4x4 dash indicator light stopped working on my father's 2002 F-350 V10 about 8 years ago and I'm trying to see if I can fix that problem.

Unsure exactly when this problem began, but when a mechanic replaced front wheel hub about 8~9 years ago, automatic hub lock stopped working. Back then, I thought that is why 4x4 indicator stopped working.
When I removed hub to replace ball joint 2 years ago, I discovered yellow O ring was not installed and that was the reason why automatic hub lock stopped working. Now I installed new O ring and vacuum seal, automatic hub lock works. However, 4x4 indicator on the dash still does not work.

I thought may be bulb went out, but when I tested dash bulbs with self test mode, 4x4 indicator light turns on. Also I believe that lamp is LED not regular bulb.

When I pulled DTC, I get P1879 - Powertrain Transmission Transfer Case 2-Wheel Drive Solenoid Circuit Failure.
From what I searched this indicates failure at vacuum solenoid for front hub locks, but it is working fine.

Any idea what is going on?

 

Hi Byoung, welcome to FTE!

That code means the PCM sees a fault in the circuit, and nothing more. The fault can exist and still engage your 4WD, or perhaps the system worked once and it wont' disengage the hubs? If they're always engaged you'll get 4WD, but that doesn't mean they are working right.

Have you checked to see if they disengage? Jack up each wheel in 2WD with the engine off and see if the shaft spins with the wheel.

 

Yes. It is disengaging. But, I did see it fail to disengaging once. Switching to 4WD then back to 2WD fixed it.

 

I don't have the shop manual for that year truck, but it sounds like you need to do some electrical testing along that circuit. I'd suggest a subscription to ALLDATAdiy.com Leading Source of Factory Automotive Repair Information, they have OEM diagnostic routines for each trouble code.

Wish I could be more help!

 

I did find 4WD service manual for different year(09), but appears to be same design. Do you know where 4x4 control module is located? I think it is somewhere near passenger side dash, but cannot figure out exactly where.