793D Off-Highway Truck Power Train: General Information

Basic diagram for the power train systems

The power train is made up of four basic systems. The following systems are the four systems:

Power Train Electronic Control Module
Torque Converter
Transfer Gears and Transmission
Differential and Final Drives

The four basic systems have a hydraulic connection, an electrical connection, a magnetic connection, or a mechanical connection.

Power is supplied from the engine to the torque converter. Power goes from the torque converter to the transfer gears. The power then goes to the transmission. If the transmission is in gear power flows from the transmission to the differential. The rear axles mechanically connect the differential to the final drives. The final drives are connected to the rear wheels. Power is then sent to the tires.

The operation of the power train begins at the Power Train Electronic Control Module (Power Train ECM). The Power Train ECM receives information of the selected speed of operation through the shift lever switch in the electrical system. The Power Train ECM uses the information from several switches and sensors in the electrical system to control the torque converter and the transmission hydraulic system. This is done by energizing the appropriate solenoids.

The rotating housing of the torque converter is fastened directly to the engine flywheel. The torque converter has a lockup clutch for direct drive and a one-way clutch for torque converter drive. During torque converter drive, the torque converter drives the transmission hydraulically. During direct drive, there is a direct connection between the engine flywheel and the transmission.

The Power Train ECM will activate the lockup clutch solenoid when direct drive is necessary. When the lockup clutch solenoid is activated, the lockup clutch is hydraulically engaged. The lockup clutch becomes a direct connection between the rotating housing and the output shaft of the torque converter. The full power from the engine flywheel is transmitted through the torque converter when the torque converter is in direct drive.

The output shaft of the torque converter is connected to a drive shaft. The drive shaft mechanically connects the torque converter to the transfer gears. The transfer gears are fastened directly to the transmission.

The Power Train ECM will activate the upshift solenoid or the downshift solenoid when shifts are needed. The upshift solenoid and the downshift solenoid hydraulically activate the rotary actuator of the transmission. Movement of the rotary actuator mechanically selects the position of the rotary selector spool. The flow through the rotary selector spool hydraulically activates the correct valves in the pressure control valve. These valves engage the correct transmission clutches. The transmission clutches mechanically connect the transmission input shaft to the output shaft and to the differential.

When the rotary selector spool is in the position to engage the correct transmission clutches, the transmission gear switch will electronically signal the Power Train ECM that the shift is complete. The Power Train ECM will stop energizing the upshift solenoid or the downshift solenoid.

When the output shaft of the transmission is rotating, the transmission speed sensor electrically signals the Power Train ECM that the machine has moved.

The transmission will not drive the output shaft unless power is flowing through the torque converter. The power that is flowing through the torque converter can be hydraulic or mechanical.

The transmission has six forward speeds and one reverse speed. The selection of reverse, neutral, or first speed is done manually. The selection of second speed through sixth speed is done automatically.

When the transmission is in reverse gear, the torque converter will stay in torque converter drive. When the transmission is in first gear, the torque converter will be in either torque converter drive or direct drive. This is dependent on ground speed. When the transmission is in any of the gears between the second gear and the sixth gear, the torque converter will be in direct drive. The torque converter will be in torque converter drive for a short time during transmission shifts. This provides smoother engagement of the transmission clutches.

The transmission output shaft is fastened directly to the differential and the bevel gear. The differential and the bevel gear are fastened directly to the rear axles. The rear axles mechanically connect the differential to the final drives. The final drives are connected to the rear wheels. Power is then sent to the tires.

The torque converter housing acts as a sump in order to supply the oil for the torque converter and for the transmission hydraulic system.

An oil pump with four sections is used in the torque converter and the transmission hydraulic system.

The torque converter charging section of the oil pump supplies oil to the torque converter. This oil goes through the torque converter hydraulic filter. Then, the oil is split. A small amount of the oil from the torque converter charging section lubricates the pump drive. Most of the oil from the torque converter charging section enters the torque converter. The torque converter inlet relief valve is located at the inlet to the torque converter in order to limit the maximum pressure of the oil inside the torque converter. If the oil pressure is too high, oil pressure will be dumped to the torque converter sump.

Oil that exits the torque converter flows through the torque converter outlet relief valve to the torque converter screen. The torque converter outlet relief valve maintains a minimum pressure inside the torque converter. Oil from the torque converter screen flows to the torque converter and transmission oil cooler. Then, the cooled oil is sent to the suction port for the torque converter and transmission gear pump. The oil that is not needed for suction and the oil that leaves the pump drive will return to the torque converter sump.

The transmission charging section of the oil pump sends oil through the transmission charging hydraulic filter. Then, the oil is split. Some of the oil goes to the torque converter lockup clutch and synchronizing valve. If the lockup clutch solenoid is not energized, this oil will drain into the torque converter sump. If the Power Train ECM energizes the lockup clutch solenoid, this oil will engage the lockup clutch.

The rest of the oil from the transmission charging section of the oil pump goes to the transmission hydraulic control. Some of this oil supplies the lockup clutch solenoid, the upshift solenoid, and the downshift solenoid. The rest of the oil supplies the rotary actuator, the selector and pressure control valve, and the pressure control valve.

Pump oil flows to the selector and pressure control valve. The selector and pressure control valve sends oil through the rotary selector spool to the pressure control valve. The pressure control valve sends oil to the correct transmission clutches. The pressure control valve also drains the transmission clutches that no longer require engagement.

The solenoids connect the electrical system to the hydraulic system. When the lockup clutch solenoid is activated, signal oil is sent to the relay valve in the torque converter lockup clutch and synchronizing valve and to Station D in the pressure control valve. The signal oil that is sent to the relay valve in the torque converter lockup clutch and synchronizing valve causes the torque converter lockup clutch to engage. Oil that is going to Station D in the pressure control valve is sent to the dual stage relief valve in the selector and pressure control valve. This lowers the pressure setting of the dual stage relief valve. The dual stage relief valve controls the system pressure in the transmission hydraulic control.

When the upshift solenoid or the downshift solenoid is activated, oil is sent to the rotary actuator. The rotary actuator mechanically turns the rotary selector spool in the selector and pressure control valve. Passages in the rotary selector spool line up with stations in the pressure control valve. Oil flows through the pressure control valve to the transmission clutches.

The rotary selector spool can be manually moved through all the positions when the engine is stopped. This is done by removing a plug on the side of the transmission case. The rotary selector spool is in the NEUTRAL-1 position when the spool is turned manually in a clockwise direction to the farthest point. The counterclockwise order of each detent position after the NEUTRAL-1 position is NEUTRAL-2, REVERSE, FIRST, SECOND, THIRD, FOURTH, FIFTH, and SIXTH. The SEVENTH speed and the EIGHTH speed are not used on this machine.

The transmission lubrication section of the oil pump lubricates the transmission and the transfer gears. A transmission lubrication relief valve controls the pressure of this oil.

The transmission scavenge section of the torque converter and transmission gear pump pulls oil from the transmission case reservoir. This oil goes through the transmission magnetic screen. The oil then returns to the torque converter sump.

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