PERFORMING RELATIVE COMPRESSION WITHOUT AN AMP CLAMP

The compression test is meant to be the first check. If the compression looks good, you can quickly move on to investigate other potential causes. If the test detects a low cylinder, you should perform a manual compression test to verify the data and determine the faulty cylinder.

With a relative compression test, you can easily determine if all the cylinders in an engine have equal compression. This test is a quick check on compression that you can do without removing the spark plugs or glow plugs. It uses the starter motor current to determine the comparative compression values on all cylinders. The advantages of doing this test are that you do not need a pressure transducer/sensor to run the test, all you need is a scope and a current clamp and that you do not have to check each individual cylinder, all cylinders can be tested in one go.

The theory behind the relative compression test is that the amount of current required to turn over the engine while a cylinder is on its compression stroke will indicate how much the cylinder is compressing or leaking. This will information makes sense when you compare all the cylinders, as you can easily see if one is weaker than the others.

It is important to remember to prepare the vehicle for testing so that the engine will not start during cranking.


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