HOW IS THE COMPRESSION TEST PERFORMED?

Compression test
Compression test

Compression testing can be done in several different ways.

PRESSURE READING FROM THE Spark Plugs: After removing the spark plugs, a pressure gauge 7 is connected and the starter is started. This process is repeated for all spark plugs and the differences are compared.

It is not useful in new technology vehicles as it is difficult to reach and remove the spark plugs.

CYLINDER COMPARISON WITH VOLTAGE / CURRENT: Cylinder comparison is made with an oscilloscope. This method is done in several different ways.
Voltage reading method: If all cylinders are in good condition while cranking, each cylinder should drop the battery voltage equally during cranking. If one or more cylinders are weak, the voltage drop of that cylinder will be low during cranking. Therefore, if the battery voltage is carefully read and recorded while cranking, weak cylinders can be detected. However, it cannot be determined how many cylinders are weak.
Synchronized voltage reading method: In the voltage method, if a connection is made to the spark plug cable or to one of the cables going to that cylinder to receive a trigger signal, then both the cylinder comparison process is performed and the number of cylinder is determined to be weak.
Current reading method: If all cylinders are in good condition while cranking, each cylinder should draw an equal amount of current from the battery during cranking. If one or more of the cylinders are weak, those cylinders will draw less current during cranking. Therefore, if the current drawn from the battery is carefully read and recorded while cranking, weak cylinders can be detected. However, it cannot be determined how many cylinders are weak.
Synchronized current reading method: In the current method, if a connection is made to the spark plug cable or to one of the cables going to that cylinder to receive a trigger signal, then both the cylinder comparison process is performed and the number of cylinders weak is determined.
CYLINDER COMPARISON WITH PRESSURE: If all the cylinders are in good condition when the starter is pressed, the gas pressure coming out of the exhaust pipe of the vehicle is always the same value. If some cylinders are weak, the pressure of the exhaust gas will be low during the ignition of those cylinders. Therefore, when the exhaust gas pressure is read with a very sensitive sensor, it will be possible to compare the cylinders with each other.

However, since the use of an oscilloscope and the evaluation of the curves on the screen by receiving the trigger signal require special training, some companies have manufactured devices that only display the cylinder values ​​on the screen as %, while some companies have produced devices that display both the oscilloscope screen and the cylinder values ​​in %.

However, the most important thing to know about these devices and during testing is the following;

These devices accept the cylinder with the highest compression as the best cylinder, ie 100%, and compare other cylinders with this cylinder.

Although it is a very difficult possibility, if the compression of all the cylinders in a vehicle is low, the device will not know this and will only compare the cylinders with each other.