The AL4 electronically controlled automatic transmission is a fully electronically controlled, fully enclosed, low-maintenance automatic transmission jointly developed and designed by the French PSA Group and Renault. This automatic transmission is mainly used in models such as the Citroën Fukang, Elysee, Picasso, Citroën C5, and other imported Citroën, Peugeot, and Renault cars. I. Features of the AL4 Automatic Transmission 1. Structural Features of the AL4 Automatic Transmission The AL4 electronically controlled automatic transmission uses a fuzzy logic automatic adaptation electronic control unit (ECU) that can automatically identify the driver type, road conditions, and vehicle load, adapting to the optimal shift pattern. Due to its intelligent functions, it can precisely control shifting and main oil circuit pressure, and control the movement of the torque converter piston through radiator flow control. If sensors or solenoid valves malfunction, warning signals will appear on the gear position display. The interconnection between the automatic transmission ECU and the engine control unit (ECU) allows them to transmit signals to each other, resulting in smoother torque converter lock-up and clutch/brake engagement, thereby greatly improving driving comfort and handling stability. In addition, the electronic control unit controls various solenoid valves to prevent driver misoperation, thus improving vehicle safety. 2. AL4 type electronic control automatic transmission mode switch The AL4 type automatic transmission has three transmission modes: "normal", "sport" and "snow". The mode is selected by the three mode selection buttons on the mode selection control panel on the right side of the automatic transmission lever. The selection operation and working status of each mode are as follows. (1) Normal mode. This mode is the default selection. No key needs to be pressed. The automatic transmission will work automatically in this mode. When the electronic control unit controls the automatic transmission to shift gears automatically, fuel saving is the priority. This mode is suitable for all kinds of ordinary road surfaces. (2) Sport mode. Press the "S" key. The "SPT" indicator light on the instrument panel will light up. At this time, the automatic transmission will enter the sport mode automatic shifting program. In this mode, when the electronic control unit controls the automatic transmission to shift gears automatically, power performance is the priority. That is, driving performance and acceleration performance are prioritized. (3) Snow mode. Press the "*" key. The "*" indicator light on the instrument panel will light up. At this time, the automatic transmission will enter the snow mode automatic shifting program. This mode is suitable for driving on low-traction roads. The electronic control unit controls the automatic transmission to start in 2nd gear to prevent the drive wheels from slipping. (4) Forced 1st gear mode. When the automatic transmission control lever is in position 2, press the "1" key on the mode selection control panel. The automatic transmission will always work in 1st gear. II. Transmission system structure of AL4 automatic transmission The AL4 automatic transmission adopts a series Simpson II planetary gear mechanism. Its front planetary carrier is connected to the rear ring gear, and the front ring gear is connected to the rear planetary carrier. The force application device consists of 2 plate clutches, 1 band brake and 2 plate brakes. The traditional Simpson planetary gear mechanism can only provide 3 forward gears and 1 reverse gear. If it is to be improved to 4 speeds, 1 planetary gear set, 1 clutch, 1 one-way clutch and 1 brake need to be added. The Simpson 11 planetary gear mechanism can achieve 4 forward gears with only a compound planetary gear set, saving 3 force application elements of the overspeed mechanism. Because the common sun gear of the compound planetary gear set does not have a drum, the automatic transmission is smaller in size, which perfectly matches the smaller displacement characteristic of French cars. The AL4 automatic transmission does not have a one-way clutch; the torque converter has friction plates on both the front and rear of the clutch disc. III. Characteristics of the AL4 Automatic Transmission Electronic Control System 1. Automatic Control Process The control unit of the AL4 automatic transmission converts parameters such as engine speed, throttle opening, vehicle speed, engine temperature, and automatic transmission fluid temperature into signals through various sensors, which are then input to the automatic transmission electronic control unit (ECU). The ECU determines the automatic transmission shift control signals based on these signals. The shift signal output by the ECU controls the corresponding solenoid valves to operate, and generates corresponding hydraulic control signals through its two-position directional valve, causing the relevant shift actuators to operate, thus achieving automatic shifting. 2. To adapt automatic shift control to changes in vehicle load, driving conditions, road conditions, and driving style, the AL4 automatic transmission electronic control unit (ECU) has 10 control rules: L1 is the economy rule, which is activated when the automatic transmission fluid temperature reaches 30°C; L2 is the intermediate rule, between L1 and L3; L3 is the sport rule, which is prioritized when the ECU detects the driver's driving style or when the driver presses the "S" key; L4 is the gentle uphill rule, which prioritizes the power required for gentle uphill driving during automatic shifting; and L5 is the steep uphill rule, which prioritizes the power required for steep uphill driving during automatic shifting. The required power; L6 is the downhill rule, which delays gear shifting compared to flat roads to facilitate engine braking; L7 is the snow rule, which is used on roads with low adhesion coefficients. The electronic control unit controls the automatic transmission to start in 2nd gear to prevent wheel slippage. Pressing the "*" key will enter this rule; L8 is the low temperature protection rule, which is entered when the automatic transmission fluid temperature is below 14℃, and the torque converter lock-up is prohibited; L9 is the high temperature protection rule, which is entered when the automatic transmission fluid temperature is above 118℃, and the torque converter locks up; L10 is the pollution prevention rule, which is entered when the automatic transmission fluid temperature is between 15℃ and 30℃, and the engine idle speed is increased to prevent pollution. 