DIRECT DRIVE
provide the lockup clutch pressure. Lockup clutch pressure depends mainly on the force of the load piston valve springs. When the solenoid is energized, pilot oil moves the selector piston down against a stop. When the load piston that compresses the springs is at the top against the selector piston, lockup clutch pressure is at its lowest controlled value. This value is called
"primary pressure." As the load piston moves down, lockup clutch pressure increases gradually until the load piston stops. Maximum lockup clutch pressure is then reached. The gradual increase in pressure, which depends on how fast the load piston moves, is called "modulation."
The speed of the load piston movement depends on how fast the oil can flow to the area above the load piston. The load piston orifice meters the flow of oil to the load piston chamber and determines the modulation time.
Primary pressure is adjusted with shims in the load piston. Final lockup clutch pressure is not adjustable. If the primary pressure is correct and final lockup clutch pressure is incorrect, the load piston should be checked to make sure that it moves freely in the selector piston. If the load piston moves freely, the load piston springs should be replaced.
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This schematic shows the flow of oil from the torque converter pump through the torque converter hydraulic system on the 789C truck.
The scavenge pump section pulls oil through a screen from the torque converter housing and sends the oil to the hydraulic tank.
The charging pump section sends oil through the torque converter charging filter to the torque converter inlet relief valve. Oil flows from the inlet relief valve through the torque converter to the outlet relief valve. Oil flows from the outlet relief valve through the converter outlet filter and the front brake oil cooler to the front brakes.
The parking brake release pump section sends oil through the parking brake release filter to the parking brake release valve and the torque converter lockup clutch valve. Most of the oil flows through the parking brake release valve and the front brake oil cooler to the front brakes.
The brake cooling pump section of the torque converter pump (789C only) sends oil through the two oil coolers located on the right side of the engine to the rear brakes.
TORQUE CONVERTER
Transmission and Transfer Gears
Power flows from the torque converter through a drive shaft to the transfer gears (1). The transfer gears are splined to the transmission input shaft.
The transmission (2) is located between the transfer gears and the differential (3). The transmission is electronically controlled and hydraulically operated as in all other ICM (Individual Clutch Modulation) transmissions in Caterpillar rigid frame trucks.
The differential is located in the rear axle housing behind the transmission. Power from the transmission flows through the differential and is divided equally to the final drives in the rear wheels. The final drives are double reduction planetaries.
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3
Oil flows from the transmission oil cooler to the transfer gears through a hose (1). Transmission lube oil flows through the transfer gears and the transmission to cool and lubricate the internal components.
The transmission lube pressure relief valve is in the transmission case near the transmission hydraulic control valve. The relief valve limits the maximum pressure in the transmission lube circuit. Transmission lube oil pressure can be measured at the tap (2).
At HIGH IDLE, the transmission lube pressure should be 140 to 205 kPa (20 to 30 psi). At LOW IDLE, the transmission lube pressure should be a minimum of 4 kPa (.6 psi).
The Transmission Output Speed (TOS) sensor (3) is located on the front of the transfer gears.
A small shaft runs from the speed sensor location through the entire length of the transmission and engages the transmission output shaft. The transmission speed sensor signal serves many purposes. Some of the purposes are:
- Transmission automatic shifting - Speedometer operation
- Traction Control System (TCS) top speed limit
- Truck Production Management System (TPMS) distance calculations - Machine speed input to VIMS to determine some warning categories
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2
3
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The transmission is a power shift planetary design which contains six hydraulically engaged clutches. The transmission provides six FORWARD speeds and one REVERSE speed.
1 2 3
4
5 6
POWER SHIFT PLANETARY TRANSMISSION
Transmission Hydraulic System
The transmission pump pulls oil through a suction screen from the transmission tank (see Visuals No. 12 and 159) located on the right side of the truck.
The three-section transmission pump is mounted on the rear of the pump drive, which is located inside the right frame near the torque converter. The three sections are:
- Transmission scavenge (1) - Transmission lube (2) - Transmission charging (3)
The transmission scavenge section pulls oil through the magnetic screens located at the bottom of the transmission. The scavenged oil from the transmission is sent to the transmission tank.
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Shown is the location of the transmission magnetic scavenge screens (arrow). These screens should always be checked for debris if a problem with the transmission is suspected.
Oil is scavenged from the transmission by the first section of the transmission pump (see Visual No. 110).
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Oil flows from the charging section of the transmission pump to the transmission charging filter (1) located on the frame behind the right front tire.
