Internal Diagnostic Checks

To ensure proper and safe shifting, the TCCM performs the following constant run-time diagnostics:

RAM/ROM Check

Whenever the ignition is turned on, the TCCM calculates a checksum for the internal program memory and compares it to a checksum stored within the program. This comparison ensures that the software is operating properly. Should the TCCM detect a fault in the internal program (a RAM/ROM failure) DTC number 4 is stored. If this DTC occurs, replace the TCCM.

Data Memory Retention Check

Next, the TCCM determines if RAM contains the same data memory as it did when the ignition was last turned off. This test determines if the RAM standby power supply to the TCCM has been interrupted, clearing the DTCs from the memory. If this check fails, the stored DTCs are cleared. DTC 1 (RAM Standby Power Failure) is then stored. DTC 1 automatically clears after cycling the ignition five times.

Encoder Signal Check

Each time the TCCM is powered up, it checks the encoder signal to verify that the transfer case is in either 2HI, 4HI, or 4L0.

Encoder Switch Monitor Check

While the transfer case is shifting, the position encoder switch within the electric-shift motor is monitored for the proper operating sequence. If at any time during a shift, the encoder changes to a condition other than the next possible position, an error counter in the TCCM is incremented by four. After the counter reaches 32, DTC 2 (Encoder Fault) is outputted and the TCCM reverts to a default rail-To-rail shift pattern in which only shifts into 2HI and 4L0 are possible.

To protect against transient, random encoder faults caused by vibration, contamination, electrical noise, etc., the fault counter is reduced by one each time a good encoder value is detected. Thus, the encoder must fail 25 percent of the time to be considered permanently bad and store a DTC 2.

Motor 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 the motor circuits do not function properly, one of the following conditions may exist:

^ An open or short in one or more of the following motor circuits:
- Supply circuit 1640
- Ground circuit 150
- Circuit 1552
- Circuit 1553
- Associated connections
^ An open or short within the motor itself
^ One of the motor relays not energizing or de-energizing

Relay Voltage Checks

If either or both relays fail to detect the proper voltage after energizing or de-energizing, the shift is aborted and DTC 3 (TCCM Motor Circuit) is stored.

It either or both relays fail to detect the proper voltage after de-energizing, both relays are turned on by the TCCM (even with the ignition off) to prevent the motor from running: The TCCM then flags DTC 3 (TCCM Motor Circuit). The status lamps flash to alert the vehicle operator that a condition that must be corrected immediately exists. Such a condition causes the battery to drain when the ignition is off.