3. AL4 type automatic transmission control program (1) Normal control program. When the automatic transmission fluid temperature is between 30℃ and 118℃, the electronic control unit automatically enters this program without pressing any key, and controls gear shifting. Under this program, the electronic control unit selects the shifting rules in L1 to L6 according to the driver's driving style, load, road conditions and other conditions to control the shifting. (2) Motion control program. After pressing the "S" key, the electronic control unit enters this program to control the shifting. Under this program, the electronic control unit prioritizes the L3 rule, and then cancels the L3 rule after pressing the "S" key. (3) Snow control program. After pressing the "*" key, the electronic control unit enters this program to control the shifting. Under this program, the electronic control unit first selects the L7 rule. 4. Control functions of AL4 automatic transmission (1) Downshifting control. The electronic control unit automatically controls downshifting according to the vehicle speed, throttle position and brake pedal condition to make full use of the engine braking effect. When the vehicle speed is stable and the accelerator pedal is fully released, it jumps downshifts (4→2; 3→1); when the accelerator pedal is quickly released, it stays in the original gear or downshifts by 1 gear; when the brake pedal is pressed, it downshifts in advance. (2) Instantly improves acceleration. When the accelerator pedal is quickly pressed to the floor under L1 or L2 rule control, the electronic control unit can instantly transition to L1 or L3 rule. When the accelerator pedal is released, it returns to the original rule control state. (3) Rapid acceleration function. When the accelerator pedal is pressed rapidly under a certain stable driving state, the electronic control unit automatically controls downshifting to obtain the power required for rapid vehicle acceleration. (4) Pressure control. Under closed-loop conditions (when the electronic control system is fault-free and the automatic transmission oil temperature reaches a certain range), the electronic control unit controls the pressure of the main oil circuit according to the speed and the torque of the turbine. (5) Shift solenoid valve connection control. When shifting gears, the electronic control unit controls the on and off of the shift solenoid valve, so that the automatic transmission first enters the neutral state. After the oil cylinder of the original gear starts to drain oil, the oil cylinder of the new gear starts to fill oil. The time interval between emptying and filling the two cylinders is determined by the electronic control unit according to the vehicle speed and the position of the throttle. (6) Lock-up and disengagement of the torque converter. The electronic control unit determines whether the torque converter should be locked based on the vehicle speed, throttle position, engine speed, and shift rules of the automatic transmission input speed, so as to avoid excessive automatic transmission fluid temperature, reduce fuel consumption, and obtain engine braking by locking the torque converter. (7) Shift torque reduction control. When shifting, the automatic transmission electronic control unit outputs information to the engine electronic control unit to reduce the output torque of the engine by reducing the ignition advance angle, so as to reduce the impact during shifting. (8) Idle speed compensation control. When the automatic transmission fluid temperature is between 15℃ and 30℃, it provides information to the engine electronic control unit to increase the idle speed of the engine. (9) Automatic transmission protection control. The AL4 automatic transmission has the following protection control functions. Reverse gear protection. When the vehicle speed is greater than 15km/h, when shifting from D to R, the electronic control unit controls the automatic transmission not to enter reverse gear, and the vehicle coasts in neutral. At the same time, the "N" indicator flashes and the brake light illuminates. Error operation protection. When the vehicle speed exceeds the shift limit speed, when shifting from D→3 or 3→2, the electronic control unit controls the automatic transmission to remain in the original position for a certain period of time before shifting. Operation is prohibited. When the engine speed is higher than a certain set speed, the electronic control unit will prohibit the shifting operation from N→D or N→R, and deceleration and delay are required for the transition. (10) Gear lever lock function. When the automatic transmission lever is in the "P" position, it can only be moved from "P" to other gears when the ignition switch is turned on to the "M" position and the brake pedal is pressed at the same time. (11) Display function on the instrument panel. The automatic transmission control system displays the following functions on the instrument panel: the automatic transmission gear and the selected program are displayed on the LCD screen on the instrument panel; the automatic transmission is indicated by the alternating flashing of the "SPT" and "*" indicator lights. (12) Automatic transmission fluid change reminder function. The automatic transmission control unit (ECU) calculates the automatic transmission fluid reading based on parameters such as the automatic transmission fluid temperature and the operating time at high temperatures. When the reading exceeds 32958, it indicates that the automatic