My coworker has a '97 F-150, 2 WD, 4 speed w/ OD, that has 242000 miles on it. The truck ran perfectly until a couple of weeks ago. The first problem that showed up was the heater fan would continue to run after the truck was turned off and the key removed from the ignition. Then we tried the turn signals and the wipers and they would also operate after the key was removed. Then the truck started shutting off while driving. Have noticed that this usually will occur at low rpm's or idle, not while running down the highway. When it shuts off, it will not restart unless the key is turned to the OFF position. If you try to restart it (after it shut down by itself), by turning the key to START, it will turn over but won't fire. Turn the key to OFF and then to START and it will fire right up. Just 2 days ago the speedometer started acting up. It will fall to zero even at 30-40 mph, and then it will start to fluctuate like there is a contact problem.

Have replaced the key switch, but the problem still remains.
Any and all help will be appreciated.
Thanks

 

Check the GEM module for codes. If it has bunches of codes in it, remove the central junction box and gem and seperate them. If you see corrosion, replace both and reseal windshield.