will_it_run?

got a 2002 f150 lariat 4x4. previous owner is a friend who had a shop problem shoot his 4x4 not engaging. he was told that the light in the dash went out which will prevent the 4x4 from engaging due to an incomplete circuit. is this true? can i just put a couple lights in the dash and wire it from the existing switch? is that even a thing? tested vacuum and its good

mbb

Your 4x4 is activated thru the GEM module. Because you can shift on fly.while moving, there's a bunch of criteria that have to be right for that to be allowed. First and foremost.... Computer must know where the shift motor actuator is...what gear the transfer case is in. Then it needs to know if all the circuits to the transfer case motor and the vacuum solenoids that control the front axle disconnect are working.

So every time you start your vehicle it performs a self test on this system. If it finds something wrong the system is disabled. No light, no relay clicky clicky in the dash, no clunk underneath.

So no, the light has nothing to do with it..... That is a symptom that the system is not working. There is a code stored in the PCM that will tell you what the problem is... Your dealer will read it for you.... You can read it if you have forscan. But quite often it's a shift motor failure. Occasionally it's a wiring failure that creates an open circuit, or even a wet gem failure.

Chasing down a wiring failure is not easy if you're not familiar with the wiring, and good at that kind of thing. Many people just spend a hundred bucks on the shift motor and that's often it. They can fail electronically.... Or they can fail internally mechanically. There were plastic stops that prevented them from traveling too far internally and when the plastic stops got brittle and broke the shift motors traveled too far and lost their ability to tell what position they were in... Then they simply quit working. When that was the cause some people repaired theirs internally to get it working again. There are YouTube videos on how to "fix" that. I took mine apart when it was not working and that was not my problem mine just cease to function electronically.

Jbrew

You mention vacuum is working. I'm going to take that as the solenoids are working and the actuator is attempting to engage the front axle. A GEM problem would screw up the solenoids therefore... vacuum.

Guessing it's the shift fork or motor. Going from the OP's post anyway.. which is very vague.

will_it_run?

didnt even think to check the fuse box when i got the truck lol, it was missing #6 which controls some part of the GEM according to the manual. i put one in and on startup the "low" light came on dim for a minute and went off. the door ajar light works now and the door beeps while open. i ran my blue driver and these are the codes

P1812 four wheel drive mode select circuit failure
P1838 transmission transfer case shift motor circuit failure
P1891 transfer case contact plate ground return open circuit

one instrument cluster code
B1356 ignition run circuit failure

i saw a video of someone taking the yellow and orange wires off the transfer case motor to a 6 prong on/off/on switch and he was able to engage 4H and 2H

manicmechanic007

Replace the TC shift motor

PapaBear1975

I went through the same issue- only come to find out in my case a mouse had chewed the wiring, haha. In the process, I swapped out the shift motor, the shift sensor at the T-case, the switch in the dash, and I had my local shop install a new GEM module and get that all programmed. Between me and my mechanic, we spent a couple weeks poking around with it until their kid at the shop who came from the local Ford stealership looked at the wiring under the dash....found the mouse damage and fixed it.......of course now my fog lights won't work. I'll take having 4 wheel drive over the stock fog lights any day.