Oil flows from the transmission charging filter to the transmission control valve located on top of the transmission. A transmission oil temperature sensor (2) is located in the tube between the transmission charging filter and the transmission control valve. The temperature sensor provides an input signal to the Transmission/Chassis ECM. The Transmission/Chassis ECM sends a signal to VIMS, which informs the operator of the transmission oil temperature.
Oil flows from the lube section of the transmission pump to the transmission lube filter (3).
Oil flows from the transmission lube filter through the transmission oil cooler to the transfer gears. Transmission lube oil flows through the transfer gears and the transmission to cool and lubricate the internal components.
An oil filter bypass switch is located on each filter. The oil filter bypass switches provide input signals to the Transmission/Chassis ECM. The Transmission/Chassis ECM sends signals to the VIMS, which informs the operator if the filters are restricted.
Transmission oil samples can be taken at the Scheduled Oil Sampling (S•O•S) tap (4).
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3 4
Oil flows from the transmission lube filter and the transmission control valve through the transmission oil cooler bypass valve (1) to the transmission oil cooler (2). The bypass valve for the transmission oil cooler permits oil flow to the system during cold starts when the oil is thick or if the cooler is restricted.
Oil flows from the transmission oil cooler to the transfer gears and the transmission to cool and lubricate the internal components.
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The transmission charging pump supplies oil to the transmission hydraulic control valve and the shift solenoids through the inlet port (1). Excess transmission charging oil either drops to the bottom of the housing to be scavenged or flows to the transmission oil cooler through the outlet hose (2).
The torque converter lockup clutch solenoid (3) is energized by the Transmission/Chassis ECM when DIRECT DRIVE (lockup clutch ENGAGED) is required. Transmission charge pump supply (signal) oil flows through the small hose (4) to the lockup clutch control valve. The lockup clutch control valve then engages the lockup clutch.
The transmission charging pressure relief valve is part of the transmission hydraulic control valve. The relief valve limits the maximum pressure in the transmission charging circuit.
Transmission charging pressure can be measured at the tap (5).
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3
4
5
Shown is the Individual Clutch Modulation (ICM) transmission hydraulic control valve.
Transmission clutch pressures are measured at the pressure taps (1).
The transmission hydraulic control valve contains a priority valve. The priority valve
controls the pressure that is directed to the selector pistons in each of the clutch stations. The transmission priority valve pressure is 1720 kPa (250 psi).
The transmission lube pressure relief valve (2) limits the maximum pressure in the transmission lube circuit.
The "D" Station (3) is used to control the dual stage relief valve setting for the clutch supply pressure. In DIRECT DRIVE, the pressure measured at the tap for station "D" will be approximately 1380 kPa (200 psi). This valve station is adjusted to obtain the correct transmission charge pressure in DIRECT DRIVE.
At LOW IDLE in TORQUE CONVERTER DRIVE, transmission charging pressure should be 2515 kPa (365 psi) minimum. At HIGH IDLE in TORQUE CONVERTER DRIVE, transmission charging pressure should be 3175 kPa (460 psi) maximum.
During torque converter lockup (DIRECT DRIVE), clutch supply pressure is reduced to extend the life of the transmission clutch seals. At 1300 rpm in DIRECT DRIVE, the clutch supply pressure should be 2020 + 240 - 100 kPa (293 + 35 - 15 psi). The corresponding transmission charge pressure is reduced to 2100 ± 100 kPa (305 ± 15 psi).
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4
disconnect the signal line (4) and install a plug in the hose and a cap on the fitting. An 8T5200 Signal Generator/Counter can be used to shift the transmission during the diagnostic tests. If a Signal Generator is not available, disconnect the upshift and downshift solenoids and rotate the rotary selector spool manually by inserting a 1/4 in. ratchet extension through the transmission case.
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Shown is a sectional view of the ICM transmission hydraulic control valve group. The rotary selector spool is in a position that engages two clutches. Pump supply oil from the lockup solenoid flows to the selector piston in station "D." Station "D" reduces the pump supply pressure, and the reduced pressure flows to the lower end of the relief valve. Providing oil pressure to the lower end of the relief valve reduces the clutch supply pressure.
Downshift
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Shown is the transmission hydraulic system. The transmission pump pulls oil through a suction screen from the transmission tank.
The three-section transmission pump is mounted on the rear of the pump drive, which is located inside the right frame near the torque converter. The three sections are:
- Transmission scavenge - Transmission lube - Transmission charging
The transmission scavenge section pulls oil through the magnetic screens located at the bottom of the transmission. The scavenged oil from the transmission is sent to the transmission tank.
Transmission