transmission fluid needs to be changed by flashing "SPT" and "*" alternately. (13) Self-diagnostic function. The ECU controls the power supply of sensors and actuators and monitors whether they are working properly. When there is a fault, the ECU has the following functions: after confirming the fault, it will warn by flashing the indicator light and store the fault information; it can communicate with the diagnostic equipment through the K-line; and it can make the automatic transmission control system run in backup mode. (14) Data update and encryption. The automatic transmission control unit can update its data through the diagnostic equipment, and can also set the external environment of the ECU according to the optional parts not installed on the vehicle. IV. Common Fault Analysis Since the AL4 automatic transmission does not have a dipstick, the method of checking the fluid level is different. The correct method is: under engine idling conditions, make the automatic transmission pause in each gear for a moment (the time should not be less than 5 seconds). When the automatic transmission fluid temperature reaches 60℃, open the overflow hole at the bottom; a small amount of automatic transmission fluid should flow out. Additionally, this automatic transmission uses a special fluid and should not be mixed. Common faults in the AL4 automatic transmission include the following: 1. Alternating flashing of "SPT" and "*": The indicator light will flash when there are faults such as overheating of the automatic transmission fluid, prolonged lack of fluid change, or interruption of communication between the automatic transmission control unit (ECU) and the instrument panel. The fault may be located in: the ECU itself, its power supply, the oil pressure sensor, the power supply to the upshift solenoid valve, the main oil pressure regulation, the multifunction switch, the upshift solenoid valve (EVS1～EVS6), the main oil pressure regulating solenoid valve (EVMPL), the torque converter lock-up solenoid valve (EVMPC), and the automatic transmission fluid flow solenoid valve (EPDE), etc. Additionally, if the accelerator pedal is not initialized, or if there are faults in the input/output shaft speed and engine speed information, the indicator light will also flash. 2. 3. **Neutral Switch Malfunction:** A malfunctioning neutral switch in an automatic transmission will cause the vehicle to have no gear when cold and be unable to move. If it functions normally after the engine warms up, it indicates that the neutral switch is damp or slightly corroded; cleaning the switch will restore normal operation. If the situation is more serious, the neutral switch needs to be replaced. 4. **Main Oil Circuit Pressure Regulating Solenoid Valve Malfunction:** A malfunctioning main oil circuit pressure regulating solenoid valve will result in poor vehicle comfort, shifting shocks, upshifting shocks, and downshifting shocks. When the automatic transmission control unit (ECU) detects a malfunction in the main oil circuit pressure regulating solenoid valve, it will put the automatic transmission into emergency standby mode. 5. **Pressure Regulation Malfunction:** When the two indicator lights flash alternately due to a pressure regulation malfunction in the automatic transmission, the cause may include: a fault related to the adaptive value of the main oil circuit pressure stored in the ECU, or a fault related to the detection information in the closed-loop regulation. For example, if the difference between the main oil circuit pressure stored in the ECU and the measured main oil circuit pressure increases, the operation of the pressure regulating solenoid valve and the automatic transmission fluid pressure sensor should be checked. The corresponding plungers in the hydraulic control unit should also be checked for proper functioning. When two indicator lights flash alternately, the automatic transmission will be downgraded to safe mode, the main hydraulic pressure will be adjusted to the maximum, and closed-loop control will be activated for emergency use. 5. Automatic transmission without forward gear or manual 1st gear: If an automatic transmission lacks forward gear or manual 1st gear, the following are common causes: A malfunction in the low-speed band brake F3; a fault in the 3rd gear clutch E2; or a malfunction in the shift solenoids EVS3 and EVS4. Another possibility is a stuck plunger in the automatic transmission hydraulic control unit. 6. Automatic transmission without forward gear or manual 2nd gear: Possible causes include: A malfunction in the low-speed band brake F3; a malfunction in the 2nd gear clutch E1; or a malfunction in the upshift solenoids EVS2 and EVS4. For manual positions, a malfunction in solenoid EVS6; or a stuck plunger in the automatic transmission hydraulic control unit. 7. 8. Automatic transmission without forward gear or manual 3rd gear: The causes of this problem are often a faulty plate clutch E1 or E2; the shift solenoids are engaged (in which case they are not engaged at all); or the plunger in the hydraulic control unit is stuck. 9. Automatic transmission without 4th gear: The causes of this problem may be a faulty overdrive plate brake F1, second gear clutch E1, shift solenoids EVS1 and EVS2, or a stuck plunger in the hydraulic control unit. 10. Automatic transmission without lock-up: There are many reasons why an automatic transmission may not lock up, including a faulty hydraulic control unit or lock-up solenoid. If the solenoid is working properly but there is no torque converter lock-up, the automatic transmission fluid temperature is very likely to be too high. In this case, the torque converter should be checked for proper operation. When the automatic transmission control unit detects slippage of the friction plates, it will cancel the lock-up function as an emergency measure. When an automatic transmission has no reverse gear, the cause of the fault may be a malfunction in the band brake F3, the plate clutch E2, the shift solenoid valve EVS5, or a stuck plunger in the hydraulic control unit.