Control Module: Description and Operation
1. The TCCM performs the following functions:^ Receives input signals
^ Processes the signal information
^ Develops output signals
^ Sends the output signal in order to control the shifting of the transfer case
2. The TCCM receives input signals from the following sources:
^ The transfer case shift select buttons
^ The NSBU switch on the vehicles with automatic transmissions
^ The clutch position switch on the vehicles with manual transmissions
^ The vehicle sped sensor calibrator module
^ The encoder/motor range and the mode feedback signals
^ The diagnostic enable
^ The front axle mechanical status signal
3. In order 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 the current internal operating program with a stored operating program. This comparison allows the TCCM in order to evaluate if the TCCM's RAM and ROM are operating properly. Should the TCCM detect a problem with the internal operating program, a diagnostic trouble code (DTC) of 4 is stored indicating the TCCM memory is damaged. Replace the TCCM.
Data Memory Retention Check
The TCCM checks the 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 in order 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) the TCCM stores a DTC of I 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 in order to verify that the transfer case is in either 2HI, 4HI, or 4LO.
Encoder Switch Monitor Check
While the transfer case shifts, the encoder within the electric-shift motor is monitored for the proper operating sequence. If during a shift, the encoder changes to an 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 an Encoder Fault. When this happens, the TCCM outputs a signal in order to default the rail shift pattern in the encoder in order to allow for only shifts into 2HI and 4LO.
In order to protect against transient, random encoder faults caused by vibration, contamination, electrical noise, etc., the error counter reduces the count by one each time a good encoder value is detected. The encoder must fail 25 percent of the time for the 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 the 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-energized and energized condition. If one or both of the 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 in order 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) in order to prevent the motor from running. The TCCM then stores a DTC of 3 in order to indicate a faulty TCCM motor circuit. All of the status lamps blink three times, stop, and repeat the blinking continuously in order 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.
Motor/Encoder Circuit Operation
The TCCM operates the motor/encoder in one direction by energizing one relay while the second relay is de-energized. The TCCM operates the motor in the reverse direction by energizing the second relay while the first relay is de-energized. The encoder part of the motor/encoder sends shift positioning signals to the TCCM.
The motor/encoder converts a shaft position, representing a mode of range selection, into electrical signals for use by the TCCM. By interpreting the four channels (P, A, B, C) of the encoder, the transfer case control module can detect what position the transfer case is in 2HI, 4HI, 4LO, or in transition between any of the two.
The four hall effect sensors of the motor/encoder are used for channels (A, B, C, and P). These sensors provide a path to ground when a magnet passes over them. A rotating magnetic ring causes the hall effect sensors voltage to drop from 5 to 0 volts. The TCCM detects the voltage of all the channels and interprets the current transfer case shift position.