4x4 Doesn't work

[EX.Nate]

Lets beat a dead horse shall we.

Ok I've see a million threads about 2002 sploders without 4x4. I have a 2001 sport trac and the 4x4 has not worked since i bought it 6 months ago. So here is what I have found through my tests.

-fuses are fine, triple checked them
-Switch is fine
-4x4 works when you unplug and plug the 4x4cm back in (lights return on dash, relays in 4x4cm click on and off, feel driveline engagement) however, when truck is placed in gear (Drive or reverse) and vehicle is moving. 4x4cm no longer works. This lead me to believe it was the Vehicle speed sensor.
-checked the Vehicle speed sensor wire from the PCM to the 4x4cm and it has continuity.
-Checked the voltage input at the 4x4cm from the PCM and it is receiving a 12v signal, and it drops to 6v when the vehicle is in motion)

Let me know if you have any tips as to what this is. i have a hard time believing it is the 4x4cm if it works after you unplug, and plug back in.

 

[John_Rock]

Are your tires the same size? Are they inflated to the same PSI? On the fly 4x4 won't engage if the tires are not rotating at the same speed. Diameter and PSI affect rotation speed.

 

[EX.Nate]

Yes they are identical tires, that are evenly inflated. Once I put the vehicle in drive and move the 4x4cm quits working, and in order to get it to work again i have to disconnect, and reconnect it.