The first system is a high-pressure system. This system supplies pressure oil to the transmission and the inter-axle differential lock (IAD) controls.
The second system is a low-pressure system. This system supplies low-pressure oil to the torque converter, planetary lube, and the output transfer gear (OTG) lube.
Note: A CWS login is required to access Caterpillar Channel1. Scan the QR code below with a QR enabled device or copy the link that follows.
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| Illustration 1 | g06273608 |
Reference: For more information, refer to “740 & 745 Articulated Truck Power Train Hydraulic System Operation” on Caterpillar Channel1.
https://channel1.mediaspace.kaltura.com/media/740+%26+745+Articulated+Truck+Power+Train+Hydraulic+System+Operation/1_gnuijpybTorque Converter, Transmission, and Output Transfer Gear (OTG)
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| Illustration 2 | g06196174 |
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(C) to Clutch
(P) Supply oil (T) to Tank (1) ECPC Valve for Clutch 1 (2) ECPC Valve for Clutch 2 (3) ECPC Valve for Clutch 3 (4) ECPC Valve for Clutch 4 (5) ECPC Valve for Clutch 5 (6) ECPC Valve for Clutch 6 (7) ECPC Valve for the Lock-up Clutch (8) Transmission Hydraulic control manifold (9) Suction screen (10) Oil sump (11) Scavenger pump (12) Lubrication Relief Valve (13) Transmission lubrication circuit (13a) Output transfer gear (OTG) lubrication (13b) Flywheel Lubrication (13c) Planetary lubrication (14) Transmission Oil Cooler (15) Torque Converter Temperature Sensor (16) Torque converter (and Lock Up Clutch) (17) Inter axle differential lock ECPC solenoid valve (18) T/C inlet relief valve (19) Transmission oil pump (20) Transmission oil filter (21) Main relief valve | |
The gear type transmission oil pump (19) draws oil from transmission oil sump (10) through suction screen (9). The pump supplies oil flow through filter (20) to the following major components:
- Transmission hydraulic control manifold (8)
- Transmission modulation valves (1-6)
- Torque converter and lockup clutch modulation valve (7)
- Main relief valve (21)
- Torque converter (with lockup clutch) (16)
The main relief valve sets the common maximum pressure for the components listed above. The torque converter inlet relief valve (18) is installed in transmission control body. The torque converter inlet relief valve limits the pressure of the oil to the torque converter to
Oil exiting the torque converter is measured for temperature by sensor (15) and is directed to oil cooler (14). The oil then goes to the transmission lubrication circuit (13). Lube relief valve (12) creates slight back pressure in the circuit from the transmission oil cooler. This back pressure ensures sufficient oil flow and pressure for lubrication of bearings and other moving parts in the transmission. Scavenger pump (11) returns the oil from the OTG back to the transmission sump.
When torque loads are low, the transmission ECM will energize solenoid (7) on the torque converter lockup clutch control valve to engage lockup clutch (16). The lockup clutch provides a mechanical connection between the engine and transmission. This direct drive mode is more fuel efficient and reduces power loss to a minimum, when conditions allow. When Engaged, the lockup clutch provides direct drive mode once the transmission speed and the engine speed are matched. The LUC stays engaged during gear shift.
Transmission pump supply oil, at main relief pressure, is delivered to each of the six transmission modulating valves, and to the torque converter lockup clutch via the transmission hydraulic control manifold. The transmission modulating valves control the engagement of the transmission clutches. The solenoids are controlled by a Pulse Width Modulated (PWM) signal from the Transmission ECM. Supply oil flows into the clutch modulating valves and through a passage in the center of the spool. Oil then flows to the tank if the solenoid is DE-ENERGIZED. When the solenoid is ENERGIZED, the spool will shift down and the clutch will begin to fill. The timing of the signal from the Transmission ECM determines when the clutch is filled. This timing is determined by the last clutch fill calibration for all six clutches.
Each individual valve ( (1) through (7)) is an Electronic Clutch Pressure Control (ECPC) valve. The ECPC valves control the pressure of the oil that is supplied to the clutches in the transmission planetary.
Electronic Clutch Pressure Control (ECPC) Valves
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| Illustration 3 | g06196192 |
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(M) Transmission Hydraulic control manifold
(22) Solenoid (23) Chamber to tank (24) Chamber to clutch (25) Supply oil Chamber | |
Oil (25) to the ECPC valves is supplied by the transmission pump through a filter. Refer to Illustrations 2 and 6. Oil is available to each modulating valve.
