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Relative Compression – Loss Of Compression Waveform Notes
The purpose of this particular waveform is two fold:
- To measure the amperage required to crank the engine
- To evaluate the relative compressions
The amperage required to crank the engine will largely depend on many factors,these include: the capacity of the engine, the number of cylinders, the viscosity of the oil, the condition of the starter motor, the condition of the starters wiring circuit and the compressions in the cylinders.
Once the engine is rotating, the current for a typical 4 cylinder petrol engine is in the region of 80 - 200 amps. As can be seen from the initial peak of current drawn, the current required by the starter motor to begin to rotate the engine from rest can be two or three times higher than when rotating.
The compressions can be compared against each other by monitoring the current required to push each cylinder up on its compression stroke. The better the compression the higher the current demand and vice versa. It is therefore important that the current draw on each cylinder is equal.
As can be seen in this four cylinder example, the compression drawn on one cylinder is much lower than the others suggesting a serious mechanical failure.
NOTE: when compression testing a petrol engine it is advisable to isolate the ignition primary circuit to avoid damage to the electronic circuitry.
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