The purpose of this test is to compare engine cylinder condition by observing the electrical current drawn by the starter motor during engine cranking.
If your engine has a cable operated intake throttle, depress and hold the accelerator pedal during cranking to maximise the air volume drawn in to the engine.
The orientation of the clamp relative to the wire will determine whether it has a positive or negative output. If a waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.
This known good waveform has the following characteristics:
The test uses the following property of a Direct Current (DC) starter motor to provide a convenient, non-invasive comparison of engine cylinder compression:
With a fixed supply voltage, the current drawn by a DC starter motor is proportional to the load on the motor; when the load increases the current increases.
During engine cranking, both the starter motor’s load and current increase as each piston travels through its compression stroke. Therefore, a continuous measurement of starter motor current indicates the relative degree of compression across the cylinders as they cycle through in their firing order.
Ideally, the cylinder compressions and their associated current draw would be equal. In reality, there will be some slight variation. Any repetitive low peak in the waveform indicates a cylinder with low compression and provides justification for further investigation.
Compression related issues can cause poor running, misfire and emissions related symptoms. Typical faults that cause compression issues are:
In-cylinder pressure testing can be used to confirm compression issues with engine cranking or running conditions.
We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.
February 18 2019
If you work alone and you have to get in to the car and back to your Computer all the time, its better to set a “single” shot trigger. Otherwise your waveform will be overwritten by the time you come back to stop the measurement. Default setting was the repeating trigger, in my opinion not the best choice for this test.