The purpose of this test is to compare engine cylinder condition by observing the electrical current drawn by the starter motor during engine cranking.
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:
An initial high circuit current, around 700 A, at the start of cranking.
The current falls and stabilises to form a uniformly repeating sawtooth pattern, as the engine continues to crank.
The sawtooth pattern troughs are around 190 A, the peaks around 315 A, and there are no repetitive anomalies.
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.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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April 01 2019
Where do I find the sensors library? I want to setup the current clamp I have 2000 A to do a test and do not know how.