting into a column so I wouldn’t just throw it at it, But once you’ve checked everything else and are ready to shot gun parts that is where I would start

And a great excuse to buy the Power Probe 3 kit with open and short finder...
 

WarWagon

Well it hits on 7 of 8...
Messages
8,965
Reaction score
3,964
Location
AZ
Does it short on parking lights before turning on headlights? I have had dimmers short out. Not sure what switch you have.
 

MrMarty51

Well-Known Member
Messages
1,791
Reaction score
2,288
Location
Miles City, Montana
Usuall if I get multiplelights, usually not all on the same circuit, doing weird things, I look for a bad ground wire.
Brought that to mind when You mentioned the four way flasher lights on dim and the security lamp coming on too.
 

svein

Active Member
Messages
82
Reaction score
190
Location
Norway
The dome light switch stopped working on my old 99 express van last summer, so I opened the switch and re-assembled it to fix it. It took a bit of patience to get it back together, but it´s absolutely doable to open and get it back together again. The headlight switch it self is just turning some copper to make contact with different paths on a print card in the back - so I guess if you have enough corrosion or something is broken inside the switch it could create a short.

By the emergency flasher indicator - do you mean the light in the switch itself?

I hate to say this, but lights being on when they shouldn't, and dimming when they should sounds like a grounding issue to me causing the currents to flow in new and "exciting" ways. No way to say that the grounding issue isn´t in your light switch, but from what @Will L. says, maybe the cruise control/turn signal/hazard light module could be the culprit?
 

FellowTraveler

Well-Known Member
Messages
2,795
Reaction score
2,310
Location
Nature Coast
The dome light switch stopped working on my old 99 express van last summer, so I opened the switch and re-assembled it to fix it. It took a bit of patience to get it back together, but it´s absolutely doable to open and get it back together again. The headlight switch it self is just turning some copper to make contact with different paths on a print card in the back - so I guess if you have enough corrosion or something is broken inside the switch it could create a short.

By the emergency flasher indicator - do you mean the light in the switch itself?

I hate to say this, but lights being on when they shouldn't, and dimming when they should sounds like a grounding issue to me causing the currents to flow in new and "exciting" ways. No way to say that the grounding issue isn´t in your light switch, but from what @Will L. says, maybe the cruise control/turn signal/hazard light module could be the culprit?
Its a weird situation sometimes the cluster lights work the turn signal indicators are both lit then just one is lit. The turn signals flash fast like one bulb is out but bulbs are all in and working.

If I put the 4 way flashers on they flash normally but the security indicator light in the cluster flashes with them too.

Have fixed the issue w/fluke multimeter so I'll be able start trouble shooting the harness and switches. I ordered a headlight switch and thinking I'll need the 4 way flasher switch too however; I'm not sure yet if it is integrated into the other switches on the steering column.
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,575
Reaction score
3,342
Location
ZEPHYRHILLS FL
With the other lights lighting up, you have a ground issue to your cluster. As to the fuse blowing, have you tried unplugging the rear chassis harness under the booster? It sounds to me like you have crossed and backfeeding your harness when you did your swap.
 

FellowTraveler

Well-Known Member
Messages
2,795
Reaction score
2,310
Location
Nature Coast
With the other lights lighting up, you have a ground issue to your cluster. As to the fuse blowing, have you tried unplugging the rear chassis harness under the booster? It sounds to me like you have crossed and backfeeding your harness when you did your swap.
More likely than not you are right about me screwing up something during the swap. I'll be unplugging that harness today.

I had installed an aftermarket headlight harness and when I put on the low beams the high beam indicator lights I switch the harness feed to the headlights at the relays and it still acts the same way so it looks like I have to pull the entire front lighting harness and to find what I screwed up.
 

FellowTraveler

Well-Known Member
Messages
2,795
Reaction score
2,310
Location
Nature Coast
I am waiting for electrical parts to isolate all accessories from under dash harness and a new headlight switch too I suspect I'll know the cause of this issue soon.
 
Top