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Using Oscilloscopes on Cars, Part 3

How to interpret the squiggly line, and a look at the Velleman handheld scope

by Julian Edgar

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This article was first published in 2004.
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When using a scope on a car there are two major dramas to overcome: how to make the scope see what you want it to see, and then how to interpret the resulting pattern. As we covered last week in Using Oscilloscopes on Cars, Part 2, if the scope hasn't got good enough specs, it's possible to see things on the screen that don't match very well with reality - but a lot of what you see depends on how you adjust the scope. In most cases, an 'auto set-up' button is just the starting point.

But anyway, how do you work out what that squiggly line actually means?

The Range of Tasks

At its simplest, you may be using a scope to confirm that a certain pin on the ECU is in fact the speed signal. You jack up the drive wheels, connect the scope's earth, backprobe the pin, and then see what pattern appears on the screen.

If the waveform is a square wave or sine wave, and if its frequency increases with road speed, you've just found the speed sensor. On the other hand, if it is a waveform (of any shape) that increases in frequency with engine speed rather than road speed, it's not the right pin! Furthermore, if the waveform is an odd shape - it has big spikes on it, for example - that's a clue that it probably isn't a speed sensor, which should have a consistent waveform.

At the more complex end of things, you may be monitoring the action of an interceptor, for example one that alters ignition timing.

Assuming that the scope has two inputs (which in other than basic scope designs, will be the case) you might be looking at the input signal (eg from the crank position sensor) on one trace, and the output signal of the interceptor on the other trace. In this case you'd be able to compare the waveform shapes and the phasing (ie the relationship of the peaks and troughs of each waveform to each other).

In between these tasks there are plenty of other uses - but the first step is to understand the trace.

What's the Trace Show?

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This is a screen dump from a Fluke Scopemeter 123. It was made with the scope's negative lead earthed, and the input probe connected to the switched side of an injector of a mid-Eighties BMW 735i.

Let's go through the display step by step.

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The Scopemeter - like most digital scopes - calculates data from the waveform. Here it is showing a frequency of 19.6Hz, that is, this injector is firing just under 20 times a second, or 1176 times a minute.

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The Fluke can also display duty cycle calculated from the waveform, and here it's at 6.6 per cent - appropriate for a car at idle.

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The injector waveform is also displayed. Remember, the waveform is just a graph of injector voltage over time, with time on the horizontal axis and voltage on the vertical axis. So when the line is horizontal, there's no change in measured voltage. When it rises quickly, the injector voltage must be rising quickly.

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Like any graph, you need to look at the scales. Here it is listed at 20 volts/division (and we know that's per vertical division because voltage is measured on the vertical scale) and 20 milliseconds per horizontal division. As we said, voltage on the vertical scale and time on the horizontal scale.

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By reading off the scale we can tell the time between the firings of this injector. Taking the time from the end of the injector firing to end of next time it fires (as arrowed), we can see it's just over 2.5 divisions - say 2.6 divisions. Each division is 20 milliseconds long, so the injector is firing about every (2.6 x 20) = 52 milliseconds, or every 0.052 seconds. We know that frequency is 1/period, so how does that stack up against the 19.6Hz that the Fluke calculated? 1/0.052 = 19.2Hz - pretty close for a manual reading of the waveform!

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But what about the voltage? The injector gets switched open every time it is pulled down to ground by the ECU - that is, one side of the injector is fed 12V all the time and is activated by the ECU connecting the other side to earth. So we'd expect to see a running car voltage of about 13.8 volts dropping to zero as the injector is switched on. These three arrows show just this process happening - the voltage graph dropping from battery voltage to zero.

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But what are these huge spikes that occur when the injector gets turned off? Reading off the vertical scale they're about 3.6 divisions, or 72 volts! Where's this huge voltage come from? It occurs when the magnetic field in the injector coil collapses as the injector is switched off - just like the voltage spike generated by the ignition coils when they're turned off.

So from this frozen screen (and all digital scopes allow you to freeze the on-screen display for easier reading) we've been able to:

  • Read off the injector duty cycle and frequency

  • Calculate the approximate frequency from the waveform (and we could have calculated the duty cycle too)

  • See the shape of the waveform (with the exception of the spike, it's a square wave - the injectors are either on or off)

  • See the collapsing magnetic field voltage spike that many of us wouldn't have even know occurs

Changing the Timebase

However, some of the above was a bit hard to see because there were three injector openings all on the one screen. It would be nice to have only one injector operation shown so it could be examined in more detail. You don't want to change the vertical scaling (or you might get the voltage spike going off-screen) but the horizontal scaling can be altered. This is called changing the time-base.

