793B Off-Highway Truck Power Train Caterpillar

Transmission Clutch Engagement

Usage:

793B 1HL

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Illustration 1	g00399440
(29) Lockup clutch solenoid (30) Downshift solenoid (31) Upshift solenoid (32) Rotary actuator (33) Pressure control valve (34) Priority reduction valve (35) Neutralizer valve (36) Rotary selector spool (37) Selector and pressure control valve (38) Dual stage relief valve (39) Selector piston for No. 2 clutch (40) Load piston for No. 2 clutch (41) Modulation reduction valve for No. 2 clutch (42) Modulation reduction valve for No. 4 clutch (43) Load piston for No. 4 clutch (44) Selector piston for No. 4 clutch (45) Selector piston for No. 1 clutch (46) Load piston for No. 1 clutch (47) Modulation reduction valve for No. 1 clutch (48) Modulation reduction valve for No. 5 clutch (49) Load piston for No. 5 clutch (50) Selector piston for No. 5 clutch (51) Selector piston for No. 3 clutch (52) Load piston for No. 3 clutch (53) Modulation reduction valve for No. 3 clutch (54) Modulation reduction valve for No. 6 clutch (55) Load piston for No. 6 clutch (56) Selector piston for No. 6 clutch (57) Selector piston for dual stage relief valve (58) Load piston for dual stage relief valve (59) Modulation reduction valve for dual stage relief valve

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Table 1
Chart for Speed Selector
Speed	Engaged Clutches
NEUTRAL-1	3
NEUTRAL-2	1
REVERSE	1 - 6
FIRST	1 - 5
SECOND	2 - 5
THIRD	1 - 4
FOURTH	2 - 4
FIFTH	1 - 3
SIXTH	2 - 3

The transmission is driven by the torque converter in NEUTRAL and in REVERSE. In FIRST speed, torque converter drive is used at lower ground speeds. When the ground speed increases, the Power Train Electronic Control Module (Power Train ECM) will activate the lockup clutch. This connects the transmission directly to the engine. Direct drive is used in FIRST gear at higher ground speeds. Direct drive is used in all other forward speeds. During shifts from FIRST speed through SIXTH speed, torque converter drive is momentarily activated in order to provide smoother shifts. When the transmission clutches are fully engaged, the Power Train ECM engages the lockup clutch and the transmission is in direct drive.

Oil for the transmission is supplied by the transmission charging pump section of the transmission gear pump. Oil is supplied to the pump from the transmission section of the hydraulic tank.

NEUTRAL-1

The electrical system prevents the machine from moving when the transmission is out of the NEUTRAL-1 position. If the engine is started with the transmission out of the NEUTRAL-1 position, the position of rotary selector spool (36) will stop the flow of pump oil to neutralizer valve (35). The spool of neutralizer valve (35) will move upward. When the spool is in this position, neutralizer valve (35) stops the flow of pilot oil from priority reduction valve (34) to rotary selector spool (36). The clutches in the transmission cannot engage.

When the engine is started with the transmission in the NEUTRAL-1 position, the Power Train ECM activates downshift solenoid (29). Downshift solenoid (29) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of rotary selector spool (36) to be locked in the NEUTRAL-1 position. This prevents any movement of rotary selector spool (36). The position of rotary selector spool (36) allows pump oil to go to neutralizer valve (35). The spool in neutralizer valve (35) moves downward. This allows pilot oil to go to rotary selector spool (36). The position of rotary selector spool (36) allows pilot oil to go to selector piston (51) of pressure control valve (33). This causes selector piston (51) and load piston (52) to move to the right. The force of the springs causes modulation reduction valve (53) to move to the right. Pump oil now goes around modulation reduction valve (53). The pump oil begins to fill the No. 3 clutch. The clutch oil also goes through a load piston orifice. The clutch oil then goes between selector piston (51) and load piston (52) .

