Transfer Case - Electronic Shift

WARNING: This page is about a different car, the 2005 Ford Pickup, 2005 Ford F550 Super Duty, 2005 Ford F450 Super Duty, and 2005 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

The four-wheel drive, electronic shift-on-the-fly feature electrically shifts the vehicle transfer case between 2WD, 4x4 HIGH, and 4x4 LOW. The system mode is selected by the operator through the mode select switch (MSS) on the instrument panel. The operator is informed which mode the system is in by two instrument cluster indicators, one for 4x4, and one for 4x4 LOW (in 4x4 LOW, both the lamps are on). Shifts into 4x4 HIGH can be made at any speed. When shifting into 4x4 HIGH with the vehicle stationary, tooth blockage may occur, preventing shift completion. When the vehicle is in motion the shift will complete. When shifting in or out of 4x4 LOW, the four-wheel drive (4WD) control module requires that the vehicle is in motion, the brake pedal be applied, and the transmission is in NEUTRAL.

The gearmotor encoder assembly is mounted externally on the transfer case. It drives a rotary cam which moves the mode fork and the range fork within the transfer case between the 4x4 HIGH, 4x4 LOW, and 2WD range positions.

The 4WD control module [also referred to as the generic electronic module (GEM)] uses two relays to control the gearmotor encoder assembly shift between 4x4, 4x4 LOW, and 2WD modes. The 4WD control module controls the pulse vacuum hublock (PVH) solenoid which supplies engagement and disengagement vacuum to control the hublocks.

The 4WD control module accomplishes shifts by interpreting inputs from:

Mode select switch (MSS).
vehicle speed signal (transmitted from the ABS system).
neutral position (from powertrain control module (PCM) - automatic and manual transmissions).
gearmotor encoder plate position.
brake pedal switch.
ignition switch.

Based on these inputs, the 4WD control module controls the shifts into 2WD, 4x4 HIGH, or 4x4 LOW with the following outputs:

Low to high relay (clockwise)
High to low relay (counterclockwise)
Pulse vacuum hublock (PVH) solenoid
Gearmotor encoder assembly

The electronic shift-on-the-fly (ESOF) system has a feature which allows the driver to override the vacuum-operated hublocks. When the front hublock dials are manually turned to the LOCK position, the hublocks are locked through the dial at all times, regardless of the position of the instrument panel mode select switch (MSS).

When the front hublock dials are manually turned to the AUTO position, the hublocks use an internal spring-locking mechanism that is vacuum-operated by the ESOF system, and are locked with the MSS in the 4H or 4L position and unlocked with the MSS in 2H position. Refer to HUBLOCK in this section for additional AUTO hublock system operation.