GMC c6500 Headlight help

[Wayne_Nosala]

I recently volunteered to fix the non-working headlights on the CORVA
dumptruck. seemed like a simple task, but 10 hours later no further
progress than I started.

Truck is 2000 GMC C-6500

Symptoms,
No manual Headlight or Running lights.

Daytime driving lights come on when key-on, in gear

Looked for obvious, checked ALL fuses & Breakers

I found relays in glovebox, If I jump-bypass relay, lights work
same for running light relay.

I changed out light switch on dash, has power on red wire, powers yellow wire when switch turned on.

Seems to me the way things act when K/O Then In gear,
Black box under dash seems to control breaking & daytime running lights.
for some reason, Daytime running light function is not letting things kick
back to manual?

I have no manual for component placement & wiring diagram, so flying blind here!

Any GMC talent / Ideas / help... would be greatly appreciated

Wayne Nosala
CORVA/FOJ
Wnosala@aol.com

 

[matt_helton]

does it have auto headlamps? im not sure if the heavy duty truck had that feature......these diagrams are for a 1 ton truck but should be very similar......if it has auto lamps......you need to look at the ambient light sensor on the dash. cover that up so it thinks its night time.....if the lights still wont come on then it could be as simple as that switch being bad. id start there.


this might also help........Voltage is available to the headlamp and panel dimmer switch at all times from the LIGHTING Fuse 8 through CKT 42 (RED). The headlamp and panel dimmer switch includes a self-resetting circuit breaker. The circuit breaker opens when the headlamp and panel dimmer switch draws too much current. When the circuit breaker opens, the circuit breaker interrupts the current flow. Without the current flow, the circuit breaker cools and resets automatically.

When the headlamp and panel dimmer switch is in the HEAD position, the headlamp and panel dimmer switch directs voltage to either the low beam headlamps on CKT 12 (TAN) or the high beam headlamps through CKT 11 (LT GRN). This depends on the headlamp and panel dimmer switch position. When the headlamp and panel dimmer switch is in the HEAD position, not only is voltage available for the headlamps, but the daytime running lamps (DRL) module receives a HEADLAMPS ON signal through CKT 10 (YEL). When the DRL module receives this signal, the module disables the DRL operation if the ignition switch is in the RUN position, the BULB TEST position, or the START position.

When the headlamp and panel dimmer switch is in the HEAD position and the headlamp and panel dimmer switch is in the LOW position, voltage is available through the closed contacts of the headlamp and panel dimmer switch to the left low beam headlamp and the right low beam headlamp. Ground G112 supplies ground to the right low beam headlamp Ground G113 supplies ground to the left low beam headlamp in vehicles with the base headlamps and in vehicles with the quad headlamps. With both power and ground applied, the low beam headlamps light.

When the headlamp dimmer switch is moved to the HIGH position and the headlamp and panel dimmer switch is in the HEAD position, voltage is available through the closed contacts of the headlamp and panel dimmer switch, and the headlamp and panel dimmer switch to the DRL module, the left high beam headlamp, and the right high beam headlamp. With the headlamp dimmer switch in the HIGH position, voltage is available to the high beam indicator. With ground G103 in vehicles with gasoline engines, or ground G104 in vehicles with diesel engines, supplying a ground to the high beam indicator, the indicator lights. Ground G112 supplies a ground for the right high beam headlamp. Ground G113 supplies a ground to the left high beam headlamp. With both power and ground applied, the high beam headlamps light at full intensity.

 

[matt_helton]

and heres your DRL module.



oh and ive seen a few problems like this in the past where you pull your hair out for hours and hours and it turns out the be a faulty ignition switch. just food for thought. lol

 

[Wayne_Nosala]

Hey Matt, thanks for all the info. Looks like you
put some time in it! I will give it another go tonight.
Will let you know how it goes.
Thanks
Wayne

 

[Brutpwr]

Wow...all that to get your lights to work. I'm glad my trucks are just a regular on/off switch thru a dimmer and then to the relays!

Jason

 

[Wayne_Nosala]

Heya Matt,

OK, see if this makes any sense

Circuit labeled B/U IGN3 is blowing fuse at fuse box.

seems to be short in this circuit

This circuit seems to feed relays in glove box and keyed accessories


If I rig a breaker in the B/U IGN3 fuse location, the headlights, running lights,A/C
all work momentarely until breaker pops.

If you have any more ideas and/or diagrams for this circuit, I will
give it another try before sending this mess to the dealer.

Thanks
Wayne Nosala

 

[matt_helton]

well without the exact diagram for this truck i dont think i can be of much help. i dont have any of the c-6500 info here. im sure its a bit different than the c-3500 diagrams cus i cant even find that circuit at all. a funny thing is though that "b/u" is an abreviation for "back up"......like back up lights....so that paired with an IGN1-2-3 type fuse is a bit strange without seeing it in person.

so yeah you deffinetly have a short. where? haha, guess i cant doo much more from my keyboard. sorry man.

 

[Wayne_Nosala]

No Prob,
Would make things easier if I could get a manual
from the local Napa or AutoZone, But not for this monster.

I probably will fiddle with it some more when I get some time,
may just get lucky unplugging components until I find this
mysterious short.

Thanks again
Wayne

 

[voylesprep]

Just a thought, but I have seen problems like this before, and the cause has almost always been a bad ground. When the system is not grounded properly, it will back feed, cause some components to work at the wrong time, and blow a breaker or fuse. Look for a ground system diagram for this truck, and check and clean all ground points. If you have the time, check all the ground wires from the ground point to the circuit point with an Ohm meter. Hope this helps.

 

[voylesprep]

Also thougt of this...if the system uses a standard GM marker/turn signal light, the bulb can be the problem. An 1156a bulb is for marker lights, and an 1157a is for turn signal AND marker lights. These are slightly different, but sometimes fit in the same socket. Check to see that the correct bulb is in the correct socket by refering to a guide for the truck you are working on. I have seen odd and unexplainable problems caused by this in years past...just a heads up.

 

[Wayne_Nosala]

Thanks for the advice guys. I got it fixed. After spending
many more hours playing "pop the beaker" while unpluging
everything I could find, turns out there is a transmission sensor
(temp or?) that has dead short. Disabled it, everything now functions perfect
Thanks again
Wayne
CORVA
Freinds Of Jawbone

 

[ShaneR]

That really sucks that a transmission temp. sensor would mess up your headlights. Can they make things anymore confusing to work on?

 

[Wayne_Nosala]

Not only headlights,
It fed all of the accessory relays, Air conditioning,Heater,back up lights & ABS

 

[ntsqd]

Yikes!!!!


Do Not not ask that question or "they" will.



Wayne, Glad you got it figured out.

 

[matt_helton]

thats GM for ya. over complicating the crap out of everything. i dont miss the days at the dealership chasing ghosts like that!

the squirt gun is much more fun.