The pressure of the clutch oil increases after the clutch is full of oil. As the pressure of the oil in the No. 3 clutch increases, modulation reduction valve (53) and load piston (52) move to the left. The load piston orifice in the supply passage to load piston (52) allows oil to go to the load piston at a specific rate. As the load piston moves to the left, modulation reduction valve (53) moves left and right. This causes the pressure that is in the clutch to gradually increase. This gradual pressure increase is known as modulation. When load piston (52) is fully against the stop to the left, modulation stops. The pressure in the No. 3 clutch is now at the maximum. The No. 3 clutch is engaged and the transmission is in NEUTRAL.

Directional Shift Management

Directional shift management reduces torque spikes in the transmission driveline. Directional shift management reduces the energy spikes of the transmission clutch. The transmission driveline is designed to withstand the torque converter stall. The torque converter stall is the maximum amount of torque that is sent to the transmission by the torque converter.

If the engine speed is above 1350 rpm and the operator shifts into the NEUTRAL position, through the NEUTRAL position, or out of the NEUTRAL position, spikes will be generated in the torque converter. The pressure of the spikes is greater than the levels of the torque converter stall. The Power Train ECM stops the pressure spikes by delaying the shift. The Power Train ECM then signals the Engine Electronic Control Module (Engine ECM) for a reduction of fuel and for a lowering of the engine speed. The Power Train ECM then shifts to the requested gear. This reduces the stress on the driveline. This also prevents wear on the transmission and premature failures of the final drive or of the differential.

The event will be recorded as a improper shift.

Manual Shifts

Shift from NEUTRAL to REVERSE

When the transmission control is moved from NEUTRAL to REVERSE, the Power Train ECM activates upshift solenoid (31). Upshift solenoid (31) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of rotary selector spool (36) to move in a clockwise direction from the NEUTRAL-1 position to the NEUTRAL-2 position. The rotor then moves to the REVERSE position. When rotary selector spool (36) arrives at REVERSE, the transmission gear switch that is connected to the rotary selector spool sends an electrical signal to the Power Train ECM. The electrical signal indicates that the shift has been made. The Power Train ECM deactivates upshift solenoid (31). Movement of the rotor that is in rotary actuator (32) and in rotary selector spool (36) stops.

Note: The NEUTRAL-1 position on rotary actuator (32) has a corresponding detent position on rotary selector spool (36). The NEUTRAL-1 position on rotary actuator (32) is N on the operator console. The NEUTRAL-2 position on rotary actuator (32) has a corresponding detent position on rotary selector spool (36). The NEUTRAL-2 position on rotary actuator (32) does not have a corresponding position on the operator console. The operator does not have control of the NEUTRAL-2 gear. Rotary actuator (32) and rotary selector spool (36) automatically go through the NEUTRAL-2 position during all shifts to NEUTRAL and from NEUTRAL.

The NEUTRAL-2 position of rotary selector spool (36) sends pilot oil to selector piston (45). Rotary selector spool (36) drains the pilot oil at selector piston (51). Selector piston (51) moves against the load piston body. This allows the oil that is between selector piston (51) and load piston (52) to drain. Load piston (52) and modulation reduction valve (53) move toward selector piston (51). Pump oil that is flowing to the No. 3 clutch is stopped by modulation reduction valve (53). The clutch oil that is in the No. 3 clutch is drained. The No. 3 clutch is then released.

The pilot oil that is flowing to selector piston (45) causes selector piston (45) and load piston (46) to move to the right. The force of the springs causes modulation reduction valve (47) to move to the right. Pump oil now goes around modulation reduction valve (47). The No. 1 clutch begins to fill. The clutch oil also goes through a load piston orifice. The oil then goes between selector piston (45) and load piston (46). Modulation occurs between load piston (46) and modulation reduction valve (47) for the No. 1 clutch. When load piston (46) moves fully against the stop, modulation stops. The pressure in the No. 1 clutch is at the maximum. The No. 1 clutch is engaged and the transmission is in the NEUTRAL-2 position.

