Valve Body: Description and Operation
HYDRAULIC CONTROL SYSTEMThe function of the hydraulic control system is to perform the actual gear-shifting operations and to determine which gear is to be selected at any time.
The system consists of a number of interacting hydraulic valves actuated by oil supplied under pressure by a pump driven by the torque converter.
The system controls the planetary drive assembly in the gearbox, ensuring that it operates in a gear appropriate to the throttle position, vehicle speed, terrain and gear position (as selected manually).
The hydraulic circuits are supplied at a constant line pressure which is converted and controlled so as to ensure that the clutches and brakes are operated in the correct order. This is achieved by means of a number of hydraulic valves assembled in three valve bodies and forming a compact hydraulic unit.
The valve locations are illustrated below.
Control System Assembly:
A Upper front valve body
B Upper rear valve body
C Lower valve body
1. Gear selector valve
2. Primary regulator valve
3. By-pass valve
4. Relief valve
5. Secondary regulator valve
6. Throttle valve
7. Kickdown valve
8. Governor modulator valve (aluminium plug) (earlier AW70/71s are fined with steel valve and spring)
9. Cutback valve
10. Shift valve 1-2
11. Downshift valve, manual 2-1
12. Modulator valve, manual 2-1
13. Shift valve 2-3
14. Downshift valve, manual 32
15. Modulator valve, manual 32
16. Detent regulator valve
17. Shift valve 34
18. Downshift valve, manual 4-3
19. Regulator valve, downshift D-2, manual
20. Control valve, rear clutch C2
21. Control valve, rear brake B3
22. Signal valve (lock-up)
23. Changeover valve (lock-up)
24. Accumulator pistons C1, C2, B2
25. Solenoid valve
AW71L, AW72L with lock-up function (signal valve and changeover valve), and with oil pressure switch for kickdown inhibitor.
AW70/71 without lock-up function or kickdown inhibitor.
Gear Selector Valve (1)
Operated directly by the gear selector mechanism, the valve determines the position (P, R, N, D, 3, 2,1) in which the gearbox operates.
The valve is supplied with oil at line pressure and distributes it to the appropriate passages according to the position selected. The governor is also supplied from the gear selector valve.
The pressure used in the actual gear-shifting operation (i.e. to engage clutches and/or brakes) is low when the load on the gearbox is low, and vice versa.
Primary Regulator Valve (2)
The primary regulator valve modifies the line pressure to suit different operating conditions. For example, a higher pressure is required in the R position and when accelerating.
By-pass Valve (3)
The by-pass valve protects the oil cooler from excessive pressure.
Relief Valve (4)
Located immediately downstream of the oil pump, the relief valve opens if the line pressure becomes excessive.
Secondary Regulator Valve (5)
The main purpose of the secondary regulator valve is to control the torque converter pressure.
Throttle Valve (6)
The throttle valve is supplied with oil at line pressure and converts it to throttle pressure, which varies with the amount of throttle. This, in turn, actuates the shift valves in combination with the governor pressure.
Kickdown Valve (7)
The effect of the throttle pressure on the shift valves is amplified, by the opening of a special kickdown circuit, when the accelerator is pressed to the floor. Together with the normal throttle pressure, this boost from the kickdown valve means that shifting takes place at higher speeds than normal.
Governor Modulator Valve (8)
The valve operates the cutback valve (9). In later transmissions, in which the valve has been replaced by an aluminium plug, the governor pressure is supplied directly to the cutback valve.
Cutback Valve (9)
The cutback valve is supplied with modulated governor pressure by governor modulator valve (8). However, in units in which valve (8) has been replaced by an aluminium plug, the governor pressure is supplied directly. The purpose of the valve is to reduce the throttle pressure and, as a result, the line pressure at normal and high speeds. The advantages include a reduction in the power required to drive the oil pump and improved quality of gear shifting.
Shift Valve 1-2 (10)
The valve controls the engagement of 1st and 2nd gears. Throttle pressure (which varies with the throttle opening) acts on one end of the valve and governor pressure (which varies with the speed of the car) on the other.
A port in the center supplies fluid to the planetary drive brakes which control 1st and 2nd gears. The gear engaged depends on the manner in which the throttle pressure and governor pressure act on the valve.
Downshift Valve, Manual 2-1 (11)
When the gear selector lever is moved to position 1, one end of downshift valve 2-1 is supplied with a modulated line pressure. Shift valve 1-2 is thereby moved to its bottom position, preventing an upshift.
In the course of its travel, the downshift valve also opens a passage which admits line pressure to apply brake B3. (This affords engine braking in position 1.) If, on the other hand, the car is in 2nd gear when the selector is moved to position 1, downshifting to 1st will take place only when the modulated line pressure is higher than the governor pressure in shift valve 1-2.
Modulator Valve, Manual 2-1 (12)
Operative only when position 1 is selected, the valve performs two functions:
- It reduces the operating pressure (line pressure) to brake B3, affording gentler engine braking.
- Operating in combination with downshift valve 2-1, it performs downshifting from 2nd to 1St with the selector in position 1, while also preventing downshifting at an excessively high speed.
Shift Valve 2-3 (13)
The valve controls the engagement of 2nd and 3rd gears. Throttle pressure (which varies with the throttle opening) acts on one end of the valve and governor pressure (which varies with the speed of the car) on the other.
