Relative compression - gasoline

The purpose of this test is to compare engine cylinder condition by observing the electrical current drawn by the starter motor during engine cranking.

How to perform the test

View connection guidance notes.

1. Disable the vehicle’s fuel injection.
2. Connect the high amp clamp to PicoScope Channel A.
3. Select the highest range and zero the clamp.
4. Connect the clamp to either the positive or negative battery lead.
5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
6. Start the scope.
7. Crank the engine for around 5 s to capture your waveform.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Notes

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.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• An initial high circuit current, around 450 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 90 A, the peaks around 125 A, and there are no repetitive anomalies.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Relative compression current.

Further guidance

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:

• Intake or exhaust valve timing or sealing faults.
• Piston, bore and/or compression ring faults.
• Blocked air filter or intake.
• Blocked exhaust.
• Turbo faults.
• Cylinder bore washing.
• Engine lubrication problems.

In-cylinder pressure testing can be used to confirm compression issues with engine cranking or running conditions.

GT004-EN