Relative compression - diesel

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

PicoScope and PicoDiagnostics

Although it is possible to measure relative compression with PicoScope, it is easier in most cases to use the Relative Compression test built into PicoDiagnostics. PicoDiagnostics is supplied with all our diagnostics kits.

How to perform the test

Plug the current clamp into Channel A.

The clamp should be positioned on one of the two battery connections (live or earth), whichever allows the easiest connection, as illustrated in Figure 1.

The current clamp needs to be switched on and facing the correct way and set to its higher range if applicable. There is an arrow pointing to the battery positive (+) on one side and an arrow pointing to the battery negative (-) on the other side. Incorrect connection will lead to an inverted oscilloscope picture. It can be seen from the example waveform that the current drawn on a diesel engine is considerably higher than that of petrol engine.

With the example waveform displayed on the screen, you can now hit the space bar to start looking at live readings.

Slight adjustment to the time base (ms/division) may be required to compensate for faster or slower cranking speeds.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.

Example waveform

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 including: the capacity of the engine, the number of cylinders, the viscosity of the oil, the condition of the starter motor, the condition of the starter's wiring circuit and the compressions 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. 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.

This test is only a comparison against each cylinder and is not a substitute for a physical compression test with a suitable gauge. Due to inaccessibility on a diesel engine, this test can be extremely useful when diagnosing problems on the compression/ignition diesel engine.

NOTE: When conducting compression tests on a diesel engine ensure that the appropriate gauge is used (diesels have much higher compression than petrol engines). Also make certain that the fuel supply to the injectors is stopped by electrically isolating the fuel cut-off solenoid.

The waveform below shows an engine with a loss of compression in one cylinder.

NOTE: When compression testing a petrol engine, it is advisable to isolate the ignition primary circuit to avoid stray high tension (HT) voltage damaging the electronic circuitry.

Technical information

It is essential to the running of the engine that there is sufficient compression. The compression provided by the rising piston will be determined by the swept area being compressed into the combustion area: this is called the compression ratio. The compression is also determined by the effectiveness of the seal between the cylinder's 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:

  1. An ineffective seal between the cylinder and the piston
  2. Poor seating of the inlet and exhaust valves
  3. Broken or seized piston rings
  4. Incorrect camshaft timing
  5. 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 ensures a tight seal between the piston and the bore, so if the compression is regained the fault lies within the piston rings, but 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.


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


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Guided test: Diesel Compression