Shift control system for an automatic transmission

A shift control system for the vehicle contains a torqueconverter, a multiple stage gear mechanism adapted for a shift of an operating state of a friction coupling means of a hydraulic pressure type which is coupled to the torque converter, a temperature sensor for sensing temperatures of the operating oil for the torque converter and the friction coupling element, and a control unit for shift control. The control unit controls operation of the friction coupling elements on the basis of a predetermined shift characteristic and shifts the shift characteristic, when the temperature of the operating oil to be sensed by the temperature sensor is lower than a given value, is in such a manner that the area of driving at least for first speed stage is narrowed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a shift control system for an automatic 
transmission. 
2. Description of Related Art 
Many kinds of automatic transmissions comprise a torque converter and a 
multiple stage shift gear mechanism coupled to the torque converter. The 
multiple stage gear mechanism performs a shift by shifting operational 
states of friction coupling elements such as clutches and brakes mounted 
thereto. Shifting the operational states of the friction coupling elements 
is performed on the basis of a predetermined shift characteristic which is 
set generally using a vehicle speed and an engine load as parameters. 
When the engine is cold, a driving state of the engine may not be rendered 
stable, so that Japanese Patent Examined Publication No. 32,941/1980 
proposes that the shift characteristic be transferred to the high speed 
side during a so-called cold time period when the temperature of an 
operating oil is below a given value. Hence, it is proposed that a driving 
range be widened, in which the driving is performed at a low speed stage 
over the cold time period. 
It is significant from the viewpoint of a prevention of shift shock, too, 
how to set the line pressure of the operating oil for the friction 
coupling elements. Therefore, generally speaking, for instance, the line 
pressure is subjected to duty control, while the duty ratio for the duty 
control is corrected in accordance with the oil pressure. 
During the cold time when the oil is below the freezing point, the 
viscosity of the operating oil may vary to a greater extent, as compared 
with an ambient atmosphere where the operating is not cold, so that it may 
be impossible to control the line pressure to a desired magnitude with the 
aid of the duty control even if the duty ratio would be corrected. When 
the line pressure is controlled by the process other than the line 
pressure control, an increase in the viscosity of the operating oil may 
change a timing for shifting, thereby worsening the shock of a shift. 
Therefore, it is important to raise the temperature of the operating oil 
and depart from a cold state as soon as possible in order to prevent the 
shock of the shift. 
SUMMARY OF THE INVENTION 
Therefore, the present invention has the object to provide a shift control 
system of an automatic transmission adapted to raise the temperature of 
the operating oil for the automatic transmission within the shortest 
possible time period. 
In order to achieve the object, the shift control system according to the 
present invention has the construction, as schematically shown in FIG. 10, 
which comprises: 
a torque converter; 
a multiple stage gear mechanism for performing a shift by shifting an 
operational state of a friction coupling element of a hydraulic pressure 
type, which is coupled to the torque converter; 
a shift control means for controlling operation of the friction coupling 
element on the basis of a predetermined shift characteristic; 
a temperature detecting means for detecting a temperature of an operating 
oil for the torque converter and the friction coupling element; and 
a shift characteristic shifting means for shifting the shift characteristic 
so as to make a driving range at least at first speed stage narrow during 
cold time when the temperature of the operating oil detected by the 
temperature detecting means is equal to or lower than a predetermined 
value. 
With the above arrangement, the driving range at least at a speed as low as 
first speed stage is narrowed over the cold time period when the oil 
temperature of the operating oil is below a given value, the vehicle is 
driven at a speed stage higher than in the driving state in which the 
vehicle is otherwise driven originally at first speed stage. This means 
that a slip of the torque converter becomes large and workload to be 
applied to the operating oil increases, thereby smoothly raising the oil 
temperature of the operating oil. 
It is preferred that the operation of the friction coupling elements is 
ensured by raising the line pressure during the cold time when the 
operting oil is cold. In this case, it is disadvantageous from the 
viewpoint of a prevention of the shock of a shift merely in terms of 
enlarging the line pressure. The present invention, however, can reduce 
the opportunity to the least possible level, in which the shock of a shift 
matters even if the line pressure would be raised during the cold time 
period, because the range of driving at a low speed stage, particularly at 
first speed stage, is narrowed, at which the shock of the shift may 
particularly cause problems. 