A port in the center supplies fluid to the planetary train brakes controlling 2nd and 3rd gears. The gear engaged depends on the manner in which the throttle pressure and governor pressure actuate the valve.
Downshift Valve, Manual 3-2 (14)
Line pressure is supplied to one end of the valve when position 2 is selected, moving shift valve 2-3 to its bottom position and preventing an upshift to 3rd. If the car is in 3rd when position 2 is selected, downshifting to 2nd will take place regardless of the throttle opening or speed of the car.
Downshift 4-2
If 4th gear is engaged, a slightly delayed but barely noticeable downshift to 2nd will take place due to the special function of manual downshift control valve D-2. (See description of valve (19) below.)
AW71L and AW72L with lock-up-function (signal valve and changeover valve), and with oil pressure switch for kickdown inhibitor.
AW70/71 without lock-up function or kickdown inhibitor.
Modulator Valve, Manual 3-2 (15)
The valve reduces the operating pressure to brake B1 for gentler engine braking in 2nd gear.
Detent Regulator Valve (16)
Located upstream of the kickdown valve, the detent regulator valve is supplied continuously with line pressure. In this position, it reduces the line pressure to a suitable circuit pressure which is then used by the kickdown valve to control the shift valves and downshift valve 4-3.
Shift Valve 3-4 (17)
Throttle pressure (which varies with throttle opening) is supplied to one end of the valve and an opposing governor pressure (which varies with the speed of the car) to the other.
Line pressure is supplied to clutch C0 and brake B0 through two ports in the middle of the valve, the position of the valve (or, in effect, the balance between the governor and throttle pressures) determining which is pressurized.
Downshift Valve, Manual 4-3 (18)
The valve is either operated manually by the switch on the gear selector lever or automatically by the kickdown function.
Under these conditions, the valve is supplied with system pressure or kickdown pressure, as appropriate, at one end.
In manual operation (by means of the solenoid valve), the downshift valve moves shift valve 3-4 to its bottom position (i.e. 4th gear is disengaged). The downshift valve remains in this position until the solenoid valve is deenergized.
In a kickdown shift, on the other hand, the valve is held in the bottom position only as long as the kickdown pressure is greater than the governor pressure. The latter acts in opposition to the kickdown pressure through shift valve 3-4.
Downshift D-2 Control Valve, Manual (19)
The valve controls the point at which 2nd gear is engaged when downshifting manually from position D to position 2, ensuring that this takes place in stages from 4th to 3rd and then to 2nd.
Line pressure (throttled) acts on one end of the valve and the operating pressure for clutch C0 on the other.
A port in the middle of the valve controls the flow of fluid to downshift valve 3-2.
The flow to downshift valve 3-2 is a function of the balance between the line pressure and clutch C0 operating pressure on the valve spool.
Control Valve, Rear Clutch, Reverse (20)
Clutch C2 is provided with two piston surfaces. While the smaller surface is sufficient to enter reverse when 3rd gear is engaged, both must be used to prevent the clutch from slipping when in position R. The function of the control valve is to ensure gentle engagement of reverse by application of the rear clutch in stages.
Control Valve, Rear Brake, Reverse (21)
The function of the valve is to ensure gentle engagement of reverse. This is achieved by application of brake B3 in stages. The brake is equipped with two hydraulic pistons, each with individual ports.
Consisting essentially of a restriction in the supply to the pistons, the valve distributes the oil flow between the ports. Since the front piston receives a higher flow than the rear piston, it receives full operating pressure before the latter, prolonging the operation of B3.
Lock-up Signal Valve (22)
Line pressure is supplied to one end of the valve and governor pressure to the other. Two line pressure inlets are provided, one for selector positions 1-3 and one for selector position 4. When 4th gear is engaged and the governor pressure (i.e. the speed of the car) increases, oil flows at line pressure from the control valve to the changeover valve (23).
Lock-up Changeover Valve (23)
The valve is actuated by oil at line pressure from the signal valve. Opening of the valve reverses the fluid flow from the torque converter, operating the lock-up clutch plate and activating the lock-up function.
Accumulator Pistons (3 Off) (24)
Each of the circuits serving the front clutch, rear clutch and brake B2 incorporates an accumulator piston.
The pistons are installed in parallel with the C1, C2 and B2 circuits.
When supplying operating pressure to B2, for example, the fluid flow is divided between the brake piston and the accumulator piston. As a result, braking takes place gently when the brake is first applied and full operating pressure is developed only when the accumulator piston is full. This ensures a smooth increase in operating pressure, resulting in gentler shifting.
Solenoid Valve (25)
Disengagement of 4th gear (overdrive) is controlled by the solenoid valve. Disengagement is performed manually using the switch on the gear selector lever.
When the switch is operated, the solenoid valve (which is supplied at constant line pressure) relieves the pressure to downshift valve 4-3, moving shift valve 3-4 to its bottom position and disengaging overdrive.
The valve does not return to its idle position until another signal is received from the switch on the gear selector or the ignition is switched off.
Oil pressure switch, kickdown inhibitor (26), (Europe only)
The oil pressure switch is actuated by the throttle/kickdown pressure. The signals supplied by the switch to the overdrive relay are delivered to the solenoid valve (25), which operates downshift valve 4-3 (18). In this manner, the pressure switch indirectly controls downshifting from 4th to 3rd.