Control of the transmission planetary is achieved by hydraulically actuated clutches that hold selected components of the planetary group stationary. This control over the planetary allows transmission of power in the selected gear (speed ratio) through the transmission.
Shifting of the transmission is controlled by the electronic control module (ECM). The ECM acts in response to various inputs.
Inputs to the ECM module include the following information:
- Engine Speed
- Clutch Speed
- Output Speed
- Ground Speed
The control of the transmission also accounts for factors such as the temperature of the transmission oil.
Shifting of the transmission gears involves the release of one clutch and the subsequent engagement of the next clutch.
Note: Some of the shifts are double clutch shifts. Double clutch shifting includes turning off two clutches and turning on two clutches.
Each clutch is engaged by hydraulic oil pressure that is supplied by the ECPC valves. The ECPC valves are controlled by pulse width modulated signals (PWM). The signals are sent by the ECM to individual solenoids on each ECPC valve.
The pressure of the oil in the clutches is proportional to the electrical current that is supplied to the solenoid that controls the clutch.
In the neutral position the solenoid (22) receives no signal from the ECM. Oil from the pump flows into the valve through the hole in chamber (25). The oil flows through the spool into chamber (23), and back to the oil sump.
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| Illustration 4 | g03741046 |
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Activated position (M) Transmission Hydraulic control manifold (22) Solenoid (23) Chamber to tank (24) Chamber to clutch (25) Supply oil Chamber | |
In the activated position the solenoid (22) receives a signal from the ECM. The solenoid pushes the spool to the right and compresses the spring. Supply oil from the pump flows into the valve through the hole in chamber (25). The oil flows through the spool into chamber (24). The oil in chamber (24) flows to the clutch.
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| Illustration 5 | g02329993 |
A high current input is made initially. The high current overcomes the inertia of the valve and the clutch components which initiates filling of the oil passage leading to the clutch piston. The current is reduced as the clutch piston is filled with oil and the plates move toward engagement. The current is held at a minimum value to allow the pressure of the oil in the clutch to increase slowly. The clearances between the plates and the disks are taken up smoothly. This period is called touchup. Once the plates and the disks are in contact, the current is increased gradually. The gradual increase of current ensures that maximum pressure is applied to the clutch piston in a smooth manner.
Electronic control of this process provides improved quality of transmission shifts.
A combination of clutches engages each of the nine forward gears and two reverse gears.
Power is supplied from the engine, through the torque converter, and then to the planetary transmission. The torque converter allows the speeds of the engine and the transmission to be matched smoothly. Once the speed of the engine and the transmission is matched, the torque converter lockup clutch engages. Engaging the lockup clutch allows direct drive between the engine and the transmission.
Oil is supplied to the torque converter by relief valve (21). Refer to Illustrations 2.
Oil from the torque converter flows through torque converter outlet relief valve to the transmission oil cooler.
Transmission shifts are dictated by operating conditions. The appropriate gear is selected for the speed of the engine and the speed of the machine. Operator input can alter the shifting of the transmission. Example of operator input is the selection of transmission hold. Features such as overspeed protection and the application of the engine compression brake can also alter the shifting of the transmission.
Transmission Oil Pump and Suction Screen
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| Illustration 6 | g06196208 |
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(9) Suction screen
(19) Oil pump (transmission) (26) Transmission manifold (27) Transmission manifold cover (28) Transmission oil outlet "TO FILTER" (29) Transmission oil inlet "FROM FILTER" (30) Transmission inlet oil flow from strainer to pump (31) Transmission inlet oil flow from pump to filter, torque converter, and transmission lubrication system (32) Transmission input shaft from torque converter | |
Refer to Illustrations 2 and 6. Transmission oil pump (19) in located inside the transmission. The transmission input shaft drives the transmission oil pump. Inlet oil and outlet oil for pump (19) is routed by transmission manifold (26). Oil is drawn from the sump through suction screen (9). Oil enters the transmission housing flowing into manifold (26). Oil enters the pump inlet, through the pump, exiting the pump outlet to the pump filter, torque converter, and the transmission lubrication system. Oil to the torque converter and the transmission lubrication system is internal within the transmission. The oil filter is external from the transmission. The inlet and outlet for the filter is shown in Illustration 6.