In order to narrow the range of driving at first speed stage, it is 
possible that, extremely speaking, driving may not be performed at all at 
first speed stage. 
Other objects, features and advantages of the present invention will become 
apparent in the course of the description of the preferred embodiments 
which follows in conjunction with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1, is a diagrammatical illustration of an outline of a shift control 
system according to an embodiment of the present invention. 
FIG. 2 is a schematic diagram of a hydraulic pressure circuit of the 
automatic transmission. 
FIG. 3 is a diagram showing the relation of the pilot pressure for 
adjusting the line pressure versus the duty ratio. 
FIG. 4 is a diagram showing the characteristic for setting the line 
pressure during normal time period when the oil temperature is not cold. 
FIG. 5 is a diagram showing the shift characteristic for the normal time 
period when the oil temperature is not cold. 
FIGS. 6, 8 and 9 are diagrams showing the shift characteristics for cold 
time period when the oil temperature is cold. 
FIG. 7 is a flow chart showing a control example according to the present 
invention. 
FIG. 10 is a block diagram showing an outline of the construction of the 
shift control system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described more in detail by way of examples 
in conjunction with the accompanying drawings. 
As shown in FIG. 1, output of an engine 1 is transmitted through an 
automatic transmission 2 to driven wheels (not shown). The automatic 
transmission 2 comprises a torque converter 3 having a lockup clutch 3A 
and a multiple stage gear mechanism 4 coupled to a turbine of the torque 
converter 3. 
The multiple stage gear mechanism 4 comprises a planetary geartrain as is 
known to the art to which the present invention pertains. In this 
embodiment, the geartrain has four forward speed ranges and one rearward 
speed range. The multiple stage gear mechanism 4 is designed to shift a 
pathway of power transmission, i.e., implements a shift, by changing a 
combination of coupling or uncoupling a plurality of friction coupling 
elements of a hydraulic type to or from each other. The shift is 
implemented by changing a combination of energizing a plurality of 
solenoids 6 incorporated into a liquid pressure circuit with deenergizing 
them. Likewise, the lockup clutch 3A is coupled or uncoupled by shifting 
between energization of the solenoid 5 incorporated into the hydraulic 
pressure circuit and deenergization thereof. By controlling a duty 
solenoid 7 incorporated in the hydraulic pressure circuit, the magnitude 
of a hydraulic pressure to be fed to the friction coupling element and the 
lockup clutch 3A, i.e., a line pressure, is changed. 
As shown in FIG. 2, reference numeral 31 denotes a manual valve to operated 
manually, and at least speed ranges D, N and R can be provided by 
displacing a spool 31a of the manual valve 31. The manual valve 31 can be 
communicated with ports a and b for supplying the line pressure, with 
ports c, d, and e as well as with drain ports f and g. 
To the ports a and b are connected a passage 32 extending from a pump P as 
a source of hydraulic pressures, and the pressure in the passage 32 is 
governed so as to reach a predetermined line pressure by means of a 
pressure governing valve 33 of a hydraulically operable type. In other 
words, a drain amount of the passage 32 is adjusted by a position in which 
a spool 33a of the pressure governing valve 33 is displaced. The pressure 
governing valve 33 accepts a pilot pressure from a pilot passage 34 
branched from the passage 32. The magnitude of the pilot pressure adjusts 
a drain amount of the pilot passage 34 with the aid of the solenoid 7. 
More specifically, the pilot pressure is altered in such a manner as shown 
in FIG. 3 by changing a duty ratio with respect to the solenoid 7. The 
line pressure from the passage 32 is normally fed to the torque converter 
3 through a passage 44. 