The R position of rotary selector spool (36) sends pilot oil to selector piston (45) and to selector piston (56). The pilot oil that is flowing to selector piston (56) causes selector piston (56) and load piston (55) to move to the left. The force of the springs causes modulation reduction valve (54) to move to the left. Pump oil now goes around modulation reduction valve (54). The No. 6 clutch begins to fill. The clutch oil also goes through a load piston orifice. This oil goes between selector piston (56) and load piston (55). Modulation occurs between load piston (55) and modulation reduction valve (54) for the No. 6 clutch. When load piston (55) moves fully against the stop, modulation stops. The pressure at the No. 6 clutch is at the maximum. The No. 6 clutch is engaged. Because selector piston (45) continues to receive pilot oil, the No. 1 clutch is still engaged. The transmission is in the REVERSE position.

Shift from REVERSE to NEUTRAL

When the transmission control is moved from the REVERSE position to the NEUTRAL position, the Power Train ECM activates downshift solenoid (30). Downshift solenoid (30) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of the rotary selector spool (36) to move counterclockwise from position R. The rotor then moves through the NEUTRAL-2 position toward the NEUTRAL-1 position. When rotary selector spool (36) arrives at the NEUTRAL-1 position, the Power Train ECM keeps downshift solenoid (30) activated. The movement of the rotor that is in rotary actuator (32) and in rotary selector spool (36) stops. Rotary actuator (32) and rotary selector spool (36) are locked in the NEUTRAL-1 position.

The NEUTRAL-2 position of rotary selector spool (36) sends pilot oil to selector piston (45). Rotary selector spool (36) drains the pilot oil at selector piston (56). Selector piston (56) moves against the load piston body. This allows the oil that is between selector piston (56) and load piston (55) to drain. Load piston (55) and modulation reduction valve (54) move toward selector piston (56). Pump oil that is flowing to the No. 6 clutch is stopped by modulation reduction valve (54). The clutch oil that is in the No. 6 clutch is drained. The No. 6 clutch is then released. Because selector piston (45) continues to receive pilot oil, the No. 1 clutch is still engaged. The transmission is in the NEUTRAL-2 position.

The NEUTRAL-1 position of rotary selector spool (36) sends pilot oil to selector piston (51). Rotary selector spool (36) drains the pilot oil at selector piston (45). Selector piston (45) moves against the load piston body. This allows the oil that is between selector piston (45) and load piston (46) to drain. Load piston (46) and modulation reduction valve (47) move toward selector piston (45). Pump oil that is flowing to the No. 1 clutch is stopped by modulation reduction valve (47). The clutch oil that is in the No. 1 clutch is drained. The No. 1 clutch is then released.

The pilot oil that is flowing to selector piston (51) causes selector piston (51) and load piston (52) to move to the right. The force of the springs causes modulation reduction valve (53) to move to the right. Pump oil now goes around modulation reduction valve (53). The No. 3 clutch begins to fill. The clutch oil also goes through a load piston orifice. The oil then goes between selector piston (51) and load piston (52). Modulation occurs between load piston (52) and modulation reduction valve (53) for the No. 3 clutch. When load piston (52) moves fully against the stop, modulation stops. The pressure in the No. 3 clutch is at the maximum. The No. 3 clutch is engaged and the transmission is in the NEUTRAL-1 position.

Shift from NEUTRAL to FIRST Gear

When the transmission control is moved from NEUTRAL to FIRST gear, the Power Train ECM activates upshift solenoid (31). Upshift solenoid (31) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of rotary selector spool (36) to move in a clockwise direction from NEUTRAL-1 through NEUTRAL-2. The rotor then moves to the FIRST gear. When rotary selector spool (36) arrives at FIRST gear, the transmission gear switch that is connected to the rotary selector spool sends an electrical signal to the Power Train ECM. The electrical signal indicates that the shift has been made. The Power Train ECM deactivates upshift solenoid (31). Movement of the rotor that is in rotary actuator (32) and in rotary selector spool (36) stops.

Position 1 of rotary selector spool (36) sends pilot oil to selector piston (45) and to selector piston (50). The pilot oil that is flowing to selector piston (50) causes selector piston (50) and load piston (49) to move to the left. The force of the springs causes modulation reduction valve (48) to move to the left. Pump oil now goes around modulation reduction valve (48). The No. 5 clutch begins to fill. The clutch oil also goes through a load piston orifice. This oil goes between selector piston (50) and load piston (49). Modulation occurs between load piston (49) and modulation reduction valve (48) for the No. 5 clutch. When load piston (49) moves fully against the stop, modulation stops. The pressure at the No. 5 clutch is at the maximum. The No. 5 clutch is engaged. Because selector piston (45) continues to receive pilot oil, the No. 1 clutch is still engaged. The transmission is in the FIRST position.

