As automotive technology marches relentlessly on, the demand for equally advanced test equipment and measurement techniques has increased in direct proportion.
PicoScope oscilloscopes sit alongside any Scan tool to support the diagnostic route taken by a technician to an accurate and cost effective conclusion.
Integrating the oscilloscope throughout the diagnostic journey requires yet another new skill set for the technician to grasp but the rewards pay dividends once the skills are mastered.
Whilst PicoScope and the associated software remain an essential diagnostic tool, applying the relevant accessories to any PicoScope opens up another set of applications that were once reserved to ‘gauge sets’ or ‘pressure kits’.
The Pico pressure transducer converts pressure values to voltage which are then relayed to the PicoScope, allowing pressure to be displayed against time.
The pressure transducer, with an array of adaptors, has the potential to not only replace the wide range of pressure gauges found in a typical workshop but also reveal infinite detail about the transitions in pressure that were never visible with typical mechanical gauges.
The compression test is one such example where using the pressure transducer coupled to the PicoScope will reveal far more than the maximum compression pressure.
Connecting the pressure transducer to a compression hose will allow for a compression measurement to be taken identical to the typical Compression tester, but with the results displayed on the scope screen rather than reading from a mechanical gauge.
The waveform illustrated here reveals compression peaks at 170 psi, as would a typical compression tester. However, we can now see repeated, even compression peaks as the crankshaft rotates and more importantly, events taking place between compressions that could never be visible with the compression tester.
Using PicoScope we can equally divide the distance between compression events to reveal the position of the crankshaft using phase markers.
If you know the position of the crankshaft, you can identify each of the four stroke cycles between compressions.
Looking at the base of each compression peak, you’ll notice an ‘expansion pocket’ dropping below the zero psi ruler indicting the cylinder pressure to momentarily drop ‘negative’ (vacuum).
This indicates adequate sealing of both intake and exhaust valves that remain closed as the piston descends down the cylinder towards the end of the compression stroke.
Using the time rulers we can also measure the time taken for the crankshaft to rotate 360º and multiply this by 60 to reveal the cranking speed, 278 RPM.
As the piston rises from BDC of the power stroke, the exhaust valve will open to release the cylinder pressure out to the atmosphere via the exhaust system.
The waveform indicates a pressure increase to around 125 psi during the exhaust stroke because the exhaust valve is not opening.
A typical compression tester cannot detect this condition and would read a normal compression value, leading the technician away from the fault.
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