The line pressure from the port c is coupled with a friction coupling 
element for rearward driving through a passage 41 which is connected on 
its halfway to an accumulator 43. A drain passage 41a branched from the 
passage 41 is communicated with the port e. This allows the port a to be 
communicated with the port c when the spool 31a of the manual valve 31 is 
displaced to the rearward speed range R, thereby feeding the line pressure 
from the port a through the passage 41 to the friction coupling element 42 
and providing a rearward speed range. If the spool 31a of the manual valve 
31 is displaced to provide a speed range other than the rearward speed 
range, the port e is communicated with the port g, thereby draining the 
pressure within the friction coupling element 42 and uncoupling the 
friction coupling element 42. 
When the manual valve 31 is displaced to the speed range D, the port b is 
communicated with the port d. The line pressure from the port d is 
normally fed to a friction coupling element (forward clutch) for selecting 
forward driving, although not shown in the drawing, while it is 
appropriately fed to a friction coupling element 46 for shifting through a 
shift valve 47. As shown in FIG. 2, the shift valve 47 serves as a 
so-called 1-2 shift valve for shifting between first and second speed 
stages, and a friction coupling element 46 serves as shifting between the 
first and second speed stages by coupling or uncoupling the shift valve 
47. In other words, the line pressure is fed to the friction coupling 
element 46 by shifting the shift valve 47, thereby coupling the friction 
coupling element 46, while the pressure within the friction coupling 
element 46 is drained to thereby uncouple the friction coupling element 
46. Such shift valves and friction coupling elements are provided in other 
positions, thereby enabling a shift of four forward speed ranges as a 
whole. This point is known to the art so that it is omitted from the 
drawing. It is a matter of course that the shift valve 47 is designed to 
shift its shift positions, for example, by switching a mode of supplying 
the pilot pressure by the solenoid 6. 
As shown in FIGS. 1 and 2, a control unit U comprises a microcomputer in 
which central processing unit, read-only memory (ROM), random access 
memory (RAM), CLOCK and so on are provided. To the control unit U are 
provided a vehicle speed signal from a sensor 21, a throttle opening angle 
signal from a sensor 22, and an oil temperature signal, i.e., an oil for 
the automatic transmission, from a sensor 23. On top of that, sensors 24 
and 25 generate signals to the control unit U and they sense driving load. 
The sensor 24 senses the gradient of a road surface and the sensor 25 
senses a load. 
The control unit U is designed to control the line pressure as well as to 
implement shift control. The shift control is performed by generating an 
upshifting signal or a downshifting signal to the solenoid 6 on the basis 
of a predetermined shift characteristic. 
The shift characteristic comprises two kinds, one being for ordinary time 
period when the operating oil has a temperature above a given value, as 
shown in FIG. 5 and the other being for the cold time period when the oil 
temperature of the operating oil is below the given value, as shown in 
FIG. 6. Each of the shift characteristics is set using a vehicle speed and 
a throttle opening angle (the same as an accelerator opening angle) as 
parameters. The shift characteristic for the ordinary time period as shown 
in FIG. 5 sets three kinds of shift lines for shifting between first and 
second speed stages, second and third speed stages, and third and fourth 
speed stages. The shift characteristic for the cold time period as shown 
in FIG. 6 has a narrow driving range for the first speed stage as compared 
with the shift characteristic for the ordinary time period, as shown in 
FIG. 5. More specifically, for the shift characteristic for the cold time, 
the shift line for shifting between first and second speed stages is set 
such that upshifting and downshifting lines are both on the low speed side 
and it takes the first speed stage only when the throttle opening angle 
becomes considerably large. It is to be noted herein that each of the 
shift lines for shifting between second and third speed stages and between 
third and fourth speed stages may be set to be substantially the same in 
FIGS. 5 and 6, although they are the same in the embodiment. 
The shift characteristic may appropriately be selected from the two kinds 
of the shift characteristics on the basis of the oil temperature of the 
operating oil. During the cold time period when the oil temperature is 
below --10.degree. C., the shift characteristic as shown in FIG. 6 is 
selected, while the shift characteristic for the ordinary time period as 
shown in FIG. 5 is selected when the oil temperature exceeds -10.degree. 
C.. 
Lockup characteristic for controlling the lockup clutch 3A may be set in a 
various manner. In this embodiment, the lockup clutch 3A is designed so as 
to be coupled only when third and fourth speed stages are selected. 
