Oil pressure control means for an automatic transmission

An oil pressure control means for an automatic transmission for vehicles having a plurality of speed shift valves for changing over supply of oil pressure to a plurality of friction engaging means for establishing various speed stages, a manual shift valve for shifting speed ranges and adapted to selectively supply control pressure to the speed shift valves, and a downshift control valve for controlling supply of the downshift control pressure to a particular speed shift valve so that a downshift operation performed by the manual shift valve does not cause abrupt two stage downshifting from the highest speed stage to the next but one lower speed stage, skipping the next lower speed stage.

BACKGROUND OF THE INVENTION 
The present invention relates to an oil pressure control means for an 
automatic transmission for vehicles, and, more particularly, to an 
improvement thereof with regard to downshift performance. 
In an automatic transmission for vehicles comprising a fluid torque 
converter and a speed shift gear means having a plurality of friction 
engaging means and adapted to establish various speed shift stages, 
engagement and disengagement of said friction engaging means are 
automatically changed over depending upon operating conditions of the 
vehicle so as to establish the most desirable speed shift stage for the 
current operating condition of the vehicle, instant by instant. This 
changing-over control of the friction engaging means is generally effected 
by an oil pressure control means which comprises a plurality of speed 
shift valves, adapted to be shifted depending upon a balance of the 
throttle pressure which varies in accordance with the stepping-on amount 
of the accelerator pedal or engine throttle opening and the governor 
pressure which varies in accordance with the vehicle speed, thereby 
selecting the most desirable speed shift stage of the speed shift gear 
means. Furthermore, the oil pressure control means includes a manual shift 
valve adapted to be operated by hand by the driver for setting a 
particular speed shift range. The speed shift ranges in usual automatic 
transmissions include D range, 2 range, and L range for forward driving. 
Assuming that an automatic transmission is designed to provide a 1st speed 
stage, a 2nd speed stage, a 3rd speed stage and an overdrive stage, which 
are automatically selected in accordance with a balance of the vehicle 
speed and the stepping-on amount of the accelerator pedal, when the manual 
shift valve is shifted to the D range, an automatic selection of speed 
stage is made out of the aforementioned four speed stages. When the manual 
shift valve is shifted to the 2 range, speed stage selection is made from 
the 1st speed stage and the 2nd speed stage. When the manual shift valve 
is shifted to the L range, the automatic transmission is fixed in the 1st 
speed stage. 
Now, let us assume that an automatic transmission includes overdrive means 
and a vehicle having such an automatic transmission is being driven in 
overdrive stage. Under this running condition, if the manual shift valve 
is abruptly shifted from D range to 2 range, the automatic transmission is 
abruptly shifted down by two stages from the overdrive stage to the 2nd 
speed stage, skipping over the 3rd speed stage. Such an abrupt two-stage 
downshifting is not desirable because it causes heavy frictional force to 
be exerted on the friction engaging elements of the friction engaging 
means, thereby unduly wearing these elements, while it further produces a 
danger of causing an over-revving of the engine. Furthermore, such an 
abrupt two-stage downshifting causes abrupt deceleration of the vehicle, 
which is not desirable in view of driving safety. 
