I have an '01 Ford F250 Superduty with 7.3L diesel. The only mod it has is a DP Tuner, that I have had in the truck for almost 8 years now. Last night it started doing some strange things. When I got in the truck the blower motor was running even though the ignition switch was off and the truck had been sitting overnight. I switched the ignition from off to on, then back to accessory, and nothing would make the blower shut off, except turning it off with the rotary switch on the selector panel.

Upon driving I noticed that turning on the parking lights or headlights made the speedometer go to 0 and then bounce back up to the current speed. Additionally the keyless entry would not work, but the electric doorlocks would. As would the power windows.

I first thought it was the ignition switch and pulled both relays in the fusebox under the hood. This did not shut off the blower motor. I then pulled the fuse under the dash for the GEM and the motor shut off.

So far I have not pulled the GEM out because it sounds like a bit of a job. Any thoughts and is there any way to diagnose the GEM without removing it from under the dash? Additionally, if it is the GEM any thoughts of getting it other than through the dealership? As long as it is from a comparable truck, would a used one suffice? Thanks.

 

Do you have clearance lights on top of the cab? If so they are most likely leaking- when they leak the water goes right to the GEM and makes it act funny. Mine did what you are describing With the heater motor and my lights I found were leaking. Sealed up the lights and the problems went away after the GEM dried up.

 

I had a windshield leak at the top, caused similar problems with the GEM. Try pressurizing the cab with a leaf blower or vacuum and spraying soapy water or tire lube outside around the front top of the windshield, clearance lights. Leaks will show up as bubbles.