JPH07229556A - Overheat countermeasure device for automatic transmission - Google Patents

Abstract

PURPOSE:To prevent worsening of operability due to performing unnecessary countermeasure control also in the other operating condition than control limited to an operating condition where overheat is truely apprehensive in an overheat countermeasure for an automatic transmission. CONSTITUTION:A controller 9 adopts throttle opening TH and a vehicle speed V as a control factor to perform the speed change control of an automatic transmission 2 based on a speed change pattern, and when hydraulic fluid temperature C becomes a value showing overheat, the speed change line of the speed change pattern is slipped to a high vehicle speed side to make a low speed stage selectable to perform overheat countermeasure control. However, this control is performed only at the time of the less variation of the throttle opening TH, that is, only the time in an operating condition where true overheat is apprehensive like in a condition of traveling on a long ascent having a sharp grade. Consequently, worsening operability, due to unnecessary overheat contermeasure control in the other operating condition, can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic transmission in which when the working fluid temperature of the automatic transmission becomes high, the automatic transmission is placed in a control state in which the calorific value is reduced so as to prevent this overheating. The present invention relates to an overheat countermeasure device.

[0002]

2. Description of the Related Art Conventionally, as a technique for preventing overheating of an automatic transmission, as disclosed in, for example, Japanese Patent Publication No. 5-51489, when the operating oil temperature of the automatic transmission becomes high, the speed of the automatic transmission is changed. It is known that the control pattern is switched to a shift pattern in which the shift point is shifted to the high vehicle speed side, and shift control of the automatic transmission is performed based on the shift pattern in the oil temperature increase prevention mode. According to this overheat countermeasure control,
It becomes easy for the automatic transmission to select the low speed stage, and the transmission state becomes reasonable. As a result, the heat generation amount of the automatic transmission is reduced, and the overheated state can be eliminated.

As another overheat countermeasure device for an automatic transmission, as disclosed in, for example, Japanese Patent Application Laid-Open No. 4-151060, a vehicle speed range equal to or higher than a lockup vehicle speed in which a torque increasing function and a torque fluctuation absorbing function of a torque converter are unnecessary. Assuming that the torque converter is in the lockup state in which the input and output elements are directly connected to each other from the converter state, when the operating fluid temperature of the automatic transmission becomes too high, There is known an overheat countermeasure control in which the heat generation due to the stirring of the hydraulic oil of the torque converter is suppressed by lowering the torque to facilitate the lockup of the torque converter.

[0004]

However, in all the conventional overheat countermeasure devices, even if this countermeasure is to shift the shift line to the high vehicle speed side like the former countermeasure, lock-up is performed like the latter countermeasure. Even if the vehicle speed is shifted to the higher vehicle speed side, these are executed based only on the angle of the rabbit and the hydraulic oil temperature of the automatic transmission, so it was confirmed that the following problems occur.

That is, except when the vehicle is driven on an uphill road having a long steep slope and is operated under a high load for a long time with the accelerator pedal being kept substantially constant, a normal level is maintained. Including long-distance driving on the uphill road, the amount of accelerator pedal operation changes frequently, so
Moreover, since the uphill road does not continue for a long time, the slip amount of the torque converter does not remain large for a long time. Therefore, even if the automatic transmission may become overheated once, it is temporary and the overheated state of the automatic transmission will be resolved soon without the above countermeasure control. Nevertheless, in the conventional device that executes the overheat countermeasure control as soon as it reaches a high temperature that indicates overheat according to only the hydraulic oil temperature of the automatic transmission, unnecessary overheat countermeasure control is frequently performed in the above normal running. Therefore, it is unavoidable that a shift shock associated with a downshift gearshift or a lockup shock associated with lockup of a torque converter will occur each time, and the drivability of the vehicle will be significantly deteriorated.

