2. Description of components

   1. Engine speed and vehicle speed sensors

The engine speed and vehicle speed sensors are inductive sensors.

The vehicle is equipped with a number of inductive sensors, such as: engine speed sensor ABS sensor vehicle speed sensor.

Operating principle

An inductive sensor is composed of a permanent magnet (1), a core (2) and a coil (3). When the inductive sensor is situated between two teeth, the lines of force of the magnetic field will run directly from the north pole to the south pole via the housing.

The moment a tooth approaches the inductive sensor, the lines of force of the magnetic field will run from the north pole to the south pole via the housing, the teeth of the wheel, and the core.

In this situation more lines of force will pass through the core, which will give an increased magnetic field intensity.

As a result of this change in the magnetic field, an alternating voltage will be generated in the coil.

The value of the generated alternating voltage depends on the rotation speed of the toothed wheel and the air gap between sensor (core) and tooth.

From alternating voltage to “pulse train”

The output signal of the inductive sensor is used by the various electronic units and analog meters/gauges (rev counter, tachograph).

The electronic unit has a microprocessor, which can only process digital signals (pulses). Therefore, the sine-wave signal should be converted into a ‘pulse train'.

Similarly, the meters/gauges (rev counter, tachograph) only respond to a 'pulse train’.

The conversion of the engine speed signal may take place in:

the rev counter (e.g. 95 series)

For the vehicle speed signal, conversion takes place within the speed sensor itself.

For the ABS sensor signal, conversion takes place in the ABS unit.

Duty cycle of “converted'’ engine speed signal

The linear characteristic of the duty cycle (%) versus the engine speed (n) is plotted in the graph opposite.

This characteristic applies to all engines.

Duty cycle of vehicle speed signal

The linear characteristic of the duty cycle (%) versus the vehicle speed (V) is plotted in the graph opposite.

This characteristic applies to all vehicle types. Checking

The alternating voltage signal can be checked with a multimeter set to the alternating voltage range.

The ‘pulse train’ (square-wave voltage) can be checked with a multimeter set to the direct voltage or duty cycle range.

2. Temperature sensors

The vehicle is equipped with a number of temperature sensors, such as: coolant temperature sensor

These sensors (also called thermistors) are resistors which are sensitive to temperature.

The resistance of these sensors changes considerably when the temperature increases or decreases.

There are two kinds of temperature sensor:

N TC resistor (Negative Temperature Coefficient).

PTC resistor (Positive Temperature Coefficient).

NTC resistor

In an NTC resistor, the resistor value decreases as temperature increases.

Application:

measuring coolant temperature.

PTC resistor

In a PTC resistor, the resistor value increases as temperature increases.

Contrary to NTC resistors, PTC resistors show a large resistor difference within a small temperature range.

Application:

measuring the air temperature in the auxiliary heating.

Checking

The temperature sensors can be checked with a multimeter set to the resistance range.