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Here the time base has been expanded from 20 milliseconds per horizontal division to 500 microseconds per division. (To make this change it's just a case of pressing a time-base button until the image looks right - no calculations are needed.) The vertical scale has been left at 20 volts/division. In this view you can clearly see how quickly the injectors switch on and off and how the voltage spike takes a while to die away. (Note that without multiple injector operations being displayed on the screen, the Scopemeter can no longer calculate frequency and duty cycle - so the numbers up the top are now blank.)

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However, not all injectors are operated in this way. This scope image, taken from the excellent Autonerdz.com site, shows an injector that uses a 'peak and hold' style of injector operation. That is, the current is reduced after the injector has been pulled open to just sufficient to keep the injector open. This Nissan uses a pulse-width modulated technique to do it - that is, during this 'hold' period it pulses the injector current really quickly to reduce the average amount of current flowing. Note the two collapsing field spikes - the first when it switches from full current to 'hold' current and the second when the injector switches right off.

Comparing Traces

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All but the simplest of scopes allow you to display two signals at once. This is idea for comparing the input and output signals of an interceptor, for example. That's what's been done here - the input and output signals from an Xede interceptor are displayed. The signal is from the crank angle sensor on an Impreza WRX - the signal that is intercepted to alter the ignition timing. In this scope image the timing has not been altered. Most important is to look at the fact that the input and output waveforms look identical in shape.

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Here three traces are shown, again originating from an Impreza WRX crank sensor. The top trace shows the original, the middle trace the intercepted output of the Xede interceptor, and the bottom trace the output of a competitor interceptor. As you can clearly see, the competitor interceptor grossly distorts the waveform, however because in this case the ECU is looking only for the point at which the voltage cross the centreline, it can still recognise the timing of the signal. However, obviously there will be specific cars where the distorted lower waveform simply isn't adequate. Without a scope you'd not have any idea why a problem might occur whenever the interceptor is in place...

The Velleman HPS10

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The Velleman HPS10 is one of the cheapest digital scopes around - in Australia it costs just AUD$349 from Jaycar Electronics (cat no QC-1916). In addition to displaying waveforms, the scope can also be used as a simple-to-set-up graphical data-logger.

As you would expect from its price, the Velleman HPS10 is not a high speed, wide bandwidth design. It has a max sampling rate of 10 megasamples/second (repetitive signals) and only 2 megasamples/second for one-shot events. The bandwidth is 2MHz.

Using the Techtronix 'rules of thumb' covered in Part 2 in this series, the highest frequency waveform that can be accurately displayed by this scope is 200 kHz, or a period of 5 micro-seconds. That's certainly not good enough for servicing much radio equipment, but in most applications it's fine for cars. Buffer memory length is 256 samples. The screen is a non-backlit design 64 x 128 pixels. The scope uses five AA-alkaline cells or can be powered by a mains adaptor. Battery life is quoted as "up to" 20 hours.

The scope has a good range of functions and is ideal for someone wanting a basic design. However, it has only one input (ie can display only one signal at a time) and the lack of backlighting of the LCD can be annoying.

But after only a little time to gain familiarity we were able to:

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Display the waveform of an idle air control valve...

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...view the injector waveform (note the lack of voltage spike shown on the far right pulse width - the result of the slower sampling speed of this scope design)...

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...and data-log the airflow meter output voltage. This screen shows 47 seconds of the airflow meter's output, of the 1 minute 31 seconds that was logged. Over that period the maximum voltage was 3.277V and the minimum, -0.32V. (This data is shown on the right of the display.)

We also lent the Velleman to John Nash, head technician at ChipTorque. John normally uses a sophisticated Tie Pie Handyscope 3 in his car work, but he thought the Velleman a very good instrument for its cost.

If you'd like to to occasionally view automotive waveforms and be able to do some data-logging, the Velleman is ideal.

Tektronix [www.tek.com]

Pico Technology [www.picotech.com]

Autonerdz [www.autonerdz.com]

Xede [www.xede.com.au]

Jaycar Electronics [www1.jaycar.com.au]

Thanks to ChipTorque for their help in assembling this article. The Velleman HPS10 was loaned to us by Jaycar Electronics.

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