I have been searching threads and diagnosing every little thing but I just can seem to narrow it down. I bought a 1997 F250 light duty, the guy had the transfer case motor taken off. I had to manually switch to 4 wheel drive a few times but it didn't work everytime. sometimes the front diff would engage sometimes it wouldn't. I went to the local pull-a-part to buy a transfer case motor and installed it. Still not working, the motor could be bad that I bought? When I plugged the motor in I am getting no vacuum to either the red or blue lines going to the front diff actuator, when I unplug the transfer case motor I get vacuum to the red all the time even with the dash switch in 4wd, but not to the blue vacuum line. Sorry this might sound confusing but I can't think of any other way to explain it..


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First thing to do is pick a motor to test. Install it and plug it in. Lift and support the front so the front wheels are off the ground. Key on, switch to 4Hi. Get under the truck and try to turn the front driveshaft. The one going from the t-case to the front diff. It should not turn. If it does turn try the other motor and repeat the test. If it still turns your t-case is not shifting into 4x4. Check the connector for corrosion. Look for water intrusion at the windshield on the driver side above the fuse box. Attached to the other side of the fuse box is the GEM (Generic Electronic Module). It controls the t-case motor based on the input from the 4x4 switch. It can get wet from a leaky windshield. The GEM also controls the front axle lock up based on signals received from the t-case shift motor assembly.

If the front driveshaft doesn't turn then the GEM and shift motor are working and the problem is with the front axle lock up. Check the vacuum source to the solenoids on the firewall. The source should always have vacuum. The GEM controls the lock up vacuum motor on the front axle with vacuum. It applies vacuum to one side while venting the other side to atmosphere. The way the solenoids work is with no current it vents the line to the axle to atmosphere while closing off the vacuum from the engine. With power applied it allows vacuum to flow from the engine through to the vacuum motor on the front axle.