Power flows from the engine to the rear wheels through the power train. The components of the power train are:
- Torque converter - Transfer gears - Transmission - Differential - Final drives
INSTRUCTOR NOTE: In this section of the presentation, component locations and a brief description of the component functions are provided.
POWER TRAIN
785C
Torque Converter
The first component in the power train is the torque converter. The torque converter provides a fluid coupling that permits the engine to continue running with the truck stopped. In converter drive, the torque converter multiplies engine torque to the transmission. At higher ground speeds, a lockup clutch engages to provide direct drive. The NEUTRAL and REVERSE ranges are converter drive only. FIRST SPEED is converter drive at low ground speed and direct drive at high ground speed. SECOND through SIXTH SPEEDS are direct drive only. The torque converter goes to converter drive between each shift (during clutch engagement) to provide smooth shifts.
Mounted on the torque converter are the inlet relief valve (1), the outlet relief valve (2), and the torque converter lockup clutch control valve (3).
A torque converter outlet temperature sensor (4) provides an input signal to the Transmission/
Chassis ECM. The Transmission/Chassis ECM sends the signal to VIMS, which informs the operator of the torque converter outlet temperature.
A Converter Output Speed (COS) sensor (5) sends an input signal to the Transmission/Chassis ECM. The Transmission/Chassis ECM uses the information to calculate shift times for the torque converter lockup clutch and the transmission clutches. The shift time information is sent to VIMS for shift time analysis.
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This sectional view shows a torque converter in CONVERTER DRIVE. The lockup clutch (yellow piston and blue discs) is not engaged. During operation, the rotating housing and impeller (red) can rotate faster than the turbine (blue). The stator (green) remains stationary and multiplies the torque transfer between the impeller and the turbine. The output shaft rotates slower than the engine crankshaft, but with increased torque.
Stator CONVERTER DRIVE
Torque Converter Lockup Oil Passage Freewheel
Assembly
Torque Converter Inlet Oil
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In DIRECT DRIVE, the lockup clutch is engaged by hydraulic pressure and locks the turbine to the impeller. The housing, impeller, turbine, and output shaft then rotate as a unit at engine rpm.
The stator, which is mounted on a freewheel assembly, is driven by the force of the oil in the housing and will freewheel at approximately the same rpm.
Stator DIRECT DRIVE
Torque Converter Lockup Oil Passage Freewheel
Assembly
Torque Converter Inlet Oil
Torque Converter Hydraulic System
The three (785C) or four (789C) section torque converter pump is located at the bottom rear of the torque converter. The four sections (from the front to the rear) are:
- Torque converter scavenge (1) - Torque converter charging (2) - Parking brake release (3)
- Rear brake oil cooling (4) (789C only)
Excess oil that accumulates in the bottom of the torque converter is scavenged by the first section of the pump through a screen behind the access cover (5) and returned to the hydraulic tank.
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Oil flows from the torque converter charging section of the pump to the torque converter charging filter (1).
An oil filter bypass switch (2) is located on the torque converter charging filter. The oil filter bypass switch provides an input signal to the VIMS, which informs the operator if the filter is restricted.
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Oil flows from the torque converter charging filter to the torque converter inlet relief valve (1).
The inlet relief valve limits the maximum pressure of the supply oil to the torque converter.
The torque converter inlet relief pressure can be measured at this valve by removing a plug and installing a pressure tap. Inlet relief pressure should not exceed 930 ± 35 kPa (135 ± 5 psi) at high idle when the oil is cold. Normally, the inlet relief pressure will be slightly higher than the outlet relief valve pressure.
Oil flows through the inlet relief valve and enters the torque converter.
Some of the oil will leak through the torque converter to the bottom of the housing to be scavenged. Most of the oil in the torque converter is used to provide a fluid coupling and flows through the torque converter outlet relief valve (2). The outlet relief valve maintains the minimum pressure inside the torque converter. The main function of the outlet relief valve is to keep the torque converter full of oil to prevent cavitation. The outlet relief pressure can be measured at the tap (3) on the outlet relief valve. The outlet relief pressure should be:
785C: 345 to 585 kPa (50 to 85 psi) at 1640 ± 65 rpm (TC Stall) 789C: 345 to 585 kPa (50 to 85 psi) at 1715 ± 65 rpm (TC Stall)
A torque converter outlet temperature sensor (4) provides an input signal to the Transmission/
Chassis ECM. The Transmission/Chassis ECM sends a signal to VIMS, which informs the operator of the torque converter outlet temperature.
