Abstract: automatic transmission is simple and labor-saving, and runs smoothly. It can realize stepless speed change in a certain range, and can automatically adapt to the changes of vehicle speed and driving resistance. Its advantage is to improve the power and fuel economy of the car. However, the automatic transmission has complex structure, high precision and difficult maintenance. In case of automatic transmission failure, it cannot be easily disassembled. First, make routine inspection and analysis, then reproduce the fault phenomenon through automatic transmission test, and then analyze and eliminate the fault. Automatic transmission test includes stall test, hysteresis test, oil pressure test and road test.

Keywords: automatic transmission; Fault analysis; test

First, the definition of stall test

Stall test refers to the engine speed when the car is parked, the transmission is put into D gear or reverse gear, the brake pedal is pressed, and the accelerator pedal is fully pressed. In the stall test, the output speed of the torque converter of the engine is 0 at a specific speed, and the torque ratio is the largest at this time.

The driving force generated by the engine under full load acts on the wheels through the automatic transmission and transmission system. Because the car stops under the action of service brake and parking brake, the turbine of the torque converter does not move, and only the torque converter housing and pump wheel rotate with the engine. At this time, all driving forces act on planetary gears, clutches, brakes and other devices. If the clutch or brake slips, the engine speed will increase due to the decrease of resistance torque. If all devices in the transmission system are in normal working conditions, the energy generated by the engine can only be absorbed by the liquid circulation of the hydraulic torque converter, and this working condition becomes a stall working condition. The engine speed at this time is called stall speed.

Second, the stall test matters needing attention

As the engine and transmission are fully loaded during the stall test, the following points should be strictly observed:

(1) should be tested after basic inspection and adjustment.

(2) Idle for a period of time to raise the oil temperature to the normal driving temperature (70℃~80℃).

③ Too high transmission oil temperature will accelerate the aging of transmission oil and seals and aggravate the wear of clutch friction materials. Therefore, the continuous test time should not exceed 5s, and the test should be repeated at intervals of about 3 minutes to prevent the transmission oil pressure from being too high.

④ If the engine exceeds the stall speed too much during the test, it means that the clutch in the transmission is slipping, and the test should be stopped immediately, otherwise the transmission will be damaged.

Thirdly, stall test step.

(1) park the car on a wide flat ground and plug the front and rear wheels with triangular wooden blocks;

② Tighten the handbrake and step on the brake pedal with your left foot;

③ Start the engine;

(4) Turn the joystick to position d;

⑤ Step on the accelerator pedal with your right foot and the brake pedal with your left foot at the same time. When the engine speed no longer rises, quickly read the engine speed at this time;

⑥ Release the accelerator pedal immediately after reading the engine speed;

⑦ Turn the joystick to the P or N position and let the engine idle 1 min.

Prevent hydraulic oil from deteriorating due to excessive temperature.

Fourth, stall test fault analysis

There are many factors that affect stall speed. The stall speed of different engines or transmissions produced by different manufacturers is different, so the stall speed standard value of this product should be known before stall test.

① If the stall speeds obtained in D and R gears are consistent with the given standard stall speed, the system is basically normal.

② When the stall speed obtained in D and R gears is lower than the given standard stall speed, it may be that the engine power is insufficient or the torque converter is not working properly.

You can tell whether the engine is faulty by disconnecting the power supply. The method is to put the gear lever in the P or N position, and the engine accelerates quickly. If the engine speed rises rapidly and steadily, it can be said that the engine is working normally.

When the stall speed is lower than the reference stall speed and the speed difference is greater than 600 rpm, it can be judged that the one-way clutch of the guide wheel of the torque converter is faulty.

③ When the stall speed of the D gear is higher than the standard stall speed, the failure causes may be as follows.

Automatic transmission clutch slipping (abnormal wear and combustion of friction materials).

The one-way clutch doesn't work well

Brake band slipping (abnormal wear, burning).

Oil pressure is too low.

Which actuator is out of order, the clutch and brake of automatic transmission can be used to assist in judging. When the D-range test is carried out, the car is not running and the transmission will not shift gears, so this test only detects the low-range actuator, not the high-range actuator. As shown in the following table, only the low-range rear clutch C2 and the one-way clutch F are working when driving. If the stall speed is too high and the actuator slips, check the above components. When the stall test is carried out in 2nd gear, there is a brake B2 in addition to the rear clutch. If slippage occurs, check the above two components carefully.

Clutch and brake worksheet

(4)R stall test, if there is slippage, it can be seen from the table that it may be caused by the failure of the high-speed reverse clutch.

⑤ If the stall speed of D and R gears is higher than the standard speed, check whether the hydraulic system pressure is too low. Otherwise, the transmission actuator will be seriously damaged.

In particular, it is not advisable to carry out too many stall tests. Because in this test process, the engine power is completely absorbed by the liquid in the torque converter, which will generate a lot of heat and exert great force or torque on various components, which will inevitably cause damage. Without knowing the operating tables of the clutch and brake after the repair of the automatic transmission and the working conditions of the actuators of the tested automatic transmission, it is impossible to correctly judge which actuator has a problem even if the above-mentioned failure phenomenon occurs.

References:

It's cloudy Structure principle and maintenance of automobile automatic transmission. Beijing: People's Communications Press, 200 1.

Ren Dong. Structure and maintenance of automobile chassis. China Labor and Social Security Press, 2004.

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