My son in law has an 03 dodge 2500 quad cab with some electrical gremlins. 1. dash lights won't dim, 2 dome light stays on all the time, 3 cargo light stays on for 10 seconds, goes out for 10 seconds, comes on again, etc. forever.
Since all three are related to the headlight switch, I replaced it. Looks like it fixed #1 (will know for sure after dark) but the other 2 are still there.
Being an old f... I suspected a short (grounding) in one of the door switches, but there aren't any.
Can someone explain how it knows the doors are closed?
Or point me in the right direction?

thanks for any help

 

All three of those issues you first mention all have common connections in the headlight switch itself. Maybe the switch needs replacing.

 

After reading your post, I got intrigued by just how the door switches work, this is what I found on the 'net:

DOOR AJAR SWITCH
"The door ajar switches are integral to the door latches on each door The switches close a path to ground for the Instrument Cluster when a door is opened.
The door ajar switches cannot be repaired and, if faulty or damaged, the door latch unit must be replaced.
The door ajar switches close a path to ground for the Instrument Cluster when a door is opened. The passenger front door is connected in a parallel-series circuit between ground and the Instrument Cluster, while the driver side front door ajar switch is connected in series between ground and the Instrument Cluster to provide a unique input. The Instrument Cluster reads the switch status, then sends the proper switch status messages to other electronic modules over the Programmable Communications Interface (PCI) data bus network. The door ajar switches can be diagnosed using conventional diagnostic tools and methods."

Looks like everything is run through the instrument cluster for grounding control to the lights. Since you said you've already replaced the headlight switch, looks like you're gonna have to break out a multimeter to chase down the door switch inputs. I would guess you're gonna have to replace the instrument cluster in the end.

 

Since you already replaced the headlight switch did you try to reset the pcm? Just thinking out loud here.

 

Thanks kingsman. Lets pretend I understood that.....
Door ajar indicator on dash, and cargo light indicator, both work correctly, But the lights stay on. Simple electrical diagram in my Haynes manual shows they have separate grounds, and both go to the instrument cluster. So, unless I have a short in the wire itself (possible on the dome light, unlikely on the controlled flashing cargo light) I think you are right. Doubt my son in law (college student) will spring for a new cluster just for this issue. Any chance I missed something else?
BTW - what's a PCM and how do I reset it?

 

Well, you had asked how the truck knows whether the door is open...gone are the days when stuff was done with a pushbutton switch and a couple of wires directly to a light!

The instrument cluster is sort of a computer itself, determining where and when different modules (as in lights) get their power. You said the cargo light flashes every 10 seconds and the dome light stays on. That sounds like a faulty computer right there - it's keeping the dome light's voltage on all the time regardless of the input from the door switch and cycling the cargo light regardless of the dashboard dimmer wheel position.

I would guess that your door switches are ok given that the door ajar light works when it should.

The PCM/ECM is the main computer module that controls all functions in the truck, from the horn to the headlights to the engine itself. Often when you have wierd problems with a vehicle, you would disconnect the battery ground cable and leave it off for 10 minutes or so. This resets the ECM and usually clears up some problems. Thus the suggestion by cwboyjohnny.

Hope this helps!