777F Off-Highway Truck: Torque Converter Hydraulic System

Control system for the chassis
(A) Inputs
(B) Outputs
(1) Engine ECM
(2) Instrument cluster
(3) LCD display
(4) PC for the VIMS
(5) Brake ECM
(6) ET Service tool
(7) Chassis ECM
(8) Key start switch
(9) Machine lockout switch
(10) Engine lockout switch
(11) Engine oil evacuation enable/Engine oil evacuation
(12) Secondary steering test switch
(13) Transmission control lever
(14) Hoist lever
(15) Fuel level sender
(16) Engine speed sensor
(17) Secondary brake pedal
(18) Inclinometer
(19) Torque converter oil temperature sender
(20) Primary steering pressure switch
(21) Secondary steering motor sensor
(22) Engine speed sensor
(23) R-terminal
(24) Transmission input speed sensor
(25) Bypass switch for the transmission charge filter
(26) Transmission output speed sensor "1"
(27) Transmission output speed sensor "2"
(28) Transmission oil level switch
(29) Location codes
(30) Pressure switch for the secondary steering system
(31) Secondary brake pressure switch
(32) Start relay
(33) Transmission solenoids "1" through "7"
(34) Solenoid for the steering system disable
(35) Proportional solenoid for the parking brake
(36) Torque converter lockup clutch solenoid
(37) Hoist raise solenoid
(38) Hoist lower solenoid
(39) Secondary steering/QuickEvac/Prelube relay
(40) Backup alarm relay
(41) Lamp for the engine lockout switch(42) Lamp for the machine lockout switch

Component locations
(24) Transmission input speed sensor
(26) Transmission output speed sensor "1"
(27) Transmission output speed sensor "2"
(33) Transmission solenoids "1" through "7"
(43) Transmission hydraulic control relief valve
(44) Transmission lubrication relief valve
(45) Transmission section of hydraulic and transmission tank
(46) Magnetic screen
(47) Hydraulic section of hydraulic and transmission tank
(48) Oil filter for the torque converter lockup clutch
(49) Torque converter outlet relief valve
(50) Torque converter inlet relief valve
(51) Relief valve for the pilot control pressure
(52) Torque converter
(53) Pressure taps for the modulating valves for the transmission clutches
(54) Charge oil filter for the transmission
(55) Charge oil filter for the transmission
(56) Bypass check valve for the transmission cooler
(57) Torque converter and transmission gear pump
(58) Modulating valve for the torque converter lockup clutch
(59) Charge oil filter for the torque converter
(60) Front brake and torque converter oil coolers(61) Transmission cooler

Rear of torque converter
(48) Oil filter for the torque converter lockup clutch
(49) Torque converter outlet relief valve
(50) Torque converter inlet relief valve
(51) Relief valve for the pilot control pressure
(52) Torque converter
(54) Charge oil filter for the transmission
(55) Charge oil filter for the transmission
(57) Bypass check valve for the transmission cooler
(58) Modulating valve for the torque converter lockup clutch
(62) Pressure tap for torque converter lockup clutch pressure
(63) Pressure tap for torque converter outlet pressure
(64) Torque converter scavenge pump section
(65) Charge pump section for the torque converter
(66) Pressure tap for torque converter lockup clutch pilot pressure
(67) Pump section for the torque converter lockup clutch(68) Pressure tap for transmission charge pressure

Bottom of the torque converter(69) Screen for the scavenge pump section

Torque Converter Operation

Neutral Operation

When the engine is started, oil goes from the hydraulic tank through the suction screen for the hydraulic section to the charge pump section for the torque converter (65) and to the pump section for the torque converter lockup clutch (67) .

Charging pump section (65) sends oil through charge oil filter for the torque converter (59) to relief valve (51) for the pilot control pressure. The pilot relief valve (51) limits the maximum pressure of the oil from the charging pump section (65) that is used as pilot oil to the hoist control valve. The oil from charging pump section (65) also flows to torque converter inlet relief valve (50) and to torque converter (52) . Inlet relief valve (50) controls the maximum pressure of the oil that goes to torque converter (52) . From torque converter (51) , oil goes to torque converter outlet relief valve (49) . Outlet relief valve (49) controls the minimum pressure of the oil in the torque converter.

Oil from torque converter (52) goes through an orifice in the torque converter housing, or oil goes around the valve spool in torque converter outlet relief valve (49) . This oil now becomes brake cooling oil.

On the 777F Off Highway Truck, oil that goes around the valve spool in torque converter outlet relief valve (49) goes through screens and through the brake oil coolers in order to cool the brakes. Oil that leaves the brakes goes through the tube assembly and the baffle to the hydraulic section of the hydraulic tank.

Any leakage in torque converter (44) also goes to the bottom of the housing. Torque converter scavenge pump section (64) pulls oil through scavenge screen (68) and the oil is sent to the hydraulic tank.

Lockup Operation (Direct Drive)

The machine is in torque converter drive when the machine is in NEUTRAL and in REVERSE speeds. SECOND through SEVENTH speeds are direct drive speeds. FIRST speed is in torque converter drive at lower ground speeds and direct drive at higher ground speeds.

In FIRST speed, torque converter (52) is used until the ground speed gets high enough for direct drive. At this time, the transmission speed sensor tells the chassis ECM that direct drive (lockup) is needed.

The chassis ECM activates the solenoid on modulating valve (80) for the torque converter lockup clutch . The modulating valve sends oil to the lockup clutch. The lockup clutch is now engaged and the machine is in direct drive FIRST speed.

When an upshift to SECOND speed is required, the chassis ECM deactivates the solenoid valve on modulating valve (58) for the torque converter lockup clutch . Pilot oil to the lockup clutch is stopped and the lockup clutch is disengaged. The machine is briefly in torque converter drive while the transmission shifts.

After the proper clutches in the transmission are engaged for SECOND speed, the lockup clutch engages again by using the same steps as described above. Between transmission shifts, the torque converter will briefly change from direct drive to converter drive in order to ensure smooth shifts. This operation is used on upshifts from SECOND through SEVENTH speeds, and on downshifts from SEVENTH through FIRST speeds.

When the machine is in direct drive in FIRST speed and ground speed decreases, the chassis ECM will deactivate the solenoid valve on modulating valve (58) for the torque converter lockup . The lockup clutch will disengage and the machine will revert to torque converter drive.

MARYGAR