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Most of the oil from the torque converter outlet relief valve flows through the torque converter outlet screen (1) located outside the left frame.
A torque converter outlet screen bypass switch (2) provides an input signal to the VIMS, which informs the operator if the torque converter outlet screen is restricted.
Oil flows from the torque converter outlet screen to the front brake oil cooler located behind the engine.
Oil flows from the parking brake release section of the torque converter pump to the parking brake release filter (3).
A parking brake release filter bypass switch (4) is located on the parking brake release filter.
The bypass switch provides an input signal to the Brake ECM. The Brake ECM sends a signal to VIMS, which informs the operator if the parking brake release filter is restricted.
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The oil from the torque converter outlet screen flows through a diverter valve (1) before flowing through the front brake oil cooler (2). When the retarder or service brakes are ENGAGED, the oil is diverted through the cooler to the brakes. When the brakes are RELEASED, the oil bypasses the cooler and flows directly to the brakes.
Diverting oil around the cooler provides lower temperature aftercooler air during high power demands (when climbing a grade with the brakes RELEASED, for example).
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Oil from the parking brake release filter flows to the parking brake release valve (1). The parking brake release section of the torque converter pump provides supply oil for several purposes:
- Release the parking brakes
- Engage the torque converter lockup clutch - Hoist valve pilot oil
- Front (789C) or rear (785C) brake oil cooling
The parking brake relief valve (2) controls the pressure for parking brake release, torque
converter lockup and hoist valve pilot oil. The parking brake release pressure is 4700 ± 200 kPa (680 ± 30 psi).
Most of the oil from the parking brake release valve flows to the front brake oil cooler on the 789C truck and to the rear brake oil coolers on the 785C truck.
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The parking brake release pump supplies oil to the torque converter lockup clutch valve through the inlet port (1). When the lockup clutch solenoid (located on the transmission housing) is energized by the Transmission/Chassis ECM, transmission pump supply oil (signal oil) enters the lockup valve through the center hose (2). The signal oil pressure is approximately 1725 kPa (250 psi). The signal oil causes the lockup valve to start the modulation process for torque converter lockup. The lockup clutch valve then supplies oil to ENGAGE the lockup clutch in the torque converter.
Torque converter lockup clutch pressure can be measured at the tap (3). Torque converter lockup clutch pressure should be 2135 ± 70 kPa (310 ± 10 psi) at 1300 rpm or higher. Do not check the torque converter lockup clutch pressure below 1300 rpm.
The Converter Output Speed (COS) sensor (4) sends an input signal to the Transmission/
Chassis ECM. The Transmission/Chassis ECM memory also contains the engine rpm and the Transmission Output Speed (TOS) for each gear of the transmission. The Transmission/Chassis ECM provides all of these input signals to the VIMS.
Using the information from the Transmission/Chassis ECM, the VIMS calculates if any
slippage exists in the torque converter lockup clutch or any transmission clutches and stores this information in the VIMS main module. This information can be downloaded from the VIMS with a laptop computer.
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Shown is a sectional view of the torque converter lockup clutch valve in DIRECT DRIVE.
Supply oil from the parking brake release pump is used to provide lockup clutch oil.
First, supply pressure is reduced to provide pilot pressure to the relay valve. Supply oil to the pressure reduction valve flows through cross-drilled orifices in the spool, past a check valve, and enters the slug chamber. The check valve dampens spool movement and reduces the possibility of valve chatter and pressure fluctuation. Oil pressure moves the slug in the right end of the spool to the right and the spool moves to the left against the spring force. The slug reduces the effective area on which the oil pressure can push. Because of the reduced effective area, a smaller, more sensitive spring can be used. Pilot pressure will be equal to the force of the spring on the left end of the spool. The spring force can be adjusted with shims. Pilot pressure is 1725 ± 70 kPa (250 ± 10 psi).
When the lockup solenoid is energized, transmission pump supply (signal) pressure is directed to the relay valve. Before moving the selector piston, the pilot oil moves a shuttle valve to the right, which closes the lower left drain passage and opens the check valve. Oil then flows to the selector piston. Moving the selector piston blocks the upper drain passage, and the load piston springs are compressed.