Using Oscilloscopes on Vehicles

WisdomAugust

Enter the scope - it pictorially shows the changing voltage over time,

drawing a trace that accurately depicts the pattern of voltage variation.

In fact a scope is the only way that you're going to be able to look at signals

coming out of camshaft and crankshaft position sensors, speed sensors and ABS

sensors, amongst others. And it's also the only way that you're going to be able to

see the signals going to injectors, idle air control valves, boost control solenoids,

auto trans pressure control solenoids, and so on.

Traditionally, scopes have been used by mechanics to look at primary (low voltage)

and secondary (high voltage) ignition signals. And that's a valuable use for a scope.

But these days a scope is far more likely to be used to look at inputs and outputs

of the ECU. In fact, most good factory workshop manuals now show sample scope

traces, so that you can use a scope to quickly find if the output signal from the sensor

or ECU looks as it should.

WisdomAugust

Think about this: if in a modified car you're changing those signals by

using an interceptor, just how good a job is the interceptor doing?

Does the waveform going into the interceptor from the crankshaft position

sensor look like the intercepted waveform coming out of the modifier

(except of course for a change in timing)? Or is the output waveform horribly

distorted - perhaps the cause of that Check Engine light problem that always

rears its ugly head when you have the interceptor working?

WisdomAugust

What if you want to make a modification in an area that few other people have played with -

for example, change the auto trans pressure for firmer shifts? These days it will be controlled

by a pulsed solenoid - but actually seeing how the solenoid is pulsed in different conditions will

require a scope.

It's important to realise that scopes have a major role to play in car modification, not just in

diagnosing obscure ignition system problems....

WisdomAugust

In this three part series we'll start with the basics of scopes. Getting your head around a signal

that's constantly varying can be quite hard to do, especially when most of us are used to reading

relatively unchanging values on a multimeter. After that we'll look at what scopes are suitable for

automotive work, then we'll get stuck into using a scope on modified and standard cars.

WisdomAugust

The Oscilloscope

An oscilloscope is basically a graph-displaying device - it draws a graph of an electrical signal.

In all automotive applications, the graph shows how signals change over time: the vertical (Y)

axis represents voltage, and the horizontal (X) axis represents time.

But don't be fooled - this simple graph can tell you many things about a signal, such as:

The time and voltage values of a signal (how many volts and when it changes)

The frequency of an oscillating signal (how fast the voltage is rising and falling)

The frequency with which a particular portion of the signal is occurring relative

to other portions (is there a part of the signal that varies more rapidly up and

down than other parts?)

Whether or not a malfunctioning component is distorting the signal (do the sine

waves look more like square waves?)

How much of the signal is noise and whether the noise is changing with time

('noise' is normally seen as a superimposed signal - jagged edges on a sine

wave, for example)

WisdomAugust

The generic term for a pattern that repeats over time is a 'wave' - sound waves, brain waves, ocean waves,

and voltage waves are all repetitive patterns. An oscilloscope measures voltage waves. One cycle of a wave

is the portion of the wave that repeats. A waveform is a graphic representation of a wave. A voltage waveform

shows time on the horizontal axis and voltage on the vertical axis.

Waveform shapes reveal a great deal about a signal. Any time you see a change in the height of the waveform,

you know the voltage has changed. Any time there is a flat horizontal line, you know that there is no change for

that length of time. Straight, diagonal lines mean a linear change - rise or fall of voltage at a steady rate. Sharp

angles on a waveform indicate sudden change.

WisdomAugust

You can classify most waves into these types:

Sine waves

Square and rectangular waves

Triangle and saw-tooth waves

Complex waves

In automotive applications, sine and square waves dominate.

Sine Waves

The sine wave is the fundamental wave shape. It has harmonious mathematical properties

- it is the same sine shape you may have studied in high school trigonometry class. Mains

AC voltage varies as a sine wave. ('AC' signifies alternating current, although the voltage

alternates too. 'DC' stands for direct current, which means a steady current and voltage,

such as a car battery produces.) Many speed sensors produce sine wave outputs - this

waveform is from an ABS inductive speed sensor.

WisdomAugust

Square and Rectangular Waves

The square wave is another common wave shape. Basically, a square wave is a voltage that turns on and off

(ie goes high and low) at regular intervals. An injector waveform is fundamentally a square wave - the injector

is either on or off. A rectangular wave is like the square wave, except that the high and low time intervals are

not of equal length. That is, the 'on' and 'off' times are not equal. Again, this is often the case with an injector,

where at low loads the 'off' time will be much longer than the 'on' time. The waveform shown here is from a Hall

Effect road speed sensor.

WisdomAugust

Waveform Measurements

Many terms are used to describe the types of measurements made with an oscilloscope.

Frequency and Period

If a signal repeats, it has a frequency. Frequency is measured in Hertz (Hz) and equals the number of

times the signal repeats itself in one second. Hertz can also be referred to as 'cycles per second'. A

repetitive signal also has a period - this is the amount of time it takes the signal to complete one cycle.

Period and frequency are reciprocals of each other, so that 1/period equals the frequency and 1/frequency

equals the period.

For example, the sine wave here has a frequency of 3 Hz and a period of 1/3 second. Some scopes can

calculate frequency and display it as a standalone number, while in other cases the period needs to be

read off the scope screen and the frequency then calculated from this.

WisdomAugust

Voltage

Voltage is the amount of electric potential - or signal strength - between two points in a circuit.

Usually, one of these points is ground, or zero volts. DC signals are measured on a scope as

you would with a multimeter - from ground to the amplitude (height) of the signal.

Automotive AC signals are often measured from the maximum peak to the minimum peak of a

waveform, which is referred to as the peak-to-peak voltage. The peak-to-peak voltage of this

inductive crank sensor is just under 16 volts.