Relative compression - diesel

The purpose of this test is to evaluate the performance of each cylinder based on the electrical load placed on the starter motor during the compression stroke.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

​​Note: You will need to disable the fuel supply to the injectors to allow the engine to crank sufficiently to obtain a suitable pattern for examination.

  1. Connect the high amp current clamp into Channel A. on your PicoScope, switch on and zero the current clamp.
  2. Position the current clamp onto one of the battery leads (+) or (-).
  3. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  4. Start the scope to see live data.
  5. Crank the engine and capture your waveform.
  6. Stop the scope.
  7. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

 Note; the orientation of the current clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Waveform notes

The waveform (Example 1) shows the current draw at the starter motor whilst the motor is turning the engine.

The initial high current spike is what is required to overcome static friction. This drops to the cranking level and then each peak in the sawtooth section of the waveform represents a piston compression stroke.

It is the comparative height of the peaks that indicates the relative compression of each cylinder.

The second waveform example (Example 2) shows the complete loss of compression on one cylinder.

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

Given the inherent difficult access for pressure testing on most diesel engines this PicoScope test is a very useful tool to help decisions to be made regarding further time consuming work.

It is essential to the running of the engine that it has sufficient compression. The compression provided by the rising piston will be determined by the swept volume being compressed into the combustion area, the compression ratio. The compression is also determined by the effectiveness of the seal between the cylinder wall and the piston, this seal is maintained by the piston rings. The same applies to the seating of both the inlet and exhaust valves.

Piston rings are made of centrifugally spun cast iron, which produces a radial pressure, forming the seal. Cast iron is also used for its excellent self lubricating properties.

If a relative compression waveform highlights a problem, it will be necessary to perform a compression test. A typical diesel compression can range from 19 bar (275 psi) to 34 bar (495 psi). This pressure tends to be slightly lower on vehicles with indirect injection and higher on direct injection systems. The diesel engine relies upon compression to generate the heat which is required to ignite the atomised fuel. Any reduction in compression will reduce the heat generated and compromise the combustion process. A low compression will eventually result in the cylinder not firing. It is therefore imperative that the valve clearances (when adjustable) are set to manufacturer's specifications.

A low compression can be caused by:

  • An ineffective seal between the cylinder and the piston.
  • Poor seating of the inlet and exhaust valves.
  • Broken or seized piston rings.
  • Incorrect camshaft timing.
  • An obstructed induction tract.

All readings should be similar. If one is lower than the others a wet test can be performed by squirting a small amount of oil into the cylinder and re-testing the compression. The inclusion of the oil improves the seal between the piston and the bore, so if the compression is regained the fault lies within the piston rings. If very little difference has been made, the fault lies within the valves.

There should not be more than 25% difference between the highest and lowest compression readings.

The amperage required to crank the engine depends on many factors including, engine capacity, number of cylinders, oil viscosity, starter and circuit condition and of course the compression in the cylinders.

The current for a typical 4 cylinder diesel engine is about 200 to 300 amps. The compressions can be compared against each other by monitoring the current required to push each cylinder up on its compression stroke. It is therefore important that the current draw on each cylinder is equal.


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.

Suitable accessories

  • 200 A / 2000 A (high amps) DC current clamp


Help us improve our tests

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.

1 comment | Add comment

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.

Your email address will not be published. Required fields are marked *

Guided test: Relative compression - diesel