no third brake light

[mjones0304]

Hello I have a 2003 4x4 2500 with 170,000 miles on it and I was just wondering if anyone had ever encountered their third brake light not working. I dont know how long it hasnt been working cause thats one of those things you dont see unless the brakes are on. A friend of mine was following me the other night and made a comment that it was not working so I started to investigate and noticed that for some reason it will come on with the brake controller ( sliding the switch) but not the foot brake. Cargo lights also work fine. I would imagine that its not a wiring issue with the third brake light since it will come on with the controller. Any help would be appriciated and thanks in advance for any replies.

 

[PBJ2500CTD]

Well, something is certainly fishy if the third brake light comes on when the brake controler switch is manually activated. The output of the controller should have no effect on the lighting. That is done through the brake light switch. I would suspect a major wiring issue. It may seem an odd question, but does the brake controller activate the trailer brakes?

 

[jelag]

My Brake Smart connects to the brake light circuit as well as a pressure transducer at the master cylinder... I'm almost positive that when you hit the button on the controller it will turn on the brake light circuit when the button is pressed... . so that the people behind you know you've applied the brakes, either with the foot pedal or button...

 

[Bob4x4]

The chmsl is a separate out put from the stop light switch. Start your troubleshooting there.

 

[Bob4x4]

Brake lamp switch - black 6 way cav circuit function

1 a108 18tn/rd fused b(+)

2 l50 18wt/tn brake lamp switch output

3 v30 20db/rd speed control brake switch output

3 v30 20db/rd (ngc) fused ignition switch output (run-start)

4 v32 20yl/rd speed control supply

4 v32 20yl/rd (5. 7l) brake switch no. 2 signal

5 z241 20bk/vt ground

6 k29 20wt/pk brake switch sense

6 k29 20wt/pk (ngc) brake switch no. 1 signal

 

[Bob4x4]

Center high mounted stop lamp/cargo lamp - 4 way cav circuit function

1 f32 20rd/yl fused b(+)

2 m3 20pk/db cargo lamp driver

3 z56 18bk ground

4 l50 18wt/tn brake lamp switch output

 

[PBJ2500CTD]

... The output of the controller should have no effect on the lightiing...

Oops... Goofball me... must remove foot from mouth... mis-statement on my part... Thanks for jarring my brain cell jelag - yer right. That sure changes my electro sumthin' degree to a "duh"-gree The manual switch supplies a voltage through the "cold" side of the brake switch to illuminate the brake lights when the pedal is NOT activated but the controller switch is. Both the CHMSL and the brake lamps should illuminate if either the brake pedal is pushed or the manual switch is activated. That same "cold" side of the brake switch receives voltage when the brake pedal is pushed letting the controller know that it must wake up and do its thing. Bob4X4 gots the right starting idea at the switch thingy for the brakes.

 

[PBJ2500CTD]

My Brake Smart connects to the brake light circuit as well as a pressure transducer at the master cylinder... I'm almost positive that when you hit the button on the controller it will turn on the brake light circuit when the button is pressed... . so that the people behind you know you've applied the brakes, either with the foot pedal or button...

Jim,

You are correct, it does illuminate the brake lights when the button is pushed. That is even stated in the BrakeSmart owner's/installation manual - http://www.trailersaver.com/pdf/brakesmart_owners_installation_manual.pdf

Gonna have to find me one of those controllers...

I don't have an '03 wiring diagram, but I don't think too much would have changed from '02. In the '02 diagram the tow brake controller connector is tied through the CHMSL circuit. The brake light signal is sensed through a Joint Connection 6 where it splits to the CHMSL lamps and off to the Multifunction Switch where the Hazard Switch and Turn Signal Switch get to play with the circuit before it gets to the brake lights.
So, if I understand the OP's issue, only the CHMSL lights with the brake controller manually activated; only the brake lights illuminate with the brake pedal pressed. If the brake lights illuminate with the pedal, then there is a good chance the multifunction switch is good. The problem may lie somewhere in Joint Connector 6 or associated wiring. That brings up another question: Is the brake controller connected to the vehicle via the appropriate harness and factory connector, or is it attached by parasitic electron sucking scotch-locks.
I would disconnect the brake controller first, then check the switch and wiring if key symptoms don't disappear. Follow the White/Tan wire that Bob4X4 listed. That is the one that routes voltage to the lamps.

 

[sag2]

Notice in Bob's post that brake switch cavity 2 is the same L50 circuit that feeds to pin 4 of the CHIMSL connector. My vote is bad brake switch, or open between the brake switch and CHIMSL on the L50 circuit. The brake controller may light the CHIMSL because they tapped into it after the brake switch.

 

[PBJ2500CTD]

Notice in Bob's post that brake switch cavity 2 is the same L50 circuit that feeds to pin 4 of the CHIMSL connector. My vote is bad brake switch, or open between the brake switch and CHIMSL on the L50 circuit. The brake controller may light the CHIMSL because they tapped into it after the brake switch.

Agreed... Those are the most logical possibilities...

 

[mjones0304]

Hello everyone I tried unhooking the brake controller and that did not fix the problem I did however notice that the brake lights only come on with the key turned on. Now that may be how they are supposed to work but all previous vehichles ive ever owned the brake lights will come on without ignition on. To make sure everyone fully understands the problem I will explain again. With foot brake pressed and key turned on both left and right rear tail lights will illuminate as should but no third breake light on top of cab. With key turned on or off sliding the brake controller will illuminate both rear tailghts and also third brake light on top of cab as it definatly should. I understand that the controller should illuminate all brake lights and that is what I want the foot brake to do as well. Also the brake controller activates the trailer brakes as it should to answer that question. The controller is the right one for the truck ( plug and play ) so its kinda hard to do anything wrong there, like I said unhooking it made no difference. Thanks again for all the help.

 

[Bob4x4]

CHMSL center high mounted stop lamp

 

[PBJ2500CTD]

Hello everyone I tried unhooking the brake controller and that did not fix the problem I did however notice that the brake lights only come on with the key turned on. Now that may be how they are supposed to work but all previous vehichles ive ever owned the brake lights will come on without ignition on. To make sure everyone fully understands the problem I will explain again. With foot brake pressed and key turned on both left and right rear tail lights will illuminate as should but no third breake light on top of cab. With key turned on or off sliding the brake controller will illuminate both rear tailghts and also third brake light on top of cab as it definatly should. I understand that the controller should illuminate all brake lights and that is what I want the foot brake to do as well. Also the brake controller activates the trailer brakes as it should to answer that question. The controller is the right one for the truck ( plug and play ) so its kinda hard to do anything wrong there, like I said unhooking it made no difference. Thanks again for all the help.

Your statement suggests everything "post" brake light switch is good.
I cannot say if your fuse numbering is the same as a 2002, but you should have brake lights w/o having to turn on the ignition. On a 2002 the battery voltage to the brake switch is supplied through fuse F5 (20A) in the PDC. Check your fuses in the PDC. If the fuses are good, check the brake lamp switch itself.