Transfer Case Control Module (TCCM) Operation
GENERAL INFORMATIONThe TCCM receives inputs signals, processes the signal information, develops outputs signals, and sends the outputs signals to control the shifting of the transfer case. The TCCM receives input signals from the following sources:
- Transfer case shift select buttons.
- NSBU switch on vehicles with automatic transmissions.
- Clutch safety switch on vehicles with manual transmissions.
- Vehicle speed sensor calibrator module.
- Encoder/motor range and mode feedback signals.
- Diagnostic enable.
- Front axle mechanical status signal.
After processing the received information, the TCCM develops and outputs signals to the following:
- Motor/encoder.
- Select illumination switch circuit.
- Diagnostic trouble codes to it's self.
To ensure the electronic shift system is operating properly, the transfer case control module (TCCM) continually performs diagnostics checks on itself and other parts of the electronic shift system when the ignition switch is in the run position. The following are different types of system checks that the TCCM continually performs.
RAM/ROM CHECK
The TCCM compares its current internal operating program with a stored operating program. This comparison allows the TCCM to evaluate if its own RAM and ROM are operating properly. Should the TCCM detect a problem with it's internal operating program, a diagnostic trouble code (DTC) of 4 is stored indicating the TCCM memory is damaged. Replace the TRANSFER CASE CONTROL MODULE.
DATA MEMORY RETENTION CHECK
The TCCM checks its stored diagnostic trouble code memory to see if the memory has changed since the ignition switch was last positioned to off. This self test checks to see if the RAM standby power supply has been interrupted. If the TCCM detects a RAM standby power supply interruption, (the loss of stored diagnostic trouble code) it stores a DTC of 1 indicating RAM standby power failure.
This condition occurs when the TCCM is disconnected from the wiring harness or battery power is removed. The DTC of 1 automatically clears from the TCCM after cycling the ignition switch on and off five times.
ENCODER SIGNAL CHECK
The TCCM checks the encoder signal to verify that the transfer case is in either 2HI, 4HI, or 4LO.
ENCODER SWITCH MONITOR CHECK
While the transfer case is shifting, the encoder within the electric-shift motor is monitored for the proper operating sequence. If during a shift, the encoder changes to a incorrect position, an error counter in the TCCM starts to count the number of times the encoder fails. If the encoder fails eight times, the TCCM produces a DTC of 2 indicating a (Encoder Fault). When this happens, the TCCM outputs a signal to default the rail shift pattern in the encoder to allow for only shifts into 2HI and 4LO.
To protect against transient, random encoder faults caused by vibration, contamination, electrical noise, etc., the error counter reduces its count by one each time a good encoder value is detected. The encoder must fail 25 percent of the time for TCCM to store a DTC of 2 and indicate a damaged encoder.
MOTOR AND RELAY VOLTAGE CHECKS
Whenever the electric-shift motor is turned on or off, the motor and its electrical circuits are tested both in the de-energized and energized condition. If one or both voltage relays fail to detect the proper voltage after energizing or de-energizing, the shift is aborted by the TCCM and a DTC of 3 is stored to indicate a faulty TCCM motor circuit.
If one or both relays fail to detect the proper voltage after de-energizing, both relays are turned on by the TCCM (even with the ignition switch positioned off) to prevent the motor from running. The TCCM then stores a DTC of 3 to indicate a faulty TCCM-motor circuit. The all the status lamps blink three times, stop, and repeat the blinking continuously to alert the driver that the electronic shift system requires immediate repair. The battery will drain when this condition occurs, regardless of the ignition switch position.