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Home >> GMC Truck >> 1997 >> K 1500 Truck 4WD V6-4.3L VIN W >> Repair and Diagnosis >> Transmission and Drivetrain >> Transfer Case >> Description and Operation >> New Venture Gear 243 Transfer Case (Electronic Shift) >> Electronic Shift System

Electronic Shift System



Mode Shifts

Mode shifts are shifts from:
- 2HI to 4HI
- 4HI to 2HI
A mode shift can be accomplished in any gear position and at any vehicle speed. If the system is in 2HI, the driver can shift into 4HI merely by pressing and the 4HI shift select button. The 4HI button will blink status whenever a 2HI or 4HI shift is initiated and will continue to blink until the TCCM completes the shift. After the shift into 4HI is accomplished, the 4HI status lamp remains lit to indicate that the system is in 4HI.

There is a difference between when the TCCM sends voltage signals to engage a four wheel drive shift and when the vehicle is truly in four wheel drive. The transfer case can be shifted into 4HI and the front axle may not be engaged. The 4HI shift select button will blink until the axle becomes engaged. During periods of cold weather, (below -12° Celsius, 10° Fahrenheit) front axle shift engagement can take much longer. Vehicles in cold weather with extreme extended front axle shift times should have the front axle gear lube replaced with a synthetic gear lube (75 W-90).

The driver can shift from 4HI back to 2HI by pressing the 2HI shift select button. The 2HI shift select button blinks until the shift to 2HI is complete. The 2HI shift select button will remain on once the shift is mechanically complete.

Range Shifts

Range shifts are shifts between the HI and LO ranges, from:
- 2HI to 4LO
- 4HI to 4LO
- 4LO to 4HI
- 4LO to 2HI

A range shift can only be made with the automatic transmission in neutral or with the manual transmission clutch fully depressed. The vehicle speed must be below three miles per hour before the shift can occur.

Whenever a shift into 4LO is initiated, the 4LO shift select button blinks. The 4LO shift select button will continue to blink until the shift is completed mechanically or until 30 seconds elapses. The 4LO shift select button must be on and not blinking prior to shifting the transmission into gear or releasing the clutch pedal.

If a range shift is initiated when the transmission is engaged or the vehicle speed is above 3 mph, the 4LO shift select button will blink for 30 seconds and no range shift actually occurs, the TCCM will default and position the transfer case into 4HI.

Transfer Case Control Module (TCCM) Operation

The 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 itself.

To ensure the electronic shift system is operating properly, the 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 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 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 indicated 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. Then 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.

Motor/Encoder 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, 4 HI, 4LO, or in transition between any of the two.

Motor/Encoder Channel Signal Table






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.









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