SUMMARY OF THE INVENTION 
It is therefore the object of the present invention to provide an improved 
oil pressure control means for automatic transmissions incorporating 
overdrive means, said oil pressure control means being adapted not to 
abruptly shift down the transmission from the overdrive stage to the 2nd 
speed stage when the manual shift valve is shifted from D range to 2 range 
when the vehicle is running in the overdrive stage, but to shift down once 
the transmission from the overdrive stage to the 3rd speed stage so as to 
cause a moderate deceleration of the vehicle in the 3rd speed stage and 
thereafter as a second operation to shift down the transmission from the 
3rd speed stage to the 2nd speed stage.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 an example of an automatic transmission incorporating an 
overdrive means is shown in the form of a diagram for the purpose of 
simplicity. However, if the more concrete structure of the transmission is 
required, reference should be made to the specifications and drawings of 
co-pending patent applications Ser. Nos. 791,575 and 791,576, both being 
assigned to the same assignee as the present application. The automatic 
transmission herein shown comprises a fluid torque converter 1, an 
overdrive means 2, a reduction gear means 3 for effecting three forward 
speed stages and one rearward speed stage and is adapted to be controlled 
by the oil pressure control means shown in FIG. 2. The fluid torque 
converter 1 is of the conventional well-known type including a pump 
impeller 5, a turbine 6, and a stator 7. The pump impeller 5 is connected 
with the crankshaft 8 of the engine (not shown) while the turbine 6 is 
connected with a turbine shaft 9 which forms an output shaft of the fluid 
torque converter. This output shaft also provides an input shaft of the 
overdrive means 2, wherein the input shaft is connected with a carrier 10 
of a planetary gear mechanism incorporated in the overdrive means. The 
carrier 10 rotatably supports a plurality of planetary pinions 14 which in 
turn are engaged with a sun gear 11 and a ring gear 15. A multi-disc 
clutch 12 and a one-way clutch 13 are provided between the sun gear 11 and 
the carrier 10, and, furthermore, a multi-disc brake 19 is provided 
between the sun gear 11 and the overdrive housing or case 16 enclosing the 
overdrive means or the planetary gear mechanism. 
The ring gear 15 of the overdrive means 2 is connected with an input shaft 
23 of the reduction gear means 3. A multi-disc clutch 24 is provided 
between the input shaft 23 and intermediate shaft 29 while a multi-disc 
clutch 25 is provided between the input shaft 23 and a sun gear shaft 30. 
A multi-disc brake 26 is provided between the sun gear shaft 30 and the 
transmission case 18. The sun gear shaft 30 has a sun gear 32 integrally 
formed therein, said sun gear meshing with a plurality of first planetary 
pinions 34 which mesh in turn with a ring gear 35 supported by a carrier 
33 thereby forming a first set of planetary gear mechanism while the sun 
gear 32 also meshes with a plurality of second planetary pinions 37 which 
mesh in turn with a ring gear 38 supported by a carrier 36, thereby 
forming a second set of planetary gear mechanism. The ring gear 35 of said 
first planetary gear mechanism is connected with the intermediate shaft 
29, while the carrier 33 thereof is connected with the ring gear 38 of 
said second planetary gear mechanism. The combination of the carrier 33 
and the ring gear 38 is connected with an output shaft 39. A multi-disc 
brake 27 and a one-way clutch 28 are provided between the carrier 36 of 
said second planetary gear mechanism and the transmission case 18. 
The automatic transmission is controlled by the oil pressure control means 
explained hereinunder in a manner such that the aforementioned clutches 
and brakes are engaged or disengaged in order to accomplish various shift 
conditions among four forward speed stages including an overdrive stage 
and one rearward drive stage in accordance with the operation of a manual 
shift lever (not shown) and/or the balance of the engine output power and 
the vehicle speed. 
Table 1 shows the operating conditions of the clutches and brakes in 
various shift conditions. 
FIG. 2 is a diagram showing an example of the standard constitution of the 
oil pressure control circuit included in the oil pressure control means 4. 
The oil pumped up from the oil reservoir 40 by an oil pump 41 is delivered 
to a line pressure regulating valve 42 and generates a line pressure Pl 
having a predetermined regulated value in an oil passage 43. The line 
pressure is supplied to a manual shift valve 44, a throttle pressure 
regulating valve 45, and a detent pressure regulating valve 46. The manual 
shift valve 44 has various shift positions such as parking (P), reverse 
(R), neutral (N), D range (D), 2 range (2), and L range (L), and is 
adapted to deliver the line pressure supplied to its input port 43a to its 
output ports 43b, 43c, 43d and 43e in accordance with the shift position 
thereof as shown in Table 2. 
