777F Off-Highway Truck Power Train: General Information

Basic diagram for the power train systems

ReferenceFor more details on the power train electrical system, refer to Service Manual, " 773F, 775F, and 777F Power Train ECM".

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

Chassis 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.

The chassis electronic control module (chassisECM) matches the transmission speed to the selected speed of the transmission shift lever. The chassis ECM gets information of the selected speed of operation through the electrical system. The chassis ECM activates the transmission hydraulic system and the torque converter hydraulic systems through the modulating valve solenoids for the transmission, and the modulating valve solenoid for the torque converter lockup clutch.

The torque converter has a lockup clutch for direct drive and a one-way clutch for torque converter drive. The torque converter is fastened directly to the flywheel of the engine.

The torque converter drives the transmission hydraulically, unless the lockup clutch is activated. When the lockup solenoid is activated, the lockup clutch is hydraulically engaged. The torque converter rotating housing is now mechanically connected to the output shaft of the torque converter. The drive shaft mechanically connects the torque converter to the transfer gears. The transfer gears are fastened directly to the transmission.

The modulating valve solenoids for the transmission hydraulically activate the transmission hydraulic control valves. The control valves activate the transmission clutches which cause the mechanical connection to the transmission output shaft. The clutches in the transmission will not drive the transmission output shaft unless the torque converter is activated hydraulically or the torque converter is activated mechanically.

The transmission has seven forward speeds and one reverse speed. The selection of speed is done manually in REVERSE, in NEUTRAL and in FIRST. The selection of SECOND through SEVENTH speeds is done automatically.

Power is sent through the REVERSE gear in torque converter drive only. FIRST has both a torque converter drive and a direct drive. SECOND through SEVENTH speeds are direct drive only with a very short time of converter drive during transmission clutch engagement in order to enable smoother shifts. The transmission is fastened directly to the differential and bevel gear. The differential and bevel gear are fastened directly to the rear axle housing.

After the transmission is connected to the torque converter, power flows from the engine through the torque converter and 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 torque converter hydraulic system uses oil that is common with the brake cooling system. These systems use the same oil from the hydraulic oil tank. The following components are part of this hydraulic system:

Torque converter inlet relief valve
Torque converter outlet relief valve
Charge oil filter for the torque converter
Oil filter for the torque converter lockup clutch
Gear pump for the torque converter
Gear pump for the torque converter lockup clutch
Modulating valve for the torque converter lockup clutch
Lockup clutch

Pressure oil that is used to engage the lockup clutch comes from the pump section of the gear pump for the lockup clutch for transmission and for the torque converter. Oil flows through the oil filter to the modulating valve for the torque converter lockup clutch. The modulating valve for the torque converter lockup clutch controls the operation of the lockup clutch.

Oil from the outlet relief valve is used to cool the wheel brakes. The oil flows through an oil cooler before flowing to the brakes. After cooling the brakes, the oil returns to the hydraulic oil tank.

The transmission has a separate hydraulic system, which uses oil from the transmission oil tank. The following components are part of this hydraulic system:

Transmission hydraulic control group
Gear pump (transmission charging section and transmission scavenge section)
Transmission charging hydraulic filter
Magnetic screen
Transmission oil cooler

The following components are part of the transmission hydraulic control group:

Modulating valves for the transmission clutch
Transmission hydraulic control relief valve
Transmission lubrication relief valve

Note: The solenoids on the modulating valves for the transmission clutches are the connection between the electrical system and the hydraulic system.

The modulating valve solenoids for the transmission clutches regulate the oil to the clutches . These solenoids are electrically activated. The modulating valves for the transmission clutches then send oil at the correct rate in order to smoothly engage the proper clutches in the transmission.

MARYGAR