I have a 1998 Expedition with the 5.4 At and 2wd, a4wd, 4wd Hi, 4wd Lo option, it currently has about 145000 miles on it and runs perfectly, the check engine light is on but the code is 401 and I will take care of that when it warms up a bit. Here is the issue, when I am traveling at highway speeds in 2wd I was getting what I though was an engine cutout for like a 10th of a second, just a slight bump. This will continue at about 5-10 second intervals, I figured out it was trying to engage the 4wd, so when this happens I switch to a4wd and the problem is solved. I can't find anything anywhere that addresses this, it seems there must be a sensor someplace that is going bad. I could just run in a4wd, but I really like having things working as they should. Does anyone have any good suggestions on where to start looking?
Thanks
Dan

 

Three things come to mind. The GEM controls the transfer case. The GEM takes its input from the switch and the speed sensors in the transfer case. Either the GEM is defective, the sensors are or the switch is. Test the Switch as follows. Remove the switch and probe the contacts with the dark blue wires at the switch, not the connector. The resistance should be in ohms as follows. 2WD 3.9k, A4WD 1.5K, 4Hi 680 and 2Lo 240. If the switch checks out then the GEM is not switching the system correctly. This may be caused by a faulty GEM or a faulty speed sensor in the transfer case.