The throttle pressure regulating valve 45 generates a throttle pressure Pth 
at its output port 50 in accordance with the amount of depression of the 
accelerator pedal or the opening of the intake throttle valve. The detent 
pressure regulating valve 46 generates a detent pressure at its output 
passage 51, said detent pressure being lower than the line pressure by a 
predetermined value and being supplied to speed shift valves for 
maintaining them at particular shift positions as explained hereinunder 
through a valve 52 incorporated in the throttle pressure regulating valve 
and adapted to establish communication therethrough when the accelerator 
pedal has been stepped on beyond a predetermined amount and through an oil 
pressure 53. A pasage 47 connected to the port 43b of the manual shift 
valve 44 is led to the clutch 24 (forward clutch), and a passage 47a 
branched from a middle portion thereof is led to a governor valve 54. The 
governor valve generates a governor pressure Pgo corresponding to the 
vehicle speed at its output port 55. 
TABLE 1 
______________________________________ 
Clutch/Brake Operation of the Conventional 
Control Means 
SHIFT POSITION 12 13 19 24 25 26 27 28 
______________________________________ 
Forward 
D 1st Speed O * X O X X X * 
Range 2nd Speed O * X O X O X X 
3rd Speed O * X O O X X X 
4th Speed X X O O O X X X 
(O/D) 
2 1st Speed O * X O X X X * 
Range 2nd Speed O * X O X O X X 
L 1st Speed O * X O X X O * 
Range 
REVERSE (R RANGE) 
O * X X O X O X 
______________________________________ 
Wherein O shows that the clutch or brake is engaged; * shows that the 
clutch or brake is engaged in the engine drive condition while it is 
disengaged in the engine brake condition; and X shows that the clutch or 
brake is disengaged. O/D means the overdrive speed stage. In D range, the 
transmission may operate in the 1st, 2nd, 3rd and 4th (O/D) speed stages. 
In 2 range, the transmission may operate in the 1st and 2nd speed stages. 
In L range, the transmission is fixed in the 1st speed stage. 
TABLE 2 
______________________________________ 
Shift Position 
Port P R N D 2 L 
______________________________________ 
43b 0 0 0 
43c 0 0 
43d 0 0 0 
43e 0 
______________________________________ 
The valves designated by 56 and 57 are a 1-2 speed shift valve and a 2-3 
speed shift valve, respectively. 58 designates an overdrive control valve. 
The 1-2 speed shift valve 56 comprises two valve elements 60 and 61 
axially opposed to each other with interposition of a compression coil 
spring 59. The valve element 60 is shifted between the lower shift 
position shown by 56A and the upper shift position shown by 56B due to the 
balance of the sum of the force applied by the spring 59 and the downward 
force (as seen in the figure) applied by the throttle pressure Pth 
supplied to a port 62 through a passage 50a and the upward force (as seen 
in the figure) applied by the governor pressure Pgo supplied to a port 63 
through a passage 55a. Ports 64 and 65 of the 1-2 speed shift valve 56 are 
supplied with the line pressure which appears at the output 43d of the 
manual shift valve 44 when it is shifted to L range through a passage 49 
and a low modulator valve 66 under the regulation applied thereby so that, 
when the pressure is supplied to these ports, the valve element 61 is 
shifted downward in the figure and compulsorily maintains the valve 
element 60 at its lower shift position 56A. 
Similarly, the 2-3 speed shift valve 57 comprises two valve elements 68 and 
69 axially opposed to each other with interposition of a compression coil 
spring 67. The valve element 68 is also shifted between the lower shift 
position designated by 57A and the upper shift position designated by 57B 
due to the balance of the sum of the downward force applied by the spring 
67 and the downward force applied by the throttle pressure Pth supplied to 
a port 70 through a passage 50b and the upward force applied by the 
governor pressure Pgo supplied to a port 71 through a passage 55b. A port 
72 is supplied with the line pressure which appears at the output port 43c 
of the manual shift valve 44 when it is shifted to 2 range through 
passages 48 and 48a so that when the pressure is supplied to the port 72, 
the valve element is shifted downward and compulsorily maintains the valve 
element 68 at the lower shift position shown by 57A. 
