Hi, Sorry for the delay in replying. Your question got me stumped so I had to spend some time doing research on the problem and possible causes.
I have the explanation for the ADD here.
ADD DESCRIPTION & OPERATION
The Automatic Disconnect Differential (ADD) allows engagement of front differential from 2WD to 4WD while vehicle is moving. On vehicles with ADD, a flange is mounted on the front hub which is splined onto axle shaft connected to side gear shaft on front differential. Locking hubs are not used with the ADD.
When transfer case is shifted into 4WD, the ADD is shifted into 4WD by operation of ADD solenoids which control vacuum supply to the ADD actuator. The ADD actuator engages clutch sleeve with intermediate shaft in front differential and side gear shaft, providing 4WD operation. When vehicle is in 4WD, the 4WD indicator light on instrument panel will be illuminated.
NOTE: ADD solenoids may also be referred to as ADD Vacuum Switching Valve (VSV).
Operation of transfer position switch and transfer pressure switch completes ground circuit which allows ADD relay to operate. When ADD relay operates, voltage is supplied to the ADD solenoids. The ADD solenoids operate and control vacuum supply to ADD actuator for 4WD operation.
Based on the schematic diagrams, one of the possible cause would be a faulty VSV4 ADD solenoid that is leaking vacumn or the solenoid is stuck in open position. Another possibility would be a faulty transfer pressure switch.
To test which is causing the problem, disconnect the VSV4 solenoid wire connector. If problem persists, it rules out the pressure switch. Disconnect and plug vacumn hose of VSV4 and retest. This should disable the 4WD functions as the ADD would not work.
Thursday, April 2nd, 2009 AT 8:21 AM