 | |
| Illustration 1 | g01189343 |
Schematic of the Power Train/Chassis Electronic Control Module | |
The Power Train/Chassis Electronic Control Module (ECM) controls the transmission shift points. The basic components and corresponding requirements for shifting of the transmission follow:
Harness Code - The harness code is from the Caterpillar Monitoring System. The strategy for shifting is different on the machines. The Caterpillar Monitoring System must know the model that the system is on in order to operate correctly. This information is provided to the Caterpillar Monitoring System by the status of the inputs to the monitoring panel. The permanent connections in the machine harness provide this information.
Shift Lever Position Sensor - The ECM must know the current gear position. The sensor for the shift lever position provides six switch positions.
Transmission Gear Position Sensor - The ECM must know the transmission gear that is currently engaged.
Transmission Output Speed Sensor - The ECM must know the output speed of the transmission. The transmission output speed sensor tells the ECM the speed of the transmission output shaft.
Upshift Solenoid and Downshift Solenoid - The ECM must electrically shift the transmission. The ECM uses two separate outputs to the solenoids in order to shift the transmission. The transmission is always shifted one gear at a time. The ECM activates the solenoids electrically in order to shift the transmission.
When the operator selects a forward gear the ECM automatically shifts the transmission. The shift is dictated by the transmission output speed. The automatic shift points are stored in the ECM. A shift occurs when the transmission output speed matches the shift point. When the transmission output speed increases to a shift point the ECM activates the proper solenoid until the transmission gear position sensor changes to the correct position.
Note: The transmission output speed is directly proportional to ground speed and engine speed. This occurs while the transmission is in direct drive. Shift points are based on the calculated engine speed.
The items that follow are exceptions.
- If a shift from a forward gear or reverse gear to NEUTRAL is made and the machine is moving faster than 5 km/h (3.1 mph), the ECM will not shift the transmission into neutral until the ground speed is less than 5 km/h (3.1 mph). This discourages coasting in neutral at high speed, which reduces transmission life. For more information, see the topic in System Operation, Troubleshooting, Testing and Adjusting, "Neutral Coast Function".
- The reverse gear is not engaged until ground speed is less than 5 km/h (3.1 mph). This helps protect the transmission from damaging directional shifts.
- The ECM does not downshift when the operator selects a speed that is not appropriate for the current transmission output speed sensor. Automatic upshifting and/or downshifting will continue. The selected gear now becomes the new top gear.
- If the transmission is in top gear, the lockup clutch is deactivated in order to protect the engine against an engine overspeed condition.
Note: All events regarding engine overspeed are recorded and available for examination. The examination of these events requires the use of the proper service tool. See the topic in System Operation, Troubleshooting, Testing and Adjusting, "Tattletale Function".