The overdrive control valve 58 comprises a valve element 74 urged downward 
in the figure by a compression coil spring 73. The valve element 74 is 
exposed to a governor pressure Pgo at its lower end, said governor 
pressure being supplied at a port 75 through a passage 55c. In addition to 
the downward force applied by the spring 73, the valve element 74 is 
selectively supplied by either the throttle pressure Pth or the line 
pressure which appears in the output port 43c of the manual shift valve 44 
when it is shifted to 2 range, said throttle pressure being supplied 
through a passage 50c, a shuttle valve 76 and a port 77 while the line 
pressure is supplied through a passage 48b, the shuttle valve 76 and a 
port 77. According to the balance of the sum of the spring force and the 
force applied by the throttle pressure or the line pressure and the upward 
force applied by the governor pressure, the valve element 74 is shifted 
between the lower shift position designated by 58A and the upper shift 
position designated by 58B. 
The port 78 of the 1-2 speed shift valve 56 is supplied with the line 
pressure through a passage 47b when the manual shift valve is shifted to D 
range. If the 1-2 speed shift valve is in its upward shift position shown 
by 56B, the line pressure supplied to the port 78 is transmitted to the 
port 79, wherefrom the pressure is conducted through a passage 47c to a 
port 80 of the 2-3 speed shift valve 57. When the 2-3 speed shift valve is 
in its downward shift position as shown by 57A, the line pressure is 
transmitted from the port 80 to a port 81, wherefrom the pressure is 
conducted through a passage 47d to the brake 26 (second brake). When the 
2-3 speed shift valve is in the upward speed shift position, the oil 
pressure supplied to the port 80 is transmitted to a port 82, wherefrom 
the pressure is conducted through a passage 47e and a shuttle valve 47f to 
the clutch 25 (direct clutch). When the manual shift valve 44 is shifted 
to L range, the oil pressure appearing at the output port 43d is conducted 
through the low modulator valve 66, the ports 64 and 83 of the 1-2 speed 
shift valve 56 and a passage 49a to the inside of the brake 27 (first 
brake). The outside of the brake 27 is supplied with the oil pressure 
which appears in the output port 43e of the manual shift valve 44 when it 
is shifted to R position. 
The port 84 of the overdrive control valve 58 is supplied with the line 
pressure through the passages 43 and 43f. When the overdrive control valve 
is in the lower shift position such as shown by 58A, the line pressure 
supplied to the port 84 is transmitted to a port 85, wherefrom the 
pressure is conducted through a passage 43g to the clutch 12 of the 
overdrive means. On the contrary, if the overdrive control valve is in the 
upper shift position such as shown by 58B, the pressure is transmitted 
from the port 84 to a port 86, wherefrom the pressure is conducted through 
a passage 43h to the brake 19 of the overdrive means. 
The operation of an oil pressure control circuit such as shown in FIG. 2 is 
conventionally well known in the art. However, for the sake of 
convenience, the operation will be summarized in the following: 
D range 
When the manual shift valve 44 is shifted to D range, the line pressure 
appears in the passage 47 and the pressure is supplied directly to the 
clutch 24. Under this condition, if the vehicle is standing or is running 
at a low speed, the governor pressure Pgo generated by the governor valve 
54 is so low that the -2 speed shift valve 56, the 2-3 speed shift valve 
57 and the overdrive control valve 58 are maintained in the lower shift 
positions such as designated by 56A, 57A and 58A, respectively, whereby 
the oil pressure supplied through the passage 47b is intercepted at the 
port 78 and is not transmitted to the subsequent passages including the 
direct clutch 25 and the second brake 26. The oil pressure conducted 
through the passage 43f to the overdrive control valve 58 is supplied to 
the clutch 12 of the overdrive means. In this condition, therefore, the 
overdrive mechanism is locked, and the reduction gear means is in the 1st 
speed stage. 
