Transfer Case Shift Control Module
The transfer case shift control module receives input signals, processes the signal information, develops output signals, and sends the output signal, in order to control the shifting of the transfer case.
The transfer case shift control module receives input signals from the transfer case control switch buttons, the park/neutral position (PNP) switch for vehicles with automatic transmissions, the clutch position switch for vehicles with manual transmissions, the powertrain control module (PCM) that supplies the vehicle speed signals, the encoder motor that provides actual mode and range information signals, the data link connector pin D3 that actuates diagnostic enable, power for the module and motor supplies, and the ground used for return lines at the module.
The transfer case shift control module sends signals to the transfer case encoder motor to initiate mode and range shifts, the transfer case control switch indicator lamps to provide transfer case status information, the diagnostic DTCs which are outputted via the shift control switch indicator lamps, and the encoder power.
In order to ensure the electronic shift system is operating properly, the transfer case shift control module continually performs diagnostics tests on itself, and other parts of the electronic shift system, when the ignition switch is in the RUN position.
Below, are the different types of system tests that the transfer case shift control module continually performs.
- Data Memory Retention Test
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The transfer case shift control module inspects the stored diagnostic trouble code memory, to find out if the memory has changed since the ignition switch was last positioned to OFF. This test determines if the RAM standby power supply has been interrupted. If the transfer case shift control module detects a RAM standby power supply interruption, the shift control module stores a DTC of 1 indicating RAM standby power failure. This condition occurs when the shift control module is disconnected from the wiring harness or battery power is removed.
- Encoder Signal Test
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The transfer case shift control module tests the encoder signal in order to verify that the transfer case is in either 2HI, 4HI, or 4LO.
- Encoder Switch Monitor Test
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While the transfer case shifts, the encoder within the encoder motor assembly is monitored for the proper operating sequence. If, during a shift, the encoder changes to an incorrect position, an error counter in the transfer case shift control module starts to count the number of times the encoder fails. If the encoder fails 8 times, the shift control module produces a DTC of 2, indicating an encoder fault. If an encoder fault is detected, the shift control module sends a signal, in order to default the rail shift pattern in the encoder, in order to allow for only shifts into 2HI and 4LO. To protect against transient, random encoder faults caused by vibration, contamination, and electrical noise, the error counter reduces the count by 1 each time a good encoder value is detected. The encoder must fail 25 percent of the time for the shift control module to store a DTC of 2 and indicate a damaged encoder.
- Motor and Relay Voltage Tests
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When the encoder motor is turned ON or OFF, the motor and the electrical circuits are tested, in the de-energized and energized condition. If 1, or both of the voltage relays fail to detect the proper voltage after energizing or de-energizing, the shift is aborted by the shift control module and a DTC of 3 is stored, in order to indicate a faulty shift control module motor circuit. If 1 or both relays fail to detect the proper voltage after de-energizing, both relays are turned ON by the shift control module, in order to prevent the motor from running. The shift control module then stores a DTC of 3, to indicate a faulty shift control module motor circuit. All of the status lamps blink 3 times, stop, and repeat the blinking continuously, in order to alert the driver that the electronic shift system requires immediate repair. The battery drains when this condition occurs, regardless of the ignition switch position.
- RAM/ROM Test
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The transfer case shift control module compares the current internal operating program with a stored operating program. This comparison allows the shift control module to evaluate if the shift control module RAM and ROM are operating properly. If the shift control module detects a condition with the internal operating program, a diagnostic trouble code (DTC) of 4 is stored indicating the shift control module memory is damaged.