New (to me) truck, light switch wire question

[KDOHM]

I joined the 3rd Gen club! I sold my trusty '96 2500 with 175K mi and bought me a nice '03 (see sig). So far I love this truck!

Anyway I wanted to throw out a question - Has anybody successfully wired a running light interupt switch in these "computer controlled" lighting systems? I would like to "communicate" with other trucks. TIA



Looking forward to many years in my new rig!

 

[rbattelle]

Maybe no one has done this, but it is a cool idea. I wonder if you'd have to reprogram one of the computers?

 

[JWillms]

Are you just looking to just use a filp switch or somehting like that to filp the lights on and off? Thats a cool idea and dosent sound to hard.

Justin<><

 

[ThomasLawrence]

It looks pretty doable. Power for the parking lamps is controlled by the parking lamp relay in the power distribution center. One choice here is to move this relay outside of the PDC, and use jumper wires with spade terminals to connect back into the PDC relay socket. You would then just wire in a switch between the relay's #87 terminal and the corresponding terminal in the PDC.



Alternatively, you would need a double-throw switch, and splice into two wires coming out of the PDC - an 18 gauge pink with violet tracer (this controls the left side) and an 18 gauge black with yellow tracer (this controls the right side and the cab clearance lights). Break these wires and the lamps will go out.



-Tom

 

[KDOHM]

Hey guys, had an unforseen road trip so I couldn't reply til now.



Thank you for the ideas, I will have to trace some wires/relays. On my '96 it was easy to do right at the switch, but on the '03 I could get no definitive test light/meter readings at switch wires. I have read posts that explained that the switch only sends pulse to ecm. Thank you Thomas, I'll start at the relay-maybe this weekend I'll find time to try. I'll let you know how it goes!

 

[ThomasLawrence]

On my '96 it was easy to do right at the switch, but on the '03 I could get no definitive test light/meter readings at switch wires.

Yeah, the 3rd Gen light switch is different. All it does it take a reference voltage in, and switch it through different valued resistors, with the output going to the FCM (Front Control Module).



The FCM says, in effect, "Hmmm... I see about 10Kohms resistance here - that must mean he has the switch in the 'headlight' position. Okay - I'll turn on the parking lamps and headlamps".



Because there's two discrete resistance values, one for parking lamps only, and one for headlamps+parking lamps (but none for headlamps without parking lamps), it's not possible to "turn off" just the parking lamps from the headlight switch.



This is much easier to understand by looking at a picture of the wiring diagram, rather than struggling to read through my written explanations You can get the wiring diagrams on-line here



-Tom

 

[KDOHM]

Thanks Tom, I'll try downloading the diagram, Kevin

 

[Spooled-up]

I had the wire cutters on what I thought was the wire to do just as you describe, but thought I had better check one more time. Turns out I would have cut the instrument cluster lights. Glad I checked. I started reading about this in my service manual and couldn't figure it out to save my life.



If you get it to work, let us know how you did it.