938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier Power Train Modulating Valve (Transmission Clutch) Caterpillar


Modulating Valve (Transmission Clutch)
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1.1. General Information
2.1. Neutral
3.1. Less Than Maximum Signal
4.1. Maximum Signal

General Information




Illustration 1g00929156



Illustration 2g00666134

Modulating Valve (Transmission Clutch)

The clutch modulating valves are mounted on the left side of the transmission under the cab. There is one modulating valve for each of the six transmission clutches. Refer to Illustration 1 for the location of the following modulating valves:

  • Modulating valve for clutch 1 (forward low) (1)

  • Modulating valve for clutch 2 (forward high) (2)

  • Modulating valve for clutch 3 (reverse) (3)

  • Modulating valve for clutch 4 (second) (4)

  • Modulating valve for clutch 5 (third) (5)

  • Modulating valve for clutch 6 (first) (6)

Each of the modulating valves are controlled by a solenoid. The solenoids are controlled by the power train electronic control module (ECM). The modulating valves are used by the power train ECM to directly modulate the oil pressure that is sent to each individual clutch. When the operator selects a direction and a speed, the power train ECM sends a pulse width modulated signal (PWM) in order to vary the current to the solenoid on the modulating valve. The amount of current that is sent to the solenoid regulates the amount of oil pressure that is supplied to the selected clutch. When the power train ECM sends the maximum current to the solenoid, the oil pressure in the clutch is at the maximum. When no current is sent to the modulating valve, the oil pressure in the clutch is at the minimum.

In order for the machine to move, one speed clutch solenoid and one direction clutch solenoid must be energized.

Note: The solenoid coils are not designed to be operated at 24 DCV directly. The power train ECM sends a 24 volt PWM signal at a duty cycle which provides an average voltage of about 12 volts to the solenoid coils. Do not energize the solenoid coils with 24 DCV (+battery). The life of the solenoid coils will be drastically reduced if the solenoid coils are energized with 24 DCV (+battery). If the solenoid coils must be energized by a source that is different from the power train ECM, use 12 DCV to energize the solenoid coils.

Neutral




Illustration 3g00922059

When the power train ECM requires a clutch to be disengaged, there is no flow of electric current to the solenoid.

Pump oil (16) flows into the valve body from pump passage (6). Oil then flows into passage (7), through orifice (2) and into chamber (13). Since there is no signal that is sent to solenoid (12), pin (11) cannot hold ball (1) against orifice (10). The oil flows through orifice (10) past ball (1) to the tank passage (9). The oil in tank passage (9) becomes return oil (17) .

Spring (5) holds valve spool (4) to the left. When valve spool (4) is shifted to the left, the oil in clutch passage (8) flows to tank passage (9). When the oil in clutch passage (8) is vented to the tank, the clutch cannot be engaged. The oil in pump passage (6) is blocked from entering clutch passage (8) .

Less Than Maximum Signal




Illustration 4g00908868

When the power train ECM requires a clutch to be engaged, a signal is sent to the solenoid (12). The strength of the signal is proportional to the desired clutch pressure.

When the signal is sent to the solenoid, pin (11) moves to the right and pin (11) forces ball (1) toward orifice (2). When ball (1) moves toward orifice (2), the flow of return oil (18) into tank passage (9) is restricted. Pilot oil pressure in chamber (13) increases and moves valve spool (4) to the right against the force of spring (5) .

When valve spool (4) moves to the right, pump oil (16) in pump passage (6) enters clutch passage (8). The valve spool blocks the flow of oil from clutch passage (8) to tank passage (9). The clutch pressure will increase due to the flow of reduced pressure oil (17) into clutch passage (8). The oil in clutch passage (8) also flows through passage (14) into chamber (15) .

Initially, the power train ECM sends a high signal to the solenoid in order to fill the clutch with oil quickly. Then, a reduced signal is sent to the solenoid in order to allow the clutch to engage smoothly.

Once the clutch engages, the power train ECM begins to increase the signal to solenoid (12) in order to further engage the clutch. When the signal to solenoid (12) increases, pin (11) moves to the right. Pin (11) pushes ball (1) toward orifice (2) and ball (1) restricts the flow of oil to tank passage (9). Oil pressure in chamber (13) will rise and valve spool (4) will move to the right. This causes the clutch pressure to increase.

Maximum Signal




Illustration 5g00908869

When the clutch is fully engaged, the power train ECM sends the maximum specified signal to solenoid (12) in order to keep the clutch fully engaged.

The signal that is sent to solenoid (12) moves pin (11) and ball (1) to the right. This further restricts the flow of return oil (17) from clutch passage (8) to tank passage (9). The pilot oil pressure in chamber (13) increases and valve spool (4) moves further to the right. The pump oil (16) from pump passage (7) flows to clutch passage (8) and the clutch will fully engage.

Once the clutch is fully engaged, the pump oil (16) in chamber (13) and the combined pressure of oil in chamber (15) and spring (5) will equalize. This will cause valve spool (4) to move to the left to a balanced position.

In the balanced position, the flow of oil from pump passage (6) to clutch passage (8) maintains the desired clutch pressure. In the balanced position, oil flows to the clutch in order to compensate for internal leakage in the clutch.

Note: The modulating valves control the engaging and the disengaging of the clutches. This valve is not a simple on/off valve. The power train ECM varies the strength of the signal that is sent to the solenoid. The signal that is sent to the solenoid is programmed into the power train ECM.

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