Shift from FIRST gear to NEUTRAL

When the transmission control is moved from the FIRST position to the NEUTRAL position, the Power Train ECM activates downshift solenoid (30). Downshift solenoid (30) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of the rotary selector spool (36) to move counterclockwise from position 1. The rotor then moves through the NEUTRAL-2 position toward the NEUTRAL-1 position. When rotary selector spool (36) arrives at the NEUTRAL-1 position, the Power Train ECM keeps downshift solenoid (30) activated. The movement of the rotor that is in rotary actuator (32) and in rotary selector spool (36) stops. Rotary actuator (32) and rotary selector spool (36) are locked in the NEUTRAL-1 position.

The NEUTRAL-2 position of rotary selector spool (36) sends pilot oil to selector piston (45). Rotary selector spool (36) drains the pilot oil at selector piston (50). Selector piston (50) moves against the load piston body. This allows the oil that is between selector piston (50) and load piston (49) to drain. Load piston (49) and modulation reduction valve (48) move toward selector piston (50). Pump oil that is flowing to the No. 5 clutch is stopped by modulation reduction valve (48). The clutch oil that is in the No. 5 clutch is drained. The No. 5 clutch is then released. Because selector piston (45) continues to receive pilot oil, the No. 1 clutch is still engaged. The transmission is in the NEUTRAL-2 position.

Automatic Upshifts

The transmission speed sensor and the Power Train ECM cause all the automatic upshifts and all the automatic downshifts. The speed sensor magnetically determines the rpm of a gear on the output shaft of the transmission. The Power Train ECM performs the conversions for the ground speed and for the engine rpm. As the ground speed of the machine increases, the Power Train ECM will activate upshift solenoid (31) at the correct speed. As the ground speed of the machine decreases, the Power Train ECM will activate downshift solenoid (30) at the correct speed.

Note: During a shift from a direct drive gear to another direct drive gear, torque converter drive is momentarily engaged. This occurs on upshifts and on downshifts. The Power Train ECM controls this function with lockup clutch solenoid (29). For more information, refer to the power train Systems Operation, "Lockup Clutch and Synchronizing Valve (Torque Converter)".

Shift from FIRST gear to SECOND gear

When the transmission control is moved from FIRST gear to SECOND gear, an upshift will occur when the output shaft of the transmission achieves the correct rpm. The Power Train ECM activates upshift solenoid (31). The Power Train ECM also deactivates lockup clutch solenoid (29). Upshift solenoid (31) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of rotary selector spool (36) to move in a clockwise direction from position 1 to position 2. When rotary selector spool (36) achieves position 2, the transmission gear switch that is connected to the rotary selector spool sends an electrical signal to the Power Train ECM. The signal informs the Power Train ECM that the shift has been made. Upshift solenoid (31) is deactivated and lockup clutch solenoid (29) is activated. Movement of the rotor in rotary actuator (32) and in rotary selector spool (36) stops.

For information on the results of activating and deactivating the lockup clutch solenoid, refer to the power train Systems Operation, "Lockup Clutch and Synchronizing Valve (Torque Converter)".

Position 2 of rotary selector spool (36) sends pilot oil to selector piston (50) and to selector piston (39). Rotary selector spool (36) drains the pilot oil at selector piston (45). Selector piston (45) moves against the load piston body. This drains the oil that is between selector piston (45) and load piston (46). Load piston (46) and modulation reduction valve (47) now move toward selector piston (45). The pump oil that is flowing to the No. 1 clutch is stopped by modulation reduction valve (45). The clutch oil that is in the No. 1 clutch is drained. The No. 1 clutch is released.