The line pressure is adjusted by means of the control unit U in accordance 
with the oil temperature. The solid line in FIG. 4 indicates a basic duty 
ratio with respect to the solenoid 7 for adjusting the line pressure. In 
this embodiment, the pilot pressure for determining the line pressure is 
set by using the throttle opening angle as a parameter. The characteristic 
indicated by the solid line in FIG. 4 is for the ordinary time period and 
is selected during the non-cold time when the oil temperature exceeds 
-10.degree. C. At this time, it is possible to correct the basic duty 
ratio in accordance with the oil temperature of the operating oil. When 
the oil temperature becomes below -10.degree. C., on the other hand, the 
duty ratio with respect to the solenoid 7 is uniformly set to 0% without 
following the characteristic indicated by the solid line in FIG. 4, 
thereby allowing the line pressure to become the largest as indicated by 
the broken line in FIG. 4, during the cold time. 
The shift characteristics and the basic duty ratio as shown in FIG. 4 are 
stored in the ROM in the control unit U. 
Description will now be made of the flow chart as shown in FIG. 7, which 
represents the control content by the control unit U. 
At step P1, the vehicle speed, the throttle opening angle, the oil 
temperature and the load are read. Then at step P2, it is judged whether 
the current oil temperature is below -10.degree. C. 
When it is decided at step P2 that the oil temperature does not exceed 
-10.degree. C., on the one hand, the duty ratio D is decided at step P3 
with respect to the solenoid 7 for adjusting the line pressure on the 
basis of the basic characteristic indicated by the solid line in FIG. 4. 
Then at step P4, the shift characteristic for the ordinary time period as 
shown in FIG. 5 is selected. Thereafter, the duty ratio D is generated 
into the solenoid 7 and, at step P6, the shift control is implemented on 
the basis of the shift characteristics selected, i.e., upshifting signals 
and downshifting signals are generated. 
When it is decided at step P2 that the oil temperature exceeds -10.degree. 
C., on the other hand, the duty ratio with respect to the solenoid 7 is 
set to 0% at step P7. Thereafter, at step P8, it is judged whether the 
driving load of the vehicle is large on the basis of output from the 
sensors 24 and 25--more specifically, ether of whether the vehicle is 
running on an up-hill road in which the gradient of the road surface is 
larger than a given value or whether the load is larger than a given value 
is satisfied. If NO at step P8, on the one hand, the shift characteristic 
for the cold time period as shown in FIG. 6 is selected at step P9 and 
then the processing at steP P5 and thereafter is performed. 
When YES at step P8, on the other hand, the flow advances to step P4 and 
the shift characteristic for ordinary time period as shown in FIG. 5 is 
selected. The reason for judging at step P8 as described hereinabove is 
based on the fact that in many occasions the driving at the first speed 
stage is particularly required when the running load is large. 
FIGS. 8 and 9 show other shift characteristics to be used for the cold time 
period when the oil temperature of the operating oil is higher than a 
predetermined value. FIG. 8 shows the shift characteristic in which there 
is provided no shift line for shifting between first and second speed 
stages, namely, in which the range of driving at first speed stage is not 
set at all. For the shift characteristic as shown in FIG. 8, the shift 
line for the speed stage higher than the second speed stage (shift line 
for shifting between second and third speed stages or for shifting between 
third and fourth speed stages) is set in the same manner as that for the 
shift characteristic for the ordinary time period as shown in FIG. 5. For 
the shift characteristic as shown in FIG. 9, the range of driving at first 
speed stage is set only at lower speed and at higher running load, as 
compared to that shown in FIG. 5. The shift characteristic as shown in 
FIG. 9 is such that the shift line for the speed stage on the high speed 
side higher than the second speed range is set on the lower speed side, as 
compared with the shift characteristic for ordinary time period as shown 
in FIG. 5. 
It is to be understood that the foregoing text and drawings relate to 
embodiments of the present invention given by way of examples but not 
limitation. Various other embodiments and variants are possible within the 
spirit of the present invention.