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| Illustration 1 | g01336844 |
| (1) Modulating valves | |
The nine modulating valves are located on top of the transmission.
Each modulating valve has a designated letter, as shown. The letters have been cast into the transmission case. The designated letters are used to correlate the modulating valve to the corresponding transmission clutch.
Reference: See Systems Operation/Testing and Adjusting, "Transmission Power Flow" for additional information.
The modulating valve provides the following control functions:
- Proportional control of pressure
- Controls oil flow to clutch
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| Illustration 2 | g00886268 |
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Schematic for Modulating Valve (Transmission) (1) Modulating valve (transmission) (2) Test port (3) Transmission clutch (4) Spring (5) Line to sump (6) Spool orifice (7) Charge pressure oil (8) Solenoid | |
Modulating valve (1) is controlled by the transmission electronic control module (ECM). The modulating valve is used by the transmission ECM to directly modulate the oil pressure that is sent to each of the nine individual transmission clutches.
When the operator selects a direction or when the operator selects a speed, the transmission ECM sends a pulse width modulated signal in order to vary the current to solenoid (8).
The distance that is travelled by the solenoid plunger is proportional to the electrical current that is sent to the solenoid. The position of the solenoid plunger controls the oil pressure that is sent to transmission clutch (3).
When the transmission ECM sends the maximum programmed current to the modulating valve, the oil pressure in the transmission clutch will be at the maximum desired pressure. When no current is sent to the modulating valve, the oil pressure in the transmission clutch will be zero.
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| Illustration 3 | g01336456 |
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Bottom View of Modulating Valve (Transmission) (5) To sump (7) Charge pressure oil (8) To transmission clutch | |
Illustration 3 shows the bottom view of the modulating valve (transmission). The bottom view shows the porting for the inlet and outlet flow of hydraulic oil.
There are three basic stages of the controlled operation of the modulating valve:
- No signal current
- Partial signal current
- Maximum signal current
The operation of the modulating valve is not a simple on/off cycle. The transmission ECM varies the strength of the signal current through a programmed cycle in order to control the movement of the proportional solenoid valve spool.
No Signal Current (Clutch Disengaged)
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| Illustration 4 | g01336785 |
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Modulating Valve (Transmission) (1) Modulating valve (transmission) (2) Test port (4) Spring (5) Line to sump (6) Spool orifice (7) Charge pressure oil (8) To transmission clutch (9) Solenoid (10) Ball (11) Valve spool (12) Drain orifice (13) Pin | |
The transmission ECM controls the rate of oil flow through modulating valve (1) by changing the strength of the signal current to solenoid (9). When signal current is not being sent to the solenoid, the solenoid will be de-energized.
Oil Flow in a De-energized Valve
Spring (4) holds the valve spool to the left in the valve body.
The valve spool opens the passage between transmission clutch (3) and the sump.
The valve spool blocks the passage between the transmission clutch and the charge pressure oil. Consequently, pressurized oil flow is blocked to the transmission clutch.
Charge pressure oil (7) flows from the charge pump into the valve body and around valve spool (11).
The oil flows through the drilled passage that is in the valve spool and into spool orifice (6). The Spool orifice is on the left side of the valve spool. The oil then flows onto drain orifice (12).
Because there is no force on pin (13), in order to hold ball (10) against the drain orifice, the oil flows through the valve spool. The oil flows through the drain orifice, around the ball and to sump (5).
Oil from the transmission clutch will drain into the sump. As a result, the transmission clutch is not engaged.
Partial Signal Current (Modulation)
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| Illustration 5 | g01328854 |
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Modulating Valve (Transmission) (1) Modulating valve (transmission) (2) Test port (4) Spring (5) Line to sump (6) Spool orifice (7) Charge pressure oil (8) To transmission clutch (9) Solenoid (10) Ball (11) Valve spool (12) Drain orifice (13) Pin | |
The transmission ECM controls the signal current in order to modulate the flow of oil through modulating valve (1). When solenoid (9) receives less than maximum programmed signal current, the oil flow is modulated.
Oil Flow in Valve with Partial Signal Current
The engagement of transmission clutch (3) begins when the transmission ECM initially sends signal current in order to energize the solenoid.
The signal current creates a magnetic field around pin (13) which will cause the pin to move to the right against ball (10).
The force against the ball will be in proportion to the strength of the signal current from the transmission ECM.
The position of the ball against drain orifice (12) restricts the oil flow to sump (5). This partial restriction causes the pressure at the left end of valve spool (11) to increase.
The oil pressure moves the valve spool to the right against spring (4). The movement of the valve spool starts to open a passage on the right end of the valve spool. As a result, charge pressure oil (7) will start to fill the transmission clutch.
Oil also begins to fill the chamber for the spring on the right end of the valve spool.
In the initial stage for filling the transmission clutch with oil, the transmission ECM sends a high pulse of current in order to quickly move the valve spool.
During this short period of time, the clutch piston will move in order to remove the clearances that are between the clutch discs and the plates. This will minimize the amount of time that is needed to fill the clutch.
The transmission ECM will then reduce the signal current which will also reduce the pressure setting of the modulating valve. The change in signal current reduces the flow of oil to the transmission clutch.
When the plates and the discs start to touch each other, this is called the touch-up point. Once the touch-up point is obtained, the transmission ECM begins a controlled increase of the signal current in order to start the modulation cycle.
The increase in the signal current causes the ball and the pin to further restrict the flow of oil through the drain orifice and to the sump. This causes a controlled movement of the valve spool to the right.
This movement of the valve spool allows the pressure to transmission clutch (3) to increase.
The modulating valve uses the variable signal current from the transmission ECM to vary the pressure output of the valve.
The sequence of partial engagement is called desired slippage. The clutches must slip a specific amount in order to provide smooth shifting. The desired slippage is controlled by the application program that is stored in the transmission ECM.
Maximum Signal Current (Clutch Fully Engaged)
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| Illustration 6 | g00774323 |
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Modulating Valve (Transmission) (1) Modulating valve (transmission) (2) Test port (4) Spring (5) Line to sump (6) Spool orifice (7) Charge pressure oil (8) To transmission clutch (9) Solenoid (10) Ball (11) Valve spool (12) Drain orifice (13) Pin | |
When the modulation cycle stops, the transmission ECM will send the maximum specified signal current in order to fully engage the transmission clutch (3).
Oil Flow in Valve with Maximum Current Signal
The constant signal current pushes pin (11) firmly against ball (10) in modulating valve (1).
The force against the ball reduces the amount of oil flow through drain orifice (12). This restriction causes an increase in pressure on the left side of valve spool (13).
The valve spool moves to the right in order to allow the charge oil to fully engage the transmission clutch.
In a short period of time, maximum pressure is felt at both ends of the valve spool. This pressure and the additional force that is produced by spring (4) causes the valve spool to move to the left until the forces on the right end and the left end of the valve spool are balanced.
The movement of the valve spool to the left balanced position reduces the flow of oil to the engaged transmission clutch. The transmission ECM sends a constant maximum specified signal current to solenoid (8) in order to maintain the desired clutch pressure.