The pilot oil that is flowing to selector piston (39) causes selector piston (39) and load piston (40) to move to the right. The force of the springs causes modulation reduction valve (41) to move to the right. Pump oil now goes around modulation reduction valve (41) and the No. 2 clutch begins to fill. The clutch oil also goes through a load piston orifice. The clutch oil then goes between selector piston (39) and load piston (40). Modulation occurs between load piston (40) and modulation reduction valve (41) for the No. 2 clutch. When load piston (40) is fully against the stop, modulation stops. The pressure in the No. 2 clutch is at the maximum. The No. 2 clutch is engaged. Because selector piston (50) continues to receive pilot oil, the No. 5 clutch remains engaged. the transmission is in SECOND gear.

Shift from SECOND gear to THIRD

When the transmission control is moved from SECOND gear to THIRD, an upshift will occur when the output shaft of the transmission achieves the correct rpm. The Power Train ECM activates upshift solenoid (31). The Power Train ECM also deactivates lockup clutch solenoid (29). Upshift solenoid (31) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of rotary selector spool (36) to move in a clockwise direction from position 2 to position 3. When rotary selector spool (36) achieves position 3, the transmission gear switch that is connected to the rotary selector spool sends an electrical signal to the Power Train ECM. The signal informs the Power Train ECM that the shift has been made. Upshift solenoid (31) is deactivated and lockup clutch solenoid (29) is activated. Movement of the rotor in rotary actuator (32) and in rotary selector spool (36) stops.

For information on the results of activating and deactivating the lockup clutch solenoid, refer to the power train Systems Operation, "Lockup Clutch and Synchronizing Valve (Torque Converter)".

Position 3 of rotary selector spool (36) sends pilot oil to selector piston (45) and to selector piston (44). Rotary selector spool 36 drains the pilot oil at selector piston (39) and at selector piston (50). Selector piston (39) moves against the load piston body. This drains the oil that is between selector piston (39) and load piston (40). Selector piston (50) moves against the load piston body. This drains the oil that is between selector piston (50) and load piston (49) .

Load piston (40) and modulation reduction valve (41) now move toward selector piston (39). Load piston (49) and modulation reduction valve (48) now move toward selector piston (50). The pump oil that is flowing to the No. 2 clutch is stopped by modulation reduction valve (41). The clutch oil that is in the No. 2 clutch is drained. Pump oil that is flowing to the No. 5 clutch is stopped by modulation reduction valve (48). The clutch oil in the No. 5 clutch and in the No. 2 is drained. The No. 2 clutch and the No. 5 clutch are released.

The pilot oil that is flowing to selector piston (45) causes selector piston (45) and load piston (46) to move to the right. The pilot oil that is flowing to selector piston (44) causes selector piston (44) and load piston (43) to move to the left. The force of the springs causes modulation reduction valve (47) to move to the right. The force of the springs causes modulation reduction valve (42) to move to the left. Pump oil now goes around modulation reduction valve (47) and the No. 1 clutch begins to fill. The clutch oil also goes through a load piston orifice. The clutch oil then goes between selector piston (45) and load piston (46). Pump oil now goes around modulation reduction valve (42) and the No. 4 clutch begins to fill. The clutch oil also goes through a load piston orifice. The clutch oil then goes between selector piston (44) and load piston (43). Modulation occurs between load piston (46) and modulation reduction valve (47) for the No. 1 clutch. Modulation occurs between load piston (43) and modulation reduction valve (42) for the No. 4 clutch. When load piston (43) is fully against the stop, modulation stops. The pressure in the No. 1 clutch is at the maximum. When load piston (43) is fully against the stop, modulation stops. The pressure in the No. 4 clutch is at the maximum. The No. 1 clutch is engaged and the No. 4 clutch is engaged. The transmission is in THIRD.

Shift from THIRD through SIXTH

The sequence of each automatic upshift is identical except that different selector pistons receive pilot oil. When the transmission control is in the SIXTH speed position, an upshift occurs from THIRD speed to FOURTH speed at the correct rpm of the transmission output shaft. The No. 1 clutch drains. The No. 1 clutch then disengages. The No. 2 clutch fills. The No. 2 clutch then engages. The No. 4 clutch remains engaged.

As the ground speed increases, an upshift occurs from FOURTH speed to FIFTH speed. The No. 2 clutch and the No. 4 clutch drain. The No. 2 clutch and the No. 4 clutch disengage. The No. 1 clutch and the No. 3 clutch fill. The No. 1 clutch and the No. 3 clutch engage.

