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Oscilloscope Basics: Setting Voltage, Timebase & Triggers

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Andy Crook describes how he set up an oscilloscope to capture the supply voltage and the camshaft sensor signal on a faulty BMW X5 in his two new technical videos, available on Autotechnician’s YouTube channel, and provides an overview of his presentations at the Big Weekend training event in Warwick this November.

A BMW X5 has an intermittent non-start fault, the only Diagnostic Trouble Code (DTC’s) stored relates to the camshaft sensor voltage.

Using the relevant wiring diagram, we have established that the sensor has 3 pins:

Pin 1 Supply Voltage

Pin 2 Signal

Pin 3 Sensor Ground

It should also be noted that power for the sensor is supplied via the main engine management relay. This relay also supplies the following;

• EGR solenoid
• Mass Airflow Sensor
• Engine Mounting Solenoid
• Turbocharger variable vane actuator • Glow Plug Control Module.

No DTC’s have been stored for any of these components. Therefore, the relay can be ruled out of our enquiries, but we still need to check the supply voltage at the sensor, the signal output and the ground.

Tool selection…

While measuring the supply and ground circuits using a multimeter is possible, the camshaft sensor signal is not, as demonstrated in the video. It displays the RMS Voltage, (Root Mean Square) or average voltage during the sampling period.

A multimeter is not capable of measuring and displaying this signal accurately enough for detailed analysis. This is why a Digital Storage Oscilloscope (DSO) is used. During the video and Big Weekend event in November, I will be using the latest Pico 4425 and the new Pico 7 automotive Beta software to demonstrate how to set up an oscilloscope to capture and analyse waveforms.

Basic oscilloscope set-up…

The two videos show how I set up the oscilloscope to capture the supply voltage and the camshaft sensor signal. Setting up the oscilloscope is the first skill that any technician needs to master – it is essential that the displayed waveform can be analysed quickly and accurately.

To test the sensor the technician could use channel A for the supply Voltage and Channel B for the Camshaft sensor signal – the sensor ground once tested can be used as the measurement ground.

This has two advantages:

1. It reduces the amount of noise on the captured waveforms

2. It keeps a channel free if required

Voltage selection…

In this example, the expected voltage on channel A (blue trace) is Nominal Battery Voltage (NBV), a fancy term used by technicians meaning the same as the battery voltage. This could range between 9 Volts during cranking and could go as high as 18+ Volts if there is a charging fault.

Channel B (red trace) is the camshaft sensor output signal, checking data sources suggests the output will be a digital signal alternating between close to NBV and Ground (just in time learning).

It should be noted that some sensors output different voltages, and the technician should always check before testing. So, in this case, both channels use the same 20 Volt scale.

Hint:

The default setting places the waveform in the centre of the screen, with equal positive and negative voltage ranges. In this case, -10 V to + 10 V. (20 V). However, we know our supply and signal should be Direct Current (DC) so we can change the scale to x2 to display more of the positive voltage on the screen.

You need to move the zero line down to display an appropriate Voltage range, I choose -2 V to 18 V (20 V). It is not good practice to place the zero line at the bottom of the screen.

Now the waveform fills more of the screen making it easier to analyse.

Timebase…

The timebase is shared with all measurements which can make selecting the best timebase tricky.

In this case, we need to observe the supply Voltage which should change slowly from cranking voltage to the charging voltage, against the camshaft sensor signal which changes relatively quickly.

So for the initial captures, we selected 200ms/div.

200ms is 200 milliseconds or 0.2 of a second. There are 10 divisions so each screen displays 2 seconds of data. The storage buffer allows more than 2 seconds to be captured for analysis depending on the sample rate. (the number of data points measured per second)

To view more detail of a single event, reduce the time base. To view more events with less detail, increase the time base.

Hint:

The Pico software allows the technician to zoom in from a longer time base (a wider view), it has a very high sample rate which allows the technician to zoom in without losing resolution. Other DSO’s may operate differently, always check the manual.

Basic triggers…

Triggers ensure that the oscilloscope captures the waveform at the right time and place while keeping the waveform in a stable position on the screen. The trigger function of an oscilloscope monitors incoming signals and decides when to begin capturing the signal. Depending on the trigger condition that you set, the scope may trigger when the signal crosses a given threshold or may wait until a more complex condition is satisfied.

A basic trigger only requires the signal pass through the trigger point – called the trigger event. If it crosses the threshold from a higher value to a lower value it is called a falling slope trigger, if it crosses the threshold from a lower value to a higher value it is termed a raising slope trigger. The technician can choose when to start displaying the image by moving the trigger point across the screen or the pre trigger % button.

Hint: Using the Auto trigger Function

Most Auto trigger functions capture data even if there is no trigger event within a reasonable time.

Single or repeat triggers usually require the trigger event to start capturing the signal. This can lead to confusion, or even suspecting components are at fault, so it is often best to start with no trigger and decide if the function is required after capturing some data.

At Autotechnician’s Big Weekend, Andy will be expanding further on what you have seen here with something for everyone, from the novice to experienced scope user.

The first session will focus on why oscilloscopes are required, taking familiar components and systems and demonstrating the benefits of using an oscilloscope when diagnosing circuit faults. Andy will combine simple tests and measurement techniques that can be used to pinpoint circuit or component failures.

The second session will focus on math channels. Andy will prove that understanding the circuit you are measuring will reduce diagnostic time. Or as he likes to put it, ‘More craft equals less graft!’

Search for Autotechnician magazine on www.youtube.com or visit the channel directly on www.youtube.com/channel/UCVpRBsEJ89t3Tfh8JrsBOxw

One and two-day tickets to the Big Weekend event in Warwick are available now at subsidised rates at: www.autotechnician.co.uk/training

 

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