In the present invention, it is really necessary to perform the overheat countermeasure control as described above for the automatic transmission, when the accelerator pedal is stepped on while keeping the accelerator pedal substantially constant as in the case of running the vehicle on a steep uphill road. Based on the fact that it is the case where the high-load operation is continued for a long time in this state, and the slip amount of the torque converter is kept large for a long time, the information on the accelerator pedal operation is controlled to prevent overheating. It is an object of the present invention to propose an overheat countermeasure device for an automatic transmission, which can solve the above-mentioned problem relating to deterioration of drivability by adding to the control factor.

[0007]

To this end, the overheat countermeasure device for an automatic transmission according to the first aspect of the present invention, as shown in the concept of FIG. 1, uses an overheat detecting means to automatically change the working fluid temperature of the automatic transmission. In the automatic transmission overheat countermeasure device that puts the automatic transmission in a control state in which the amount of heat generated decreases while it is detected that the temperature is equal to or higher than the set temperature that indicates overheating of the machine, A stable load condition detecting means for detecting a stable load condition of the prime mover in which the degree of change in the load condition is smaller than the reference is provided, and responds to the signal from the overheat detecting means only while the stable load condition of the prime mover is detected by the means. The above-mentioned overheat countermeasure device is configured to perform overheat countermeasure control of the automatic transmission.

A second aspect of the invention is characterized in that the automatic transmission countermeasure against overheating shifts the downshift transmission line to the high vehicle speed side so that the automatic transmission can be easily put into the low speed stage. To do.

A third aspect of the invention is the addition of the second aspect of the invention to the addition of hunting detection means for detecting the occurrence of hunting in the shift control of the automatic transmission, and when hunting in the shift control is detected by the means. The upshift transmission line is also configured to be shifted to the high vehicle speed side to prevent hunting in the shift control.

According to a fourth aspect of the present invention, the overheat countermeasure control of the automatic transmission in the first aspect shifts the lockup line of the torque converter to the low vehicle speed side so that the torque converter is easily locked up. It is characterized by.

In a fifth aspect based on any one of the first to fourth aspects, the stable load state detecting means detects the load of the prime mover and the weighted average value of the load is smaller than a set value. It is characterized in that time is regarded as a stable load state of the prime mover.

A sixth aspect of the invention is the shift determination according to any one of the first to fifth aspects of the invention, for determining whether or not the overheat countermeasure control of the automatic transmission causes a shift of the automatic transmission. Means and an overheat countermeasure control prohibiting means for issuing a command for prohibiting the overheat countermeasure control of the automatic transmission to the overheat countermeasure control device when the gear shift is determined to occur by the means. .

[0013]

In the first aspect of the invention, basically, when the temperature of the working fluid of the automatic transmission is equal to or higher than the set temperature indicating the overheating of the automatic transmission, the overheating is detected in response to the signal from the overheat detecting means for detecting the overheating. The countermeasure device puts the automatic transmission in a controlled state in which the amount of heat generated is reduced. Then, the stable load state detection means
Only when the stable load state of the prime mover is detected from the degree of change in the load state of the prime mover, the overheat countermeasure device executes the overheat countermeasure control of the automatic transmission, and does not perform this control except in the stable load state of the prime mover. .

Therefore, as in the case where the vehicle is traveling on a steep uphill road, the accelerator pedal is kept substantially constant for a long time under high load operation, and the slip amount of the torque converter is long. The above-mentioned overheat countermeasure control of the automatic transmission is executed only when it is kept increased over time, that is, only in a traveling state where the overheating of the automatic transmission is really concerned. The overheat prevention function of can be used effectively.

Under normal running conditions other than when the prime mover is in a stable load condition, the above-mentioned overheat countermeasure control of the automatic transmission is not performed even if the working fluid temperature may temporarily become high. Therefore, it is possible to solve the above-mentioned problem of the conventional device that a shock is generated due to unnecessary control against overheat and the drivability is deteriorated.

As the overheat countermeasure control for the automatic transmission, as in the second aspect of the invention, the control for shifting the downshift transmission line to the high vehicle speed side so that the automatic transmission can be easily put into the low speed stage is performed. It is useful.

