Module-Transfer Case Control: Operation: Notes

WARNING: This page is about a different car, the 2005 Dodge Durango. However, it is still accessible from the selected car via links, so may be relevant.

The Front Control Module (FCM) utilizes the input from the transfer case mounted mode sensor, the instrument panel mounted selector switch, and the following information from the vehicle's CAN bus to determine if a shift is allowed.

Engine RPM and Vehicle Speed
Diagnostic Requests
Manual Transmission Clutch Switch
Brake Applied
PRNDL
Ignition Status
ABS Messages

Once the FCM determines that a requested shift is allowed, it actuates the bi-directional shift motor as necessary to achieve the desired transfer case operating mode. The FCM also monitors the mode sensor while controlling the shift motor to determine the status of the shift attempt.

Several items can cause the requested shift not to be completed. If the FCM has recognized a fault (DTC) of some variety, it will begin operation in one of four Functionality Levels. These levels are:

Level Zero - Normal Operation.
Level One - Only Mode Shifts Are Allowed.
Level Two - Only Mode Shifts and Shifts Into LOW Are Allowed (No Neutral Shifts Are Allowed).
Level Three - No Shifts Are Allowed

The FCM can also be operating in one of three possible power modes. These power modes are:

Full Power Mode is the normal operational mode of the module. This mode is achieved by normal CAN bus traffic being present and the ignition being in the RUN position.
Reduced Power Mode will be entered when the ignition has been powered off. In this state, the module will shut down power supplied to external devices, and to electronic interface inputs and outputs. From this state the module can enter either Sleep Mode or Full Power Mode. To enter this mode, the module must receive an ignition message denoting that the ignition is off, or not receive any messages for 5 +/-0.5 seconds. To exit this mode, the module must receive one ignition message that denotes that the ignition is in the RUN position.
Sleep Mode will be entered, from the Reduced Power Mode, when no CAN traffic has been sensed for 20 +/-1 seconds. If during Sleep Mode the module detects CAN bus traffic, it will revert to the Reduced Power mode while monitoring for ignition messages. It will remain in this state as long as there is traffic other than run or start messages, and will return to Sleep mode if the bus goes without traffic for 20 +/-1 seconds.