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| Illustration 7 | g06196219 |
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Location of transmission oil filter is left the side under the cab. (20A) Filter base (20B) Filter element housing (32) Bypass valve (33) Pressure test port (34) Bypass switch (35) SOS sample port | |
The transmission oil filter is located under the left rear section of the operator station.
The transmission oil filter base contains S-O-S test port (35) that is used to collect oil samples from the transmission oil system. The filter base also contains filter bypass valve (32). If the filter becomes impregnated with contaminants, the pressure on the bypass valve will increase. When the pressure differential between the filter inlet and outlet is greater than
Then Normally Closed bypass switch (34) will open at the same pressure of
Pressure test port (33) is also contained in the filter base. This test port can be used to test the transmission pump supply pressure to the transmission hydraulic control manifold.
The S-O-S port is installed in the inlet to the filter. The pressure test port is installed in the outlet from the filter.
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| Illustration 8 | g06196236 |
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Transmission Oil Cooler (11) Scavenger pump for OTG oil recovery (14) Oil cooler (14A) Top section of oil cooler that cools the transmission oil (14B) Bottom section of oil cooler that cools oil from the hydraulic arrangement | |
The oil-to-water type transmission oil cooler (14) is located beneath the front right-hand side of the operator station. The oil is cooled by coolant supplied by the engine cooling package.
Oil from the torque converter flow through a bundle of small tubes inside the cooler. The oil is cooled by engine coolant flowing around the bundle of oil filled tubes. The cooled oil enters the transmission lube oil circuit and drains into the transmission oil sump. The remainder of the oil is returned directly to the transmission oil sump. Refer to Illustration 8 for details.
Upper portion (14A) of the cooler handles the oil for the transmission and OTG. The lower portion (14B) handles oil from the hydraulic arrangement. The water lines from the engine cooling package flows through both sections of the cooler.
Output Transfer Gears and Differentials
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| Illustration 9 | g06196247 |
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(8) Transmission Hydraulic control manifold
(9) Suction screen (10) Oil sump (11) Scavenger pump (12) Lubrication Relief Valve (13) Transmission lubrication circuit (13a) Output transfer gear (OTG) lubrication (13b) Flywheel Lubrication (13c) Planetary lubrication (14) Transmission Oil Cooler (15) Torque Converter Temperature Sensor (16) Torque converter (and Lock Up Clutch) (17) Inter axle differential lock ECPC solenoid valve (18) T/C inlet relief valve (19) Transmission oil pump (20) Transmission oil filter (21) Main relief valve (36) Interaxle Differential (37) Output Transfer Gears | |
A second hydraulic circuit supplies lubrication for the output transfer gears (OTG). The circuit also controls the interaxle differential lock within the OTG.
Oil from the sump of the output transfer gears (37) is transferred by scavenging pump (11) to oil sump (10). The scavenger pump is located left of the engine on the top of the flywheel housing. Refer to Illustration 7.
Oil in sump (10) is drawn by transmission pump (19) through suction screen (9). The magnetic screen will remove any metallic particles that may be present in the oil.
Pump (19) supplies high-pressure oil through filter (20) to transmission hydraulic control (8). Oil from the filter also supplies high-pressure oil to interaxle differential lock (17).
Oil pressure to the clutch solenoids in manifold (8) is limited by relief valve (21). Relief (21) also limits the oil pressure to IAD valve (17).
Some of the low-pressure oil in relief (21) is directed to the torque converter along with low-pressure oil from pump (19). This oil to the torque converter is controlled by torque converter inlet relief (18). This oil flows through the torque converter and provides lubrication to the following components:
- output transfer gear lubrication (13a)
- flywheel lubrication (13b)
- planetary lubrication (13c)
Solenoid (17) is mounted on the output transfer gear case and controls interaxle differential lock (36). Operation of solenoid (17) applies oil pressure to the lockup clutch in the interaxle differential. Applying oil pressure to the lockup clutch engages the interaxle differential lock.
The IAD and the cross axle differential (XAD) system are two separate systems. The Series C articulated truck does not use the transmission hydraulic oil circuit to supply oil to the XAD system.
All the gears and bearings in the OTG are pressure lubricated. A tee fitting in the transmission cooler outlet directs oil from the cooler to the OTG manifold.