As the ground speed increases, an upshift occurs from FIFTH speed to SIXTH speed. The No. 1 clutch drains. The No. 1 clutch then drains. The No. 2 clutch fills. The No. 2 clutch then engages. The No. 3 clutch remains engaged.

Automatic Downshifts

The transmission speed sensor and the Power Train ECM cause all the automatic downshifts and the automatic upshifts. The speed sensor magnetically determines the rpm of a gear on the transmission output shaft. The Power Train ECM performs the conversions for ground speed and for engine rpm. As the ground speed of the machine decreases, the Power Train ECM will then activate downshift solenoid (30) at the correct speed.

Shift from SIXTH through FIRST gear

When the transmission control is moved from SIXTH gear to FIFTH gear, a down shift will occur when the output shaft of the transmission achieves the correct rpm. The Power Train ECM activates downshift solenoid (30). The Power Train ECM also deactivates lockup clutch solenoid (29). Downshift solenoid (30) sends pump oil to rotary actuator (32). This causes the rotor of the rotary actuator and of rotary selector spool (36) to move in a counterclockwise direction from position 6 to position 5. When rotary selector spool (36) achieves position 5, the transmission gear switch that is connected to the rotary selector spool sends an electrical signal to the Power Train ECM. The signal informs the Power Train ECM that the shift has been made. Downshift solenoid (30) is deactivated and lockup clutch solenoid (29) is activated.. Movement of the rotor in rotary actuator (32) and in rotary selector spool (36) stops.

This sequence is followed for each shift. The procedure for clutch engagement is identical to the procedure that is described for upshifts. The procedure for releasing a clutch is identical to the procedure that is described for upshifts.

Pilot oil is sent to selector piston (51) and to selector piston (45). The pilot oil at selector piston (39) drains. The No. 2 clutch is released. The No. 3 clutch remains engaged. The No. 1 clutch is engaged. The transmission is now in FIFTH speed.

Pilot oil is sent to selector piston (39) and to selector piston (44). The pilot oil is drained at selector piston (45) and at selector piston (51). The No. 1 clutch and the No. 3 clutch are released. The No. 2 clutch and the No. 4 clutch are engaged. The transmission is now in FOURTH speed.

Pilot oil is sent to selector piston (45) and to selector piston (44). The pilot oil is drained at selector piston (39). The No. 2 clutch is released. The No. 4 clutch remains engaged. The No. 1 clutch is engaged. The transmission is now in THIRD speed.

Pilot oil is sent to selector piston (39) and to selector piston (50). The pilot oil is drained at selector piston (45) and at selector piston (44). The No. 1 clutch and the No. 4 clutch are released. The No. 2 clutch and the No. 5 clutch are engaged. The transmission is now in SECOND speed.

Pilot oil is sent to selector piston (45) and to selector piston (50). The pilot oil is drained at selector piston (39). The No. 2 clutch is released. The No. 5 clutch remains engaged. The No. 1 clutch is engaged. The transmission is now in FIRST speed.

The automatic downshifts are the opposite of the automatic upshifts. The transmission will automatically downshift from SIXTH speed through FIRST speed.

Neutral Coast

This function restricts coasting into NEUTRAL from high speeds. This restriction extends the life of the transmission. The transmission speed sensor measures the rotation of the gear teeth on a gear that is fastened to the transmission output shaft. The Power Train ECM uses the input signal from the transmission speed sensor to determine the ground speed. If the measured speed is greater than 8 km/h (5 mph), the transmission will not shift into NEUTRAL.

Note: The operator can bypass the restriction against neutral coasting. However, if the ground speed is greater than 16 km/h (10 mph) and the transmission is in NEUTRAL, the event will be recorded in the Power Train ECM.

Shift Inhibiting

If the transmission is in any forward speed and the transmission control is moved to REVERSE, the transmission will immediately make a shift to NEUTRAL. When the ground speed decreases to approximately 5 km/h (3 mph), the transmission will make a shift into REVERSE from the NEUTRAL position.