From Advanced Automotive Fault Diagnosis, Second Edition

4.1 Introduction

This chapter outlines the methods used and the results of using an oscilloscope to test a variety of systems. It will be a useful reference as all the waveforms shown are from a correctly operating system. The chapter is split into three main sections: sensors, actuators and ignition.

Author's Note

The waveforms in this chapter were captured using the PicoScope automotive oscilloscope. I am most grateful to the PicoTech team for supplying information and equipment to assist in the production of this chapter (

4.2 Sensors

4.2.1 ABS speed sensor waveform

The anti-lock braking system (ABS) wheel speed sensors have become increasingly smaller and more efficient in the course of time (Figure 4.1). Recent models not only measure the speed and direction of wheel rotation but can be integrated into the wheel bearing as well.

Figure 4.1: ABS wheel speed sensors ( Source: Bosch Press)

The ABS relies upon information coming in from the sensors to determine what action should be taken. If, under heavy braking, the ABS electronic control unit (ECU) loses a signal from one of the road wheels, it assumes that the wheel has locked and releases that brake momentarily until it sees the signal return. It is therefore imperative that the sensors are capable of providing a signal to the ABS ECU. If the signal produced from one wheel sensor is at a lower frequency than the others the ECU may also react.

The operation of an ABS sensor is similar to that...

Products & Services
Variable Reluctance Linear Position Sensors and Switches
Variable reluctance linear position sensors and switches are non-contact devices that use variable reluctance based technology, and whose output signal represents the distance between an object and a reference point.
Brake Controllers
Brake controllers are used to control braking systems. They can simply engage and disengage the brake, or control the speed of engagement, breakaway torque, and other variables.
Wheel and Track Drives
Wheel and track drives are a gearbox, motor and brake assembly which power wheeled or tracked vehicles. Motors can be either electric or hydraulically powered.
Linear Velocity Sensors
Linear velocity sensors measure the linear velocity of an object using either contact or non-contact techniques.
Rotary Position Sensors
Rotary position sensors include potentiometers, resolvers and a variety of magnetic and capacitive technologies. Sensors are designed for angular displacement less than one turn or for multiturn displacement.

Topics of Interest

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