In this case, the downshift transmission line approaches the upshift transmission line and the hysteresis between the two becomes smaller, so that hunting is likely to occur in the shift control of the automatic transmission. When the hunting detection means detects the occurrence of hunting in the shift control of the machine, the upshift transmission line is also configured to be shifted to the high vehicle speed side, so that the hunting of the shift control does not occur.

As the overheat countermeasure control for the automatic transmission, instead of the second invention, as in the fourth invention, the lockup line of the torque converter is shifted to the low vehicle speed side so that the torque converter is easily locked up. It is also useful.

Further, as in the fifth aspect of the invention, the stable load state detecting means detects the load of the prime mover, and when the weighted average value of the load is smaller than a set value, it is the stable load state of the prime mover. It is useful to have

In the sixth aspect of the invention, the following effects can be obtained by the newly added gear shift determination means and overheat countermeasure control prohibition means. In other words, the shift determination means determines whether or not the automatic transmission countermeasure control for the automatic transmission will cause a shift of the automatic transmission, and when the means determines that a shift will occur, the overheat countermeasure control prohibition means A command for prohibiting the overheat countermeasure control of the automatic transmission is issued to the overheat countermeasure device. Therefore, when the gear shift occurs due to the overheat countermeasure control, the control is not performed, and it is possible to eliminate the uncomfortable feeling that the shock caused by the overheat countermeasure control and the shock caused by the gear shift come close to each other.

[0021]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 2 shows an overheat countermeasure device for an automatic transmission according to an embodiment of the present invention, in which 1 is an engine (motor) and 2 is an automatic transmission. It is assumed that the automatic transmission 2 receives the power of the engine 1 via the torque converter 3, shifts the input rotation at a gear ratio according to the selected shift speed, and transmits it to the output shaft 4.

In the automatic transmission 2, the shift solenoids 6 and 7 in the control valve 5 are turned on and off.
The selected shift stage is determined by the combination of the above, and the torque converter 3 is in the converter state in which the input / output elements are not directly connected or the lockup state in which the input / output elements are directly connected by the duty control of the lockup solenoid 8 in the control valve 5. Shall be set.

ON / OFF of shift solenoids 6 and 7,
And the drive duty D of the lockup solenoid 8
Controls them by the controller 9, and the controller 9 receives signals from the throttle opening sensor 10 for detecting the throttle opening TH (engine load state) of the engine 1, the rotation speed of the transmission output shaft 4, and the vehicle speed. A signal from the vehicle speed sensor 11 that detects V and the transmission operating oil temperature C
The signals from the oil temperature sensor (working fluid temperature detecting means) 12 for detecting

Although not shown, the controller 9 performs the following shift control and lock-up control by well-known calculations based on these input information. First, to explain the shift control, the controller 9 determines from the throttle opening TH detected by the sensor 10 and the vehicle speed V detected by the sensor 11,
The optimum shift speed for the current operating state is obtained by a look-up method from the table data corresponding to the shift diagram shown by the solid line (upshift shift line) and the broken line (downshift shift line) in FIG. The shift solenoids 6 and 7 are turned on and off so that the gear is selected, and a predetermined gear shift is performed.

Next, the lock-up control will be described. In this lock-up control, the controller 9 performs table data corresponding to the lock-up vehicle speed diagram (L / U is the lock-up area) shown by the alternate long and short dash line in FIG. 6, for example. From the look-up method, it is determined which of the lockup region and the converter region the traveling state is based on the throttle opening TH and the vehicle speed V, and the duty control of the lockup solenoid 8 is performed according to the determination result. The torque converter 3 is locked up or put into a converter state. The lockup of the torque converter 3 achieves this by setting the drive duty D of the solenoid 8 to 95%, and the converter state of the torque converter 3 achieves this by setting the drive duty D of the solenoid 8 to 5%. And

