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ABS Speed Sensor
Air Flow Meter / Sensor
Air Intake Pressure Sensor
Alternator Current and Voltage
Amplifier Earth
Camshaft Sensor
Carbon Canister Solenoid Valve
Crankshaft sensor
Coolant Temperature sensor
Diesel Glow Plugs
Digital ECM to Ignition Amplifier Signal
Distributor Pickup
Dual Trace
Electronic Fuel Pump
Exhaust Gas Recirculation
Idle Speed Control Valve
Injectors
Knock Sensor
Lambda Sensor
MAP Sensor
Primary
Relative Compression
Secondary
Supercharger
Throttle
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Relative Compression - Petrol Diagnostics
(scroll for Diesel Diagnostics)

The purpose of this particular test 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 starter's wiring circuit and the compressions in the cylinders.

The current for a typical 4 cylinder petrol engine is in the region of 80 - 200 amps.

While testing, the initial peak of current (approx 400A) is the current required to overcome the initial friction and intertia to rotate the engine. Once the engine is rotating, the current will drop.

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.

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 it has 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, this 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.

When compression testing on a petrol engine, it is important to fully open the throttle to allow a larger quantity of air to pass into the cylinders.

A typical compression will be between 120 and 200 psi. 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 ensures a tight 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.

It is generally accepted that there should not be more than 25% difference between the highest and lowest compression readings.

A higher than average compression can be caused by :-

Carbon build up within the combustion chamber (reducing its area).

Excessive 'skimming' of the cylinder head.

Incorrect thickness of the head gasket.




Relative Compression - Diesel Diagnostics


The purpose of this test 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 starter's wiring circuit and the compressions in the cylinders.

The current for a typical 4 cylinder diesel engine is in the region of 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.

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, this 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 between 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 ensures a tight 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.

It is generally accepted that there should not be more than 25% difference between the highest and lowest compression readings.