Odd Light Issue

[Cummins12V98]

My 01 when connected to boat trailer with lights off has the LED running lights glowing very dim. Voltage at 7 pin light post is 2.1 volts. The trucks 7 pin MPOAR plug is a year old and connections are all clean. Trailer brake post has .033 volts and all others 0.00 volts.

I went ahead and ordered a new MOPAR headlight switch. I have done nothing that could have caused a pinched wire.

Thoughts???

Guess I could have tested to output at the headlight switch to confirm......

 

[brucejohnson]

Disconnect your brake controller first. If no change, reconnect.
Which truck plug pin has the ~2vdc? 3 is tail lights, 4 is 12vdc. They are next to each other. Measure voltage to ground for each. 4 should be truck's battery voltage, 3 should be 0 unless lights are on.
What happens when you turn the lights on?...turn signals - w/lights on and off?
Using 2002 FSM and assuming 2001 and 2002 are very similar in build, the lighting voltage is supplied by a Trailer Tow relay in the PDC. When the light switch is placed to an active position a relay is activated supplying power from the PDC to the trailer's tail light pin (3) in the connector.

I would open the connectors and look for terminal corrosion and possible cross tracking of a power source to the lighting circuits.

Trailer Tow relay in PDC? Bad/hung contacts?​

Do you have a emergency brake battery? How is the switch? (not too likely an issue)

Check your grounds, both trailer and truck. Almost everything in the truck is switched grounds, so you may be setting up a ground loop when you hook up the trailer. (not too likely an issue)

 

[Cummins12V98]

The running light pin is 2.1v with lights OFF.

The small voltage on brake controller pin is a way to tell if it’s connected.

turn lights on and there is proper voltage.

 

[brucejohnson]

#ad

I am assuming the voltage you are measuring is with the trailer disconnected.
" " " " the "mysterious" 2.1vdc is sensed on blade 3 - Tail/Running lights on the truck's connector.
The voltage to illuminate the tail/running lights comes from the Trailer Tow Relay in the PDC when the lights are turned on.
The battery supplied 12vdc on blade 4 comes from a fuse, 40A Fuse 8 in the PDC which branches/parallels to the contacts of the Trailer Tow Relay to blade 3 via Joint Connector 2 in the PDC.
Blades 3 and 4 are in close proximity and "may" have a stray strand, or foreign material/moisture bridging a high resistance circuit between the conductors in the shell of the connector..
Equally possible is chafed insulation between the two conductors (wires).
Since blades 3 and 4 are from essentially the same 12vdc potential point, with the Trailer Tow Relay contacts breaking the circuit for blade 3 when the lights are turned off, an assumption would indicate that a stray voltage is making its way to the conductor (wire) at or after the Trailer Tow Relay.
I would remove the fuse that supplies the battery voltage to blades 3 and 4, (Fuse 8, 40A in the PDC) then using an ohmmeter check the continuity between blades 3 and 4. It should read an open circuit. If there is any continuity you have a short between conductors/wires.
Replace fuse removed earlier.
Swap relays in the PDC (I believe they are all the same relay number, so any relay will work in the swap). There may be issues with the contacts not fully opening when the relay's coil is de-energized.
Hopefully somewhere in the mess above it will get resolved.

 

[Cummins12V98]

Thanks for the info Bruce!

Yes trailer is disconnected. Yes it's blade 3.

I am going to replace the light switch since I already ordered it. Maybe I will get lucky, if not then I will follow your suggestions.

 

[brucejohnson]

Fair enough. I changed mine a couple years ago when the "night light" went out and dash light rheostat quit dimming properly. Left hand was scared to reach into the dark...

 

[Cummins12V98]

Launched boat today and parked. Was panning to unplug the plug. No dim lights! Weird!

 

[brucejohnson]

Launched boat today and parked. Was panning to unplug the plug. No dim lights! Weird!

Tiny spiders with wet webs...
Check the boat's lights...quick! They are hitching a ride...

 

[Cummins12V98]

Should have my replacement switch today. Will be interesting if it fixes the issue. BUT now that the light issue has become intermittent..........