In the two-dot chain line in FIG. 6, 3 ←
The two-dot chain line related to the 4 downshift transmission line facilitates the automatic transmission to shift 4 to 3 downshifts when the automatic transmission is overheated, thereby reducing the heat generation amount of the automatic transmission and eliminating the overheated state. Of the 3 ← 4 downshift transmission line to the high vehicle speed side, and the two-dot chain line related to the 3 → 4 upshift transmission line is obtained by moving the 3 ← 4 downshift transmission line as described above. As a result of the reduction in the interval (hysteresis) between this and the 3 → 4 upshift transmission line indicated by the solid line, hunting occurs in the shift control in which the shift is repeated between the third speed and the fourth speed, but this hunting is prevented. 3 illustrates an example of the moving position of the 3 → 4 upshift transmission line to the high vehicle speed side for the purpose of

The controller 9 further executes the control program shown in FIGS. 3 to 5 by a timed interrupt to perform the following overheat countermeasure control when the automatic transmission overheats. In FIG. 3, first, in step 20, the transmission operating oil temperature C detected by the sensor 12 is read, and in step 21, which corresponds to the next overheat detecting means, this oil temperature C indicates a set temperature representing the overheated state of the automatic transmission 2. It is determined whether the temperature is in a high temperature region of Cs or higher. Here, the transmission hydraulic oil temperature C is the set temperature C
If it is s or more, the automatic transmission 2 is in the overheated state, and if it is less than the set temperature Cs, the automatic transmission 2 is not in the overheated state and is in the normal temperature range.

While the transmission operating fluid temperature C is in the high temperature range representing the overheated state of the automatic transmission 2, step 22 in FIG. 3 is selected, where the weighted average value α of the engine throttle opening TH within a fixed time is selected. Is less than the reference value α _S , it is checked whether the engine 1 is in a stable load state (the degree of change in load, that is, the change in accelerator pedal operation amount is small). Therefore, step 22 corresponds to a stable load state detecting means. It should be noted that the operation of the engine 1 is controlled by the operation of a known constant speed traveling device (ASCD), and it is assumed that the engine 1 is in a stable load state even while the vehicle is traveling at a constant speed. .

When it is determined in step 22 that the engine 1 is in a stable load state (the change in the accelerator pedal operation amount is small), in step 23 corresponding to the shift determining means, as shown in FIG.
It is determined whether or not a gear shift will occur when the above-mentioned gear shift control is performed by applying the high temperature 3 ← 4 downshift gear shift line indicated by the two-dot chain line in FIG. By executing the command, an instruction is given to perform the shift control based on the 3 ← 4 downshift shift line at the high temperature. When a gear shift occurs, the flag F for prohibiting switching to the 3 ← 4 downshift shift line at high temperature is turned on in step 25 corresponding to the overheat countermeasure control prohibiting means,
By executing step 26 based on this, as shown in FIG.
6 instructed to execute the shift control based on the normal 3 ← 4 downshift shift line indicated by a broken line in FIG.
Switching from the normal 3 ← 4 downshift transmission line indicated by the broken line to the high temperature 3 ← 4 downshift transmission line indicated by the two-dot chain line is prohibited.

If it is determined in step 21 that the temperature is in the normal temperature range, or if it is determined in step 21 that the automatic transmission is overheated, but it is determined in step 22 that the engine 1 is not in the stable load state, At 27,
It is determined whether or not a shift occurs when the shift control described above is performed by applying the normal 3 ← 4 downshift shift line indicated by the broken line in FIG. 6, and only when it is determined that the shift is not generated, the step 26 is performed. By executing the command, an instruction is given to perform the shift control based on the normal 3 ← 4 downshift shift line. If a gear shift occurs, then in step 28, the flag N for prohibiting the switching to the 3 ← 4 downshift shift line in the normal state is turned on, and step 24 is executed based on this, so that the two points shown in FIG. A command is issued to execute the above-mentioned shift control based on the high temperature 3 ← 4 downshift shift line indicated by the chain line, and in fact, it is indicated by a broken line from the high temperature 3 ← 4 downshift shift line indicated by the two-dot chain line in FIG. During normal operation, switching to the 3 ← 4 downshift transmission line is prohibited.