Starting from this condition, when the vehicle speed gradually increases, 
the governor pressure Pgo gradually increases and, at a certain vehicle 
speed, the 1-2 speed shift valve 56 is shifted to the 56B position, 
whereby the line pressure is supplied to the port 79, wherefrom the 
pressure is conducted through the ports 80 and 81 of the 2-3 speed shift 
valve 57 and the passage 47d to the second brake 26 which is then engaged. 
In this condition the transmission is shifted to the 2nd speed stage. 
When the vehicle speed further increases, the 2-3 speed shift valve 57 is 
shifted to the 57B position. The oil pressure supplied to the port 80 is 
then transmitted to the port 82, wherefrom the pressure is conducted 
through the passage 47e and the shuttle valve 47f to the direct clutch 25 
which is then engaged. On the other hand, the oil pressure which has been 
supplied to the brake 26 is now exhausted through the passage 47d, the 
port 81 and a drain 81a. In this condition, the transmission is shifted to 
the 3rd speed stage, i.e. direct connection. 
When the vehicle speed further increases, the overdrive control valve 58 is 
also shifted to the 58B position, whereby the oil pressure supplied to the 
port 84 is now switched from the port 85 to the port 86, wherefrom the 
pressure is conducted through the passage 43h to the brake 19, while the 
pressure which has been supplied to the clutch 12 is now exhausted through 
the passage 43g, the port 85 and a drain port 85a. In this condition, the 
overdrive means 2 is put in operation and the transmission operates in the 
overdrive stage. 
In the above explanation, for the sake of convenience the changing over of 
the speed stages has been explained in relation to the increase of the 
vehicle speed. However, in actual operation, the changing over of the 
speed stages is effected depending upon the balance of the governor 
pressure and the throttle pressure oppositely applied to the valve element 
of the individual speed shift valves and, therefore, the shift point 
varies not only in accordance with the vehicle speed, but also in 
accordance with the amount of depression of the accelerator pedal. When 
the vehicle speed gradually lowers, the overdrive control valve 58, the 
2-3 speed shift valve 57, and the 1-2 speed shift valve 56 are 
successively shifted from 58B to 58A, 57B to 57A, and 56B to 56A, 
respectively, in the reverse manner, thereby successively establishing 
lower speed stages. 
2 Range 
When the manual shift valve 44 is shifted to 2 range, the line pressure 
appears at the output port 43b as well as at the output port 43c, 
wherefrom the pressure is conducted through the passages 48a and 48b to 
the port 72 of the 2-3 speed shift valve 57 and the port 77 of the 
overdrive control valve, respectively, driving the valve elements 69 and 
74 downward to maintain the 2-3 speed shift valve and the overdrive 
control valve at shift positions 57A and 58A. In this condition, 
therefore, the overdrive mechanism is maintained in the locked condition 
and the reduction gear mechanism operates either in the 2nd speed stage or 
in the 1st speed stage. 
L Range 
When the manual shift valve 44 is shifted to L range, the line pressure 
also appears at the output port of 43d, wherefrom the pressure is 
conducted through the low modulator valve 66 to the ports 64 and 65 of the 
1-2 speed shift valve 56, driving the valve element 61 downward in the 
figure to maintain the 1-2 speed shift valve in the lower shift position 
56A. In this condition, the automatic transmission is maintained in the 
1st speed stage. 
As obvious from the foregoing explanations, if the manual shift valve 44 is 
abruptly shifted from D range to 2 range when the vehicle is running under 
the overdrive condition, the overdrive control valve 58 is forcibly 
shifted from position 58B to position 58A by the line pressure delivered 
from the port 43c of the manual shift valve and, simultaneously, the 2-3 
speed shift valve 57 is also forcibly shifted from position 57B to 
position 57A by the same line pressure. In this case, therefore, the 
automatic transmission is abruptly shifted down by two speed stages from 
the overdrive stage to the 2nd speed stage, skipping over the 3rd speed 
stage, thereby causing various problems such as explained above. 
