LBZ electrical issues

[Wturnbo]

Having electrical issues with 2006 gmc 2500hd dmax. Hazard lights on they alternate between brake light and turn. Third brake blinks as well. With headlights turned on does the same thing except now license plate lights flash. I look like a construction truck. Turn signals on mirrors work perfect until touching brakes then half of turn signal dims when flashing. Stock lights checked all bulbs. Checked grounds. New flasher relay. Could it possibly be the turn signal switch?

 

[Wturnbo]

https://youtu.be/B4shxYIWsFc

 

[TheBac]

Moved your posts into their own thread in LBZ.

 

[2004LB7]

I would guess a bad ground for the turn/brakes

 

[Wturnbo]

I have checked most of the grounds i haven't checked the grounds inside the cab yet. I have heard there is one on the a pillar that messes up sometimes.

 

[dndj]

I have checked most of the grounds i haven't checked the grounds inside the cab yet. I have heard there is one on the a pillar that messes up sometimes.

A bit more info to help troubleshoot, please.

I watched the video, and I can't tell if the license plate lights are supposed to be ON and shouldn't be switching OFF, or they are supposed to be OFF and shouldn't switching ON. Were your park/headlamps on at the time?

If left turn/right turn/flashers are all turned off, do the tail and brake lights otherwise work normally?

What is happening on the front turn / marker / park lamps?

This info will help get us pointed to the right schematics. License plate and tail lamps are on the park lamp circuit yet you're seeing your center high brake light flashing. The rear tail / brake filaments in the bulb share a common ground (park / turn filaments in the front), if that's not connected well the kinds of things you're seeing are possible.

 

[JoshH]

Do you have any LED bulbs installed?

 

[04chase]

continuity on the ground can test ok but when a load is applied it may have insufficent supply. you can easily add grounds with jumper cables or jumpers from batt to the body,bed ,etc just hook them up to the negative and not the positive . helps determine if its lack of ground as the culprit.

led's also can do weird things

 

[Wturnbo]

With no lights on and turn signal on turn signal and brake alternate with each other Brake and turn working same time hyper flashes first blink then back to alternating and 3rd brake light flashing. Hazards on alternating brake and turn signal and 3rd brake light flashing. After a whule of it being on 3rd brake light stops flashing. Cruise on use turn signal turns cruise off today tested and just did it when left blinker turns on but then didnt do it consecutive. With lights on everything goes the way you see in video. Only leds are daytime running and license plate

 

[dndj]

Ok. Looking at the video again, the telltale symptom here is that with the headlights on, the license plate and tail lights go out when the turn signal (amber) comes on. Since the license/tail lights are on the PARK circuit and always supplied with +12v when the headlights are ON, the only way these lights can be going OFF is if +12v is being applied erroneously to the GND side of the license/tail bulbs when the turn signal is ON, which implies there is a missing ground. This +12v might even be sourced through the turn signal bulb(s) if they are not properly grounded, after all an incandescent automotive bulb like a 3157 is like 5 or 6 ohms when turned on.

My best guess, since your center brake light is involved as well: there is a junction block on the rear most cross member, facing the rear, drivers side. The tail/brake/license/reverse/turn signal harnesses all tie in here, there are 4 different connectors. I would dissasemble, inspect, and clean this very well. Look for corrosion or other possible bad connections/shorts. The junction block should be a sealed part I think, if it's been damaged or cracked, moisture could get inside and cause all kinds of havok. Good luck.

 

[Wturnbo]

I did unplug all of those they did have alot of dirt in the connectors i cleaned them a little but not great ill clean them really good and see. All of this happened after a decent amount of snow fell here.

 

[dndj]

Also, definitely undo, clean and re-ground G401. This is the main ground point for all the rear lighting on main rear frame, on top of frame by the spare tire hoist. Many GND connections in the 4 connector junction block ground here.

Wouldn't hurt to take a look in the trailer connector as well.

 

[Wturnbo]

Didnt find that ground yet will definitely look at that and i unplugged the trailer connector completely. More diagnostics today hopefully i can find it.

 

[2004LB7]

It should be bolted to the spare tire crossmember

 

[Wturnbo]

Got frustrated with wiring so went back to the fuse block under hood saw a fuse missing so put on in turn the key on immediately popped it looked at what it controlled and it said fuel heater. I have been told there isnt one on these trucks and ive been told its the fuel head.

 

[2004LB7]

Take a look at your fuel filter head. If you have a wire connection at the top then you have the heater

 

[Wturnbo]

Never found a ground by the spare tire did however find some schematics that showed they're being one in between the frame and the gas tank I finally found that one is where the cab and bed meet in that space I also cut out the trailer connector that goes into the bed that was spliced in just to double-check make sure that wasn't the issue going to put new connectors and heat shrink on it this time and then wrap it up make sure there's no shorts in that.

 

[dndj]

So I've been doing a little research. The 2003 FSM and GM upfitter schematics for 03-07 classic GMT800 describe 3 possible grounding points for the rear lights / rear junction block depending on model.

• 3500: G302, left side frame on rear body mount (or front bed mount?), circuit 1550 - ground zone 15
• 3500 chassis cab: G403, same location as above, circuit 1650 - ground zone 16
• 1500/2500: G401, top of spare tire hoist assembly, circuit 1750 - ground zone 17

I checked my 03 3500 and found G302 on the left front bed mount, facing forward, in the area between the rear of the cab and front of the bed. It's a ring crimp connector under a bolt with a black wire. I also checked my dad's 04 2500HD, there was no ground on top of the spare tire hoist that I could find after a pretty good search, so I could guess maybe the ground point is also the front left bed mount and the manuals weren't updated? Regardless of the GND configuration on your truck, one of these 3 GND connections is GND for the rear lighting junction block, coming in on junction block connector C1, pin E. C1 connector body is blue, pin E is the black wire.

Suggest pulling C1 off the rear junction block and connect a test lamp between pin E (connector C1) and +12v to verify GND. Or, and ohmmeter between C1 pin E and chassis to verify GND.

 

[Wturnbo]

Finally found it!!!!!! Someone had relocated front turn signal ground to underneath battery rubbed into. And on that goes above fuel tank had rubbed into as well. Lights are all working correctly now what a relief. I appreciate all the help you have given.

 

[dndj]

Finally found it!!!!!! Someone had relocated front turn signal ground to underneath battery rubbed into. And on that goes above fuel tank had rubbed into as well. Lights are all working correctly now what a relief. I appreciate all the help you have given.

Excellent, glad to hear you got it tracked down and all is well. Thanks for posting the root cause, because others will find this thread when searching for similar problems in the future. :thumb: