How to Electronically Modify Your Car, Part 11

Measuring analog signals is straightforward - you cannot damage any sensor by measuring its output with a digital multimeter.  About the only thing to be wary of is when signals are changing rapidly - many multimeters won't be able to keep up and so you may not get a true reading. The more expensive the meter, usually the better it is at dealing with rapidly changing analog signals. 

Measuring Digital Signals

Measuring digital signals can be much more difficult than measuring analog signals. This is because, if you are not sure what sort of signal is present, it is easy to be deceived by the multimeter's reading. 

Let's have a look at this idea in more detail. 

As we described in How to Electronically Modify Your Car, Part 10, and as shown in this diagram, a digital signal is usually either on or off. When it is ‘on', it might have a DC voltage of 5V, when it is ‘off', it has a voltage of zero. Since it is a varying voltage, we initially set the meter to read ‘Volts DC'. 

If we want to measure the signal's frequency, a ‘Hz' button is then pressed on the meter. If we want to measure duty cycle, a ‘%' button is pressed. 

(Depending on the model of multimeter, there may be some minor variations in how the meter settings are configured.)

That all sounds straightforward - and often it is. For example, measuring injector duty cycle is just a case of:

So where's the problem with measuring digital signals? OK, what if you are trying to find a signal but you don't know whether it's analog or digital?

A digital signal that uses a high frequency and variable duty cycle can easily be mis-read as a changing analog voltage. That's because the meter will average the voltage, and if it's going on and off at high frequency with (say) 75 per cent duty cycle, a 12V feed will be read as (0.75 x 12) = 9V. At 50 per cent duty cycle it will ‘look like' a 6V analog signal. 

A digital signal that uses a 50 per cent duty cycle but varies in frequency will ‘look like' a fixed voltage! That last one can be very confusing - it's easy to think: how come this sensor never changes its output?

You can of course for every single reading switch the multimeter to voltage, and then frequency, and then duty cycle, but that is laborious and unwieldy. 

When working with digital signals, what you really need is an instrument that ‘draws a graph' of the way the signal varies over time. That way, you can see if it's in fact a digital signal, and if it is, you can then see what happens to its frequency and duty cycle in different conditions. You can also see the shape of the signal waveform. Such a measuring instrument exists - it's called an oscilloscope. 

Oscilloscopes

Oscilloscopes come in many different types but for car use, a handheld digital scope is the one to get. These scopes are now available from about AUD$350. 

If you aren't doing much electronic car modification, you can certainly get away without having a scope - just be careful when using a multimeter to measure digital signals. However, if you are doing a lot of modifications, and/or working across a range of cars, a scope can save an enormous number of headaches. 

In the past, scopes could be complex and difficult, but these days they're pretty simple to operate. 

A ‘scope screen consists of X and Y axes, just like a graph. The vertical axis is calibrated in voltage, and the horizontal axis in time.  

So for example, each vertical division might be 1 volt, and each horizontal division 1 second. By changing the ‘volts per division' you can alter the vertical scale, and by changing the ‘time-base', you can alter the horizontal scale. You connect the scope to the sensor, just as you would a multimeter (ie the negative lead of the scope to ground, the positive probe of the multimeter to the sensor signal output.)

If connected to a sensor, the scope might show this type of display. You read it just like a graph - the signal starts at 2V, takes a few seconds to rise to 6V, then declines to 4 volts in another second - and so on. This is an analog signal slowly varying, without any repetitious pattern occurring. The shape of the signal is the sort you might get from a throttle position sensor with the accelerator pedal being slowly moved.

This scope display shows a very different type of signal - a digital signal. The maximum voltage that is reached is 5V and the lowest is 0V, so the signal has a peak-to-peak voltage of 5V. The 5V is reached every 2 seconds, so its frequency is 0.5Hz. The length of ‘on' (5V) and ‘off' (0V) times are the same so it has a 50 per cent duty cycle. 

Let's have a look now at some real scope screen grabs. As you'll soon see, many signals that you can measure with   a scope aren't just regular, nicely shaped signals like we've shown above!

This is a screen dump from a Fluke Scopemeter 123. It was made with the scope's negative lead grounded, and the input probe connected to the switched side of a fuel injector of a mid-Eighties BMW 735i. (Note that the purple line shows zero volts.)

Let's go through the display step by step. 

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.

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 that uses fairly small injectors for its power.

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.

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.

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. 

But what about the voltage? One side of the injector is fed 12V all the time, and it is turned on by the ECU connecting the other side to ground. (That is, the ECU just acts as a switch.) The scope is connected as shown.

From the above circuit, 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.

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 spark ignition coils when they're turned off. 

However, because there were three injector openings all shown on the one screen, some details were a little hard to see. 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. 

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.)

Conclusion

It's very important that you are able to measure signals, both analog and digital. 

In by far the majority of cases, a relatively cheap, good quality multimeter will do a great job. You'll be able to accurately measure analog signals and also often measure the frequency and duty cycle of digital signals. However, when you're working in the dark as to the type of signals that you're measuring, or if it's important that you can see the shape of a digital or analog signal, a scope is the tool to use. 

Next week we'll look at some of the issues in intercepting and changing analog and digital signals.