FIG. 3 is a view similar to FIG. 2 and showing an oil pressure control 
means improved in accordance with the present invention. In FIG. 3, the 
portions corresponding to those shown in FIG. 2 are designated by the same 
reference numerals. In the oil pressure control system shown in FIG. 3, a 
downwshift control valve 87 is provided at a middle portion of the oil 
passage 48 which transmits the line pressure from the second speed port 
43c of the manual shift valve 44 to the port 72 of the 2-3 speed shift 
valve 57. The downshift control valve 87 is adapted to be controlled by 
the governor pressure so as to intercept the passage 48 when the governor 
pressure is higher than a predetermined value. The downshift control valve 
87 comprises a valve element 89 resiliently driven rightward in the figure 
by a compression coil spring 88, said valve element being adapted to be 
shifted leftward in the figure when the governor pressure supplied to a 
port 90 is higher than said predetermined value. By the provision of the 
downshift control valve, when the vehicle is running in the overdrive 
stage, the valve element 89 is in the leftward shift position, due to a 
relatively high governor pressure supplied to the port 90, thereby 
intercepting the passage 48. Therefore, even if the manual shift valve 44 
is abruptly shifted from D range to 2 range, the line pressure delivered 
from the port 43c of the manual shift valve is supplied only to the 
overdrive control valve 58 so as to shift it from shift position 58B to 
shift position 58A while the line pressure is not supplied to the 2-3 
speed shift valve 57. Therefore, the automatic transmission is shifted 
down for a time to the 3rd speed stage. By this downshifting, the vehicle 
is effected with engine braking and gradually reduces its speed. When the 
vehicle speed has lowered to a predetermined value, the force of the 
compression coil spring 88 in the downshift control valve overcomes the 
force of the governor pressure exerted on the right end of the valve 
element 89 so that the valve element 89 is shifted rightward in the figure 
thereby opening the passage 48. Thus, the 2-3 speed shift valve 57 is now 
shifted down from position 57B to position 57A thereby effecting 
downshifting from the 3rd speed stage to the 2nd speed stage. 
FIG. 4 is a view similar to FIG. 2 or 3 showing still another embodiment of 
the present invention. Also in FIG. 4 the portions corresponding to those 
shown in FIG. 2 or 3 are designated by the same reference numerals. In 
this embodiment, the downshift control valve 87 adapted to be controlled 
by the governor pressure in the first embodiment is replaced by a 
downshift control valve 91 of a timer type. The downshift control valve 91 
comprises a valve element 93 urged leftward in the figure by a compression 
coil spring 92. When the line pressure is not delivered to the port 43c of 
the manual shift valve, the valve element 93 is in the leftward shift 
position wherein it intercepts the passage 48. When the line pressure 
delivered from the port 43c is applied to the left end of the valve 
element 93, it is driven rightward and after the lapse of a predetermined 
time it opens a passage connected to the passage 48a. Therefore, it will 
be appreciated that by the provision of the downshift control valve 91 of 
a timer type, it is effected stepwise to downshift the automatic 
transmission first from the overdrive stage to the 3rd speed stage and 
second from the 3rd speed stage to the 2nd speed stage, even when the 
manual shift valve 44 has been abruptly shifted from D range to 2 range. 
From the foregoing, it will be appreciated that the present invention 
provides an improved oil pressure control means for automatic 
transmissions which prevents abrupt downshifting from the overdrive stage 
to the 2nd speed stage by an incorporation of a very simple downshift 
control valve in the conventional oil pressure control means, thereby 
providing great advantages such as ensuring longer life of the friction 
engaging means, precluding over-revving of the engine and ensuring safety 
in driving free from any abrupt deceleration caused by downshifting of the 
manual shift valve during high speed running of the vehicle. 
Although the invention has been shown and described with respect to some 
preferred embodiments thereof, it should be understood by those skilled in 
the art that various changes and omissions of the form and detail thereof 
may be made therein without departing from the scope of the invention.