Hey everyone.

I'm working on a friend of mines 1998 ford expedition. 5.4l 4awd, 4 high, 4 low transfer case. Back in December he said his truck wouldn't shift into 4 high or low but would shift into 4awd. So we replaced the motor. After replacing the motor it would shift fine but wouldn't shift into 4awd or 4x4 on the fly (didn't before the motor was replaced either). Has to come to a complete stop. So he just drove it.

Now the 4awd light flashes. Another friend got his hands on the ford ids scan tool and we have a gem code for a faulty front shaft speed sensor in the transfer case. Looked at the P.I.D while driving and verified no signal from the front shaft sensor. Rear shaft sensor is reading. So before ruling out a bad sensor I switched the rear and front shaft sensor and verified that it still didn't have any signal from the front shaft and both sensors were working.

So he bit the bullet and bought a new GEM module. After installing the gem I gained signal from both sensors and was able to shift on the fly and everything was working flawlessly. Took it on the road and from 20mph and up it had a very loud whine coming from the transfer case through all 4x4 modes and 2wd.

Baffled me so I took the t case motor back off and made sure everything was in line. Checked the transfer case fluid to make sure it was full. It was full and a little dirty so I flushed that out. While under the truck I just spun the front and rear drive shafts around to make sure the case wasn't bound up and everything seamed normal. So I started the truck up and spun it up and the whine was gone. Figured maybe the fluid got hot and was so dirty it was messing something up.

Well I took it for a drive and back to not being able to shift into 4x4 while moving and now the scan tool shows no reading from both front and rear shaft sensors!

Sorry for this long post but I'm at Whits end with this thing! Please help me

 

Have you considered a bad connection to the sensor or a fault in the wiring harness?