Son's 4wd is driving me nuts!
 

New to the forum and in desperate need of direction or help.

Found a 1998 truck that my son liked, 3 inch lift, stock motor with low mileage. Only problem was the 4wd would not engage. Had a mechanic look at it and said it was the GEM module. Ordered a re-furbished one from dealership out of Kansas with a 12 month warranty, installed it, had local dealership hook it up to their computer. No 4wd. At this point, started chasing things down on my own.

This is what I know so far:
1. The dash 4x4 indicator does not light up, regardless if the selector switch is in 4H or 4L.
2. Both solenoids on the firewall click when the ignition is turned on, so they are getting power.
3. Have tried two different selector switches to rule out that out as a possible problem.
4. Have disconnected the negative wire from the batter and let it sit 15 minutes, then re-connect and turn ignition switch on part way and let vehicle re-initialize itself (found this suggestion in another post).
5. Prior to installing new GEM module, made sure all the contacts were clean and corrosion free.
6. Fuses 17 and 23 are good.

I am guessing the next item in the circuit to check is the 4wd transfer case relay? Is this what sends the signal to the vacuum solenoids switches in #2 above?

Appreciate any input on this. Let me know if any additional information is needed.

Hi. Check the sticky in this forum on 4wd troubleshooting. Also check YouTube, good videos on Ford products by "fordtechmagliocco" (or something like that.).

IIRC switch goes to GEM which goes to vacuum solenoids which go to front axle actuator.

ALSO from the GEM I believe a signal goes to shift the transfer case via the shift motor.

Good luck.

Are you hearing anything at the transfer case when you shift into 4H? I've replaced several motor on the transfer case. But you would want to verify that you are getting power to the motor.

No sound at the transfer case motor. I will disconnect the pigtail tomorrow and see if 12 volts is making it there.

I almost guarantee it is the transfer case motor itself, just replaced mine and the symptoms were identical, You may want to watch a video on how to replace it on youtube, you have to do some electrical connection swaps, but otherwise it is just 3 bolts. They aren't too expensive either.

That looks fairly easy.

Just watched FordTechMakuloco's video on troubleshooting the system. Once my son gets home, going to check for any vacuum leaks and be sure the solenoids are engaging when the dash switch is turned from 2wd to 4wd H. Will report back tonight what we find.

Thanks for the suggestions.

Checked vacuum on both lines going to the solenoid and felt suction on both. Solenoids click when ignition was turned on, so did the trouble light check. With selector switch in 2WD, one wire on the negative terminal of battery, the right pin on both plugs caused the light to turn on. Switched the clip to the positive pole on the battery, no light on the 2WD and 4WD solenoid left pin. Turned the switch to 4WDH and ended up with the same results; no light.

At this point, I went back and confirmed both fuse 17 and 23 are still not blown. Would a bad transfer case motor cause these types of symptoms?

The ground comes from the GEM module. You're still getting no 4WD lights either, right? You've tried 2 switches and 2 GEM modules. I think you may have a wire problem in your instrument panel. Did previous owner change radio or anything? , Maybe a loose connection? Good luck.

Agree with AustinJ. bought my son a 1998 SCab 4x4 in late '13. Previous owner thought the X-fer case was bad, only paid $800 for the truck. Came home and removed the motor, pulled it apart and cleaned the contacts with electrical cleaner. Also checked fuses (they were all good). Re-installed the motor and BBAAAMMMM, working 4x4 and light in 4H ONLY. Pulled apart dash, replaced numerous bulbs, and all is well.

Remember, to go to 4L, you have to be stopped, with the trans in neutral....same for going back to 4H or 2wd.

Still had the factory radio in it, but that does not mean previous owner may have been poking around in the dash. Correct, no 4wd indicator light in the dash.

I guess the next thing to do is trace the wiring harness back from the dash switch to wherever and see if anything looks suspect. I also think I will get him under the truck and pull the transfer case motor and check it out....nothing to loose at this point.

Problem solved! Transfer case motor was frozen. Replaced that and everything appears to be working correctly now. Thanks a lot for the suggestions on what to look for....very much appreciated:-X22

Glad you got it fixed!