FIG. 4 shows a control program for resetting (OFF) the above-mentioned shift line switching prohibition flags F and N, and if either of the shift line switching prohibition flags F and N is ON in step 30. When making a determination, in step 31, the previously read value TH (O
LD) and the read value TH (NEW) this time are the same, that is, as long as it is determined that the accelerator pedal operation amount is not changed, steps 32 to 34 are skipped, and the 3 ← 4 downshift transmission line is switched. The control of FIG. 3 is performed, and the prohibition of the switching of the 34 downshift transmission line is continued while the ON states of the flags F and N are maintained.

By the way, the comparison result of the previous read value TH (OLD) and the current read value TH (NEW) of the throttle opening TH in step 31 is TH (NEW).
When the accelerator pedal depression is detected at> TH (OLD), in step 32, a command is issued to execute shift control based on the 3 ← 4 downshift shift line at high temperature shown by the two-dot chain line in FIG. 6, and TH ( When NEW) <TH (OLD) is detected and the foot release from the accelerator pedal is detected, in step 33, a command is issued to perform gear shift control based on the normal 3 ← 4 downshift gearshift line shown by the broken line in FIG. After the execution of step 32 or 33, the flags F and N are turned off in step 34, thereby releasing the prohibition of switching the 3 ← 4 downshift transmission lines.

FIG. 5 shows the 3 ← 4 downshift transmission line at high temperature when the 3 ← 4 downshift transmission line in FIG. 6 is moved from the broken line position to the high speed side to the two-dot chain line position by the above control. As a result of the decrease in hysteresis between the 3 → 4 upshift shift line indicated by the solid line, hunting occurs in the shift control in which the shift is repeated between the third speed and the fourth speed. → 4 Upshift Shift line to high vehicle speed side 3 at high temperature indicated by the two-dot chain line in Fig. 6
→ 4 Upshift This is a control program for moving up to the shift line. For this reason, first in step 40,
It is determined whether or not the shift control based on the high temperature 3 ← 4 downshift shift line indicated by the two-dot chain line in FIG. 6 is instructed, and the shift control based on the high temperature 3 ← 4 downshift shift line is instructed. In the meantime, step 41 corresponding to hunting detection means.
Check whether or not the above hunting occurs.

Here, in determining hunting,
The number of 3-4 shift occurrences within a certain period of time is counted, and when the count value is equal to or greater than the set number of times, it is determined that hunting is occurring, or the throttle opening TH during the above certain period of time is integrated. However, when the value obtained by dividing the count value of the number of times of occurrence of 3-4 shifts by the integrated value is equal to or greater than the set value, it can be determined that hunting is occurring with this. Further, instead of the integration of the throttle opening TH, the sum of absolute values of changes in the throttle opening TH during a certain period of time may be used.

If it is determined in step 41 that hunting has occurred, then in step 42, the normal 3 → 4 upshift transmission line shown by the solid line in FIG. 6 is replaced by the two-dot chain line in the figure. Command to perform shift control based on 3 → 4 upshift shift line at high temperature. Thus, even if the shift control based on the high temperature 3 ← 4 downshift shift line indicated by the two-dot chain line moved to the high vehicle speed side in FIG. 6 is instructed,
Since the 3 → 4 upshift transmission line is moved to the higher vehicle speed side as shown by the chain double-dashed line in the figure, the hysteresis between the 3 ← 4 downshift transmission line and the 3 → 4 upshift transmission line must be increased. Therefore, it is possible to prevent the hunting of the shift.

