LINK AS 1985719 - Caterpillar

Control Logic (Closed Loop and Open Loop)
During normal operation, transmission ECM (7) controls the speed of the machine using closed-loop logic. In this logic, the transmission ECM calculates the target speed and applies a steering factor to the speed of each track. The transmission ECM then generates appropriate output signals to the propel solenoids on the pumps.Transmission ECM (7) monitors the input from the speed sensors in order to determine if the actual speed is equal to the target speed. If the ECM detects a discrepancy between actual speed and target speed, the ECM modifies the output signal to the appropriate propel solenoid. This action aligns the actual speed with the target speed.Left speed sensor (26) and right speed sensor (22) each provide two input signals to transmission ECM (7). The input signals from a speed sensor are at the same frequency but the signals are out of phase. The transmission ECM equates the frequency of a speed sensor to the speed of the track. The transmission ECM equates the phase of the signal to the direction of the track.If the target speed is not zero and the actual speed of one or both tracks is zero, transmission ECM (7) controls the speed of the machine using open-loop logic. In this case, the transmission ECM calculates the desired speed as is done during normal operation. However, in the "travel" mode, the maximum speed of the machine is limited to one-half of normal maximum speed. A fault from one or both of the speed sensors can cause the transmission ECM to invoke open-loop logic.Since the speed sensors do not provide input to transmission ECM (7) during open-loop control, the target and actual machine speeds cannot be reconciled. Therefore, the machine may not drive straight.Hill Hold Mode
When the operator has enabled the hill hold function, transmission ECM (7) evaluates the following parameters to determine when to release the parking brake:
Propel mode
Parking brake status
Position of active propel lever
Output current to propel solenoids
Elapsed timeThe operator can enable the hill hold function at any time. However, transmission ECM (7) only initiates the control logic when the machine is in the pave mode. The hill hold status remains active through a key cycle.The parking brake status must be set to "released" for transmission ECM (7) to allow the machine to move. Hill hold mode does not affect the operation of the parking brake switch.Transmission ECM (7) evaluates the position of the active propel lever. The transmission ECM only initiates the hill hold control logic when input from the propel lever indicates that the lever is in the forward range.Transmission ECM (7) simultaneously evaluates the output current to the propel solenoids and elapsed time. The ECM maintains the parking brake while the output current to the solenoids increases. The current increases at the same rate as normal propel operation. The transmission ECM releases the parking brake when one of the following occurs:
The propel calibration offset current is achieved.
The duration of the rollback brake timer has