How to check the speed sensor - 3 easy ways
Content
The speed of the car must be known not only to inform the driver. Numerous electronic systems use the speed value as an input parameter for the correct control of the subordinate units. There are several ways to determine this value, most often a separate sensor is used in the transmission.
Purpose and location of the DS
The vehicle speed sensor (DS) performs several system functions:
- gives a signal to the dashboard to inform the driver in an easily read digital or arrow format;
- reports the speed to the engine control unit;
- provides a speed value to the vehicle's general information bus for use by driver assistance systems.
In parallel, speed information can be taken from the ABS wheel speed sensors, the data will be compared by electronic units.
The DS is located on one of the transmission elements, it can be a gearbox or transfer case. Sometimes direct drive from one of the wheels was used.
The principle of operation of the speed sensor
In fact, the DS measures not the speed, but the rotational speed of the part on which there is a ring gear. This value can be converted into speed mechanically or electronically, since the transmission has an unambiguous and known relationship between frequency and speed with the standard wheel size.
Installing tires or wheels of a different size leads to an error in measuring the speed. As well as the refinement of the transmission with a change in gear ratios after the DS.
Sensors can be mechanical or electronic. Mechanical DS is no longer used; earlier it consisted of a gear-type device ending with a sheathed cable. The rotation of the cable was transmitted to the dashboard, where a magnetic system was connected to it.
An alternating magnetic field induced currents in the coils, which were measured with a pointer milliammeter calibrated in speed values.
The resulting speedometer was usually combined with a mechanical rev counter - an odometer that recorded the total and daily mileage of the car.
Electronic sensors can use various principles in their work:
- optical, when the beam passes through slots in a rotating disk;
- a magnetoresistive, rotating multipole magnet causes a change in the electrical parameters of the sensing element;
- induction, metal parts cyclically change the field of a permanent magnet, which causes an alternating current in the measuring coil;
- on the Hall effect, an alternating magnetic field is fixed by a magnetically sensitive semiconductor crystal, after which the shaper creates a sequence of receiving pulse blocks that are convenient for operation.
Most often in modern technology, devices with a Hall effect and a built-in magnet are used, capable of “counting” the teeth of any metal crown passing by.
Symptoms of a malfunction
If the DS fails, the electronics will immediately notice this, the matter will not be limited to the absence of indications on the instrument panel. An error will be displayed with the issuance of the corresponding code, the unit will go into emergency mode, which will immediately affect the work.
The engine will start to stall in neutral while driving, consumption will increase and power will decrease. The electric power steering that uses speed information will fail. The trip computer will stop working.
3 ways to check the speed sensor
First of all, it is worth checking the power supply and signal wiring. Here, the most common are oxidation of contacts, violation of the termination of wires into connectors, corrosion and mechanical damage to wires. Then proceed to check the sensor itself.
You should also connect the diagnostic device to the ECU and diagnose for errors. For example, this can be done using the Rokodil ScanX universal autoscanner.
If there are no errors on the speed sensor, it is necessary to compare the readings of the speedometer and the sensor in the application to the scanner while driving. If the results match, most likely the sensor is in perfect order.
Using a tester (multimeter)
The signal at the output of the DS according to the Hall principle should change with the rotation of the drive gear of the sensor. If you connect a multimeter in voltmeter mode and rotate the gear, you can notice a change in readings (pulse signal) in the operating range of a particular sensor.
- remove the sensor from the car;
- connect the connector and check the presence of a positive power supply and contact with ground;
- connect a voltmeter to the signal wire and spin the drive to observe the change in readings.
All the same can be checked at the connector of the instrument panel or engine controller, so the wiring will also be checked.
without removing the controller
You can not remove the DS, by bringing its drive into rotation in a natural way. To do this, the driving wheels of the car are hung out, the engine starts, after which at low speed it is possible to determine the presence or absence of a signal according to the readings of the connected voltmeter.
Checking with a control or a light bulb
The output of the sensor is usually an open collector circuit. If you connect a control indicator with an LED or a low-power bulb between the power plus and the signal contact of the sensor, then after spinning up, as described above, you can check for the blinking of the control indicator.
How to check the drive DS
Often, DS drive gears are made of plastic, which leads to tooth wear. If the sensor is electrically sound, then it is necessary to check for engagement.
This can be noticed visually when inspecting the teeth, or by scrolling the posted drive wheel, to observe the presence of rotation of the sensor rotor.
Replacement Instructions
Replacing the sensor is not difficult; it is usually fixed in the gearbox housing with a flange screw. By unscrewing this screw and removing the connector, the sensor can be removed and a new one installed.
For sealing, a regular gasket or sealant is used. After replacement, it is necessary to reset the current errors with a scanner or by briefly removing the terminal from the battery.
Before carrying out the operation, it is necessary to thoroughly clean the box body around the sensor in order to avoid abrasives entering the crankcase. Adjacent surfaces are wiped from dirt, oil and oxides.