Systems Operation
784C, 785C and 785C HAA Off-Highway Truck/Tractors Power Train
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 has a hydraulic system that uses oil that is also common with the brake cooling system, the parking brake release system, and the body hoist system. These systems use the same section of the hydraulic tank.
Some of the components in the torque converter hydraulic system include a torque converter inlet relief valve, a torque converter outlet relief valve, a torque converter, parking brake release, and brake cooling gear pump, a torque converter hydraulic filter, and a torque converter lockup clutch and synchronizing valve.
Pressure oil that will engage the lockup clutch comes from the parking brake release section of the torque converter and parking brake release gear pump. Oil will go through the parking brake release hydraulic filter to a tee at the parking and secondary brake valve. Some of the oil will be sent to the torque converter lockup clutch and synchronizing valve. When the torque converter lockup clutch and synchronizing valve gets a pressure signal from the lockup solenoid, pump oil from the parking brake release pump section will be sent through the torque converter lockup clutch and synchronizing valve. This oil engages the torque converter lockup clutch.
Oil from the torque converter charging pump section goes through the torque converter hydraulic filter. This oil is split. Some of the oil is sent to the pump drive for lubrication. Most of the oil goes through the torque converter inlet relief valve and into the torque converter. Oil exits the torque converter and flows through the torque converter outlet relief valve. Oil from the outlet relief valve is sent to the torque converter and front brake cooling screen.
The oil from the torque converter and front brake cooling screen goes to the torque converter and front brake cooling diverter valve. The diverter valve will direct the oil through the torque converter and front brake cooling oil cooler or the diverter valve will divert the oil around the oil cooler. The cooling oil is then sent to the front wheel brakes. The oil that cools the front wheel brakes returns to the torque converter, hoist, and brake section of the hydraulic tank.
The torque converter scavenge pump section pulls oil from the bottom of the torque converter housing through a screen. This oil is returned to the torque converter, hoist, and brake section of the hydraulic tank.
The transmission has a separate hydraulic system. The transmission will use oil from the transmission section of the hydraulic tank. Other components in this system include a transmission hydraulic control, a transmission gear pump, a transmission charging hydraulic filter, a transmission lubrication hydraulic filter, a transmission magnetic screen, and a transmission oil cooler.
The transmission charging section of the transmission gear pump sends oil through the transmission charging hydraulic filter. The oil from the transmission charging pump section then goes to the transmission hydraulic control. The basic components of the transmission hydraulic control are the rotary actuator, the selector and pressure control valve, and the pressure control valve. This oil also supplies the lockup clutch solenoid, the upshift solenoid, and the downshift solenoid.
The solenoids connect the electrical systems and the hydraulic systems. 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 Station D in the pressure control valve. Signal oil that is sent to the relay valve in the 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 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 turns the rotary selector spool in the selector and pressure control valve. This sends oil to the pressure control valve. The pressure control valve sends oil at the correct rate so that the correct clutches in the transmission are engaged smoothly.
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 transmission gear pump sends oil to the transmission lubrication hydraulic filter. This oil combines with oil from the dual stage relief valve. The dual stage relief valve is located in the selector and pressure control valve which is part of the transmission hydraulic control. The combined oil is sent to the transmission oil cooler. This oil is then used to lubricate the transmission and the transfer gears. The transmission lubrication relief valve controls the pressure of this oil system. If the pressure is too high the transmission lubrication relief valve will dump oil to the transmission case reservoir.
The transmission scavenge section of the transmission gear pump pulls oil from the transmission case reservoir. This oil goes through the transmission magnetic screen. The oil then returns to the transmission section of the hydraulic tank.
