Using Oscilloscopes on Vehicles

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

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

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

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

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

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

We will see a low kilovolt because of low compression, directly followed by hash

caused by air flow distorting the spark. Depending on frequency, the problem could

be traced to worn valve guides, carbon deposits, weak valve spring or poor lubrication.

Conclusive diagnosis depends on how many cylinders are affected and if the problem

is intermittent or permanent.

The value of a scope is apparent only when the potential is fully understood. Let us

assume that every cylinder performed as it was designed. The air inhaled is determined

by bore and stroke of the piston. This air is matched with a precise amount of fuel and the

process of total combustion matches the coil output energy with very little residual energy

to spare. All cylinders are created equal and should follow that precise pattern of combustion

efficiency. Any deviation will affect the firing time and alter the residual coil energy. Because

the lab scope is time based, there is no better tool to measure, compare and display combustion

efficiency per cylinder.

WisdomAugust

The other reason for analyzing at idle in drive is because the spark occurs near TDC

when compressed gas movement is at its peak. The effect of these escaping gasses

can be seen on the scope as a hash at the beginning of the firing line.

WisdomAugust

Valve seating:

This is a common but often neglected problem. When a valve is not seated,

it does not cool, which might lead to a variety of possible failures. It is easily

spotted with a scope at idle, preferably in drive. A non-seating valve, whatever

the cause, allows compressed gases to escape. Everyone knows that a burned

valve is very noticeable at idle, but at 60 mph, that small crack does not make

much difference, because there is not enough time to escape. For that reason

testing at low speed is essential.

WisdomAugust

We can tell that it was not lean at the start, because kilovolt demand is normal and

the firing line is not elevated. We can readily see it is not a dirty screen or an odd spray

pattern. If this pattern remains permanent and does not vary, it is not cleanable and is

likely a high electrical resistance problem getting a late start. A simple replacement of

the injector would suffice. The fact that a scope can provide all that information in detail

eliminates trial and error repair methods.