According to the control of this example described above with reference to FIGS. 3 and 4, the transmission operating oil temperature C becomes equal to or higher than the set temperature Cs, and step 21 detects the overheated state of the automatic transmission 2 from this. However, unless it is determined in step 22 that the engine 1 is in the stable load state, the overheat countermeasure control of the automatic transmission by switching the downshift transmission line is not performed in step 24. Therefore, as in the case of running the vehicle on a steep uphill road, the accelerator pedal is kept substantially constant for a long period of time under high load operation, and the torque converter slips for a long period of time. The above-mentioned overheat countermeasure control of the automatic transmission is executed only when the vehicle is kept at a large size, that is, only in a driving state where the overheat of the automatic transmission is really concerned. The control can function effectively.

In a normal running condition other than when the engine 1 is in a stable load condition, even the transmission operating oil temperature C
Even if the temperature becomes higher than the set temperature Cs, it is temporary and settles in the normal temperature range in a short time. Therefore, in such a normal running state, the above-mentioned overheat countermeasure control of the automatic transmission is not performed. . Therefore, in a normal traveling state, it is possible to solve the above-mentioned problem of the conventional device that a shock is generated by the unnecessary overheat countermeasure control and the drivability is deteriorated.

If the above-mentioned overheat countermeasure control causes a gear shift, the shock due to the gear shift and the shock due to the overheat countermeasure control occur before and after the shift because the shift is prohibited by the determination in step 23. It is possible to avoid discomfort. Similarly, when the above-mentioned overheat countermeasure control is ended, if a gear shift occurs due to this, the end of the overheat countermeasure control is prohibited by the determination of step 27. Therefore, the shock due to the gear shift and the end of the overheat countermeasure control are ended. It is also possible to avoid a sense of discomfort that occurs before and after the shock caused by.

Further, according to the control of the present embodiment described above with reference to FIG. 5, in the case where hunting of gear shift is caused in the overheat countermeasure control of the automatic transmission by switching the downshift gear shift line in step 24 of FIG. In response to the detection of this in step 41, the upshift gear shift line is also shifted to the high vehicle speed side in step 42, so that hunting of gear shift can be avoided.

In the above example, as the overheat countermeasure control,
In order to make it easier for the automatic transmission to select the low speed stage, the control for shifting the shift line to the high vehicle speed side has been exemplified, but instead of this or together with this, the lockup vehicle speed diagram shown by the one-dot chain line in FIG. It goes without saying that the torque converter 3 can be easily locked up by shifting to the vehicle speed side, and as a result, heat generation due to slip of the torque converter can be reduced to take measures against overheating of the automatic transmission.

[0041]

As described above, according to the overheat countermeasure device of the first invention, in addition to the condition that the working fluid temperature of the automatic transmission is equal to or higher than the set temperature representing the overheat of the automatic transmission, Since the load condition of the prime mover is a stable load condition, the automatic transmission is set to the control condition in which the heat generation amount is reduced only when both conditions are met.
Like when driving a vehicle on a steep uphill road,
The above-mentioned overheat countermeasure control of the automatic transmission can be performed only when the accelerator pedal is kept substantially constant and high-load operation is continued for a long time, that is, only when the automatic transmission is overheated. In normal driving conditions other than that, even if the working fluid temperature temporarily rises to high temperature, the above-mentioned overheat countermeasure control of the automatic transmission is not performed unnecessarily and is unnecessary. It is possible to solve the above-described problem of the conventional device that a shock is generated by the overheat countermeasure control and the drivability is deteriorated.

As the overheat countermeasure control of the automatic transmission, as in the second aspect of the present invention, the downshift gear shift line is shifted to the high vehicle speed side so that the automatic transmission can be easily put in the low speed stage. It is even more useful to employ the control so as to do so.

In this case, the downshift gear shift line approaches the upshift gear shift line and the hysteresis between the two becomes smaller, so that hunting is likely to occur in the gear shift control of the automatic transmission. As described above, when it is detected that the shift control of the automatic transmission causes hunting, the upshift shift line is also shifted to the high vehicle speed side, so that the problem of hunting of the shift control can be solved. .

