Purpose and principle of operation of the main automatic transmission sensors
Car transmission,  Car device

Purpose and principle of operation of the main automatic transmission sensors

The vehicle's automatic transmission is controlled by an electro-hydraulic system. The process of shifting gears in the automatic transmission is due to the pressure of the working fluid, and the electronic control unit controls the operating modes and regulates the flow of the working fluid using valves. During operation, the latter receives the necessary information from sensors that read the driver's commands, the current speed of the vehicle, the work load on the engine, as well as the temperature and pressure of the working fluid.

Types and principle of operation of automatic transmission sensors

The main goal of the automatic transmission control system can be called the determination of the optimal moment at which the gear change should occur. For this, many parameters must be taken into account. Modern designs are equipped with a dynamic control program that allows you to select the appropriate mode depending on the operating conditions and the current driving mode of the car, determined by the sensors.

In an automatic transmission, the main ones are speed sensors (determining the speed at the input and output shafts of the gearbox), pressure and temperature sensors of the working fluid and a selector position sensor (inhibitor). Each of them has its own design and purpose. Information from other vehicle sensors can also be used.

Selector position sensor

When the position of the gear selector is changed, its new position is fixed by a special selector position sensor. The received data is transmitted to the electronic control unit (it is often separate for the automatic transmission, but at the same time it has a connection with the car engine ECU), which starts the corresponding programs. This activates the hydraulic system according to the selected driving mode (“P (N)”, “D”, “R” or “M”). This sensor is often referred to as an “inhibitor” in vehicle manuals. Typically, the sensor is located on the gear selector shaft, which in turn is located under the hood of the vehicle. Sometimes, to obtain information, it is connected to the drive of the spool valve for selecting the driving modes in the valve body.

The automatic transmission selector position sensor can be called “multifunctional”, since the signal from it is also used to turn on the reverse lights, as well as to control the operation of the starter drive in the “P” and “N” modes. There are many designs of sensors that determine the position of the selector lever. At the heart of the classic sensor circuit is a potentiometer that changes its resistance depending on the position of the selector lever. Structurally, it is a set of resistive plates along which a movable element (slider) moves, which is associated with a selector. Depending on the position of the slider, the resistance of the sensor will change, and hence the output voltage. All this is in a non-separable housing. In the event of a malfunction, the selector position sensor can be cleaned by opening it by drilling rivets. However, it is difficult to set up the inhibitor for repeated operation, so it is easier to simply replace the faulty sensor.

Speed ​​sensor

As a rule, two speed sensors are installed in an automatic transmission. One records the speed of the input (primary) shaft, the second measures the speed of the output shaft (for a front-wheel drive gearbox, this is the speed of the differential gear). The automatic transmission ECU uses the readings of the first sensor to determine the current engine load and select the optimal gear. The data from the second sensor is used to control the operation of the gearbox: how correctly the commands of the control unit were executed and exactly the gear that was needed was turned on.

Structurally, the speed sensor is a magnetic proximity sensor based on the Hall effect. The sensor consists of a permanent magnet and Hall IC, located in a sealed housing. It detects the rotational speed of the shafts and generates signals in the form of AC pulses. To ensure the operation of the sensor, a so-called "impulse wheel" is installed on the shaft, which has a fixed number of alternating protrusions and depressions (quite often this role is played by a conventional gear). The principle of operation of the sensor is as follows: when a gear tooth or a protrusion of a wheel passes through the sensor, the magnetic field created by it changes and, according to the Hall effect, an electrical signal is generated. Then it is converted and sent to the control unit. A low signal corresponds to a trough and a high signal to a ledge.

The main malfunctions of such a sensor are the depressurization of the case and oxidation of the contacts. A characteristic feature is that this sensor cannot be “ringed out” with a multimeter.

Less commonly, inductive speed sensors can be used as speed sensors. The principle of their operation is as follows: when the gear of the transmission gear passes through the magnetic field of the sensor, a voltage arises in the sensor coil, which is transmitted in the form of a signal to the control unit. The latter, taking into account the number of teeth of the gear, calculates the current speed. Visually, an inductive sensor looks very similar to a Hall sensor, but it has significant differences in signal shape (analog) and operating conditions - it does not use a reference voltage, but generates it independently due to the properties of magnetic induction. This sensor can be “ringed”.

Working fluid temperature sensor

The temperature level of the transmission fluid has a significant effect on the operation of the friction clutches. Therefore, an automatic transmission temperature sensor is provided in the system to protect against overheating. It is a thermistor (thermistor) and consists of a housing and a sensing element. The latter is made of a semiconductor that changes its resistance at different temperatures. The signal from the sensor is transmitted to the automatic transmission control unit. As a rule, it is a linear dependence of voltage on temperature. The sensor readings can only be found using a special diagnostic scanner.

The temperature sensor can be installed in the transmission housing, but most often it is built into the wiring harness inside the automatic transmission. If the permissible operating temperature is exceeded, the ECU can forcibly reduce the power, up to the transition of the gearbox to emergency mode.

Pressure sensor

To determine the rate of circulation of the working fluid in an automatic transmission, a pressure sensor can be provided in the system. There can be several of them (for different channels). The measurement is carried out by converting the pressure of the working fluid into electrical signals, which are fed to the electronic control unit of the gearbox.

Pressure sensors are of two types:

  • Discrete - fix the deviations of the operating modes from the set value. During normal operation, the sensor contacts are connected. If the pressure at the sensor installation site is lower than required, the sensor contacts open, and the automatic transmission control unit receives a corresponding signal and sends a command to increase the pressure.
  • Analog - converts the pressure level into an electrical signal of the corresponding magnitude. Sensitive elements of such sensors are capable of changing resistance depending on the degree of deformation under the influence of pressure.

Auxiliary sensors for automatic transmission control

In addition to the main sensors related directly to the gearbox, its electronic control unit can also use information obtained from additional sources. As a rule, these are the following sensors:

  • Brake pedal sensor - its signal is used when the selector is locked in the "P" position.
  • Gas pedal position sensor - installed in the electronic accelerator pedal. It is required to determine the current drive mode request from the driver.
  • Throttle Position Sensor - Located in the throttle body. The signal from this sensor indicates the current working load of the engine and influences the selection of the optimal gear.

The set of automatic transmission sensors ensures its correct operation and comfort during vehicle operation. In the event of sensor malfunctions, the balance of the system is disturbed, and the driver will be immediately alerted by the on-board diagnostics system (ie the corresponding “error” will light up on the instrument cluster). Ignoring malfunction signals can lead to serious problems in the main components of the car, therefore, if any malfunctions are found, it is recommended to immediately contact a specialized service.

2 comment

  • Ali Nikro XNUMX

    Hello, don't be tired. I have a XNUMXXXNUMX luxury automatic car. I have been driving it for a while. It is in normal condition. It remembers the gas automatically and the brakes don't work. Or if I apply it manually, it stops. When I press the brake pedal several times, the car returns to normal. The repairmen didn't bother me. I changed the automatic shaft sensor XNUMX year ago. Can you give me some advice, where is it from? Thank you.

  • Hamid Eskandari

    greetings
    I have a Persia model 5 tuXNUMX. For some time, when the engine temperature has not risen too high, when I drive, it makes a noise and the engine sound changes, and the XNUMXrd gear does not shift, but the engine revs high. Can you tell me the reason? Thank you

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