Oscilloscope Basics: Setting Voltage, Timebase & Triggers Andy Crook
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.
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.
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
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.
SCOPING
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
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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