As the overheat countermeasure control of the automatic transmission, instead of the control of the second invention, the lockup line of the torque converter is shifted to the low vehicle speed side as in the fourth invention described in claim 4. It is also very useful if the torque converter is locked up easily.

Further, the stable load state of the prime mover is the stable load state of the prime mover when the weighted average value of the load of the prime mover is smaller than the set value, as in the fifth aspect of the present invention. It is useful to have

A sixth aspect of the present invention is as described in claim 6.
When the automatic transmission overheat countermeasure control is performed to cause a shift of the automatic transmission, the overheat countermeasure control is configured to be prohibited, so when the gear shift occurs, the overheat countermeasure control is not performed, It is possible to eliminate the discomfort caused by the shock caused by the overheat countermeasure control and the shock caused by the gear shift coming close to each other.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an overheat countermeasure device for an automatic transmission according to the present invention.

FIG. 2 is a system diagram showing an embodiment of an overheat countermeasure device for an automatic transmission according to the present invention.

FIG. 3 is a flowchart showing overheat countermeasure control of an automatic transmission, which is executed by a controller in the same example.

FIG. 4 is a flowchart showing a control program of an overheat countermeasure control prohibition flag, which is executed by the controller.

FIG. 5 is a flowchart showing an upshift transmission line control program executed by the controller to prevent hunting of a shift caused by overheat countermeasure control.

FIG. 6 is a shift line diagram of an automatic transmission illustrating a normal time shift line and an overheated shift line in the same example together with a lockup vehicle speed line.

[Explanation of symbols]

 1 engine 2 automatic transmission 3 torque converter 5 control valve 6 shift solenoid 7 shift solenoid 8 lockup solenoid 9 controller 10 throttle opening sensor 11 vehicle speed sensor 12 oil temperature sensor (working fluid temperature detection means)

Claims (6)

[Claims] 1. While the overheat detecting means detects that the working fluid temperature of the automatic transmission is equal to or higher than a set temperature representing overheating of the automatic transmission, the automatic transmission is placed in a control state in which a calorific value is reduced. In the overheat protection device for an automatic transmission, a stable load state detection means for detecting a stable load state of the prime mover in which the degree of change in the load state of the prime mover for inputting power to the automatic transmission is smaller than the reference is provided, and the prime mover is provided by the means. The overheat countermeasure of the automatic transmission is controlled by the overheat countermeasure device in response to the signal from the overheat detecting means only while the stable load state of is detected. apparatus. 2. The overheat countermeasure control for the automatic transmission according to claim 1, wherein the downshift transmission line is shifted to a high vehicle speed side so that the automatic transmission can be easily put into a low speed stage. Characteristic automatic transmission overheat prevention device. 3. A hunting detection means for detecting the occurrence of hunting in the shift control of the automatic transmission according to claim 2, wherein the upshift shift line is also a high vehicle speed when the hunting of the shift control is detected by the means. A device for preventing overheating of an automatic transmission, which is configured to be shifted to the side to prevent hunting in shift control. 4. The overheat countermeasure control of the automatic transmission according to claim 1, wherein the lockup line of the torque converter is shifted to a low vehicle speed side so that the torque converter is easily locked up. A device to prevent overheating of an automatic transmission. 5. The stable load state detecting means according to claim 1, wherein the stable load state detecting means detects the load of the prime mover, and when the weighted average value of the load is smaller than a set value, An overheat countermeasure device for an automatic transmission, characterized in that it is in a stable load state. 6. The shift determination means for determining whether or not the overheat countermeasure control of the automatic transmission causes a shift of the automatic transmission according to any one of claims 1 to 5, and the shift is determined by the means. An overheat countermeasure device for an automatic transmission, characterized in that an overheat countermeasure control prohibiting means for issuing a command to prohibit overheat countermeasure control of the automatic transmission to the overheat countermeasure device when it is determined to occur is added.