Dynamic Compression Test Using DSO Function (CTT 00-07-01)

DYNAMIC COMPRESSION TEST USING DSO FUNCTION

TECH TIPS

SERVICE INFORMATION

All Models, Dynamic Compression Test using DSO Function 

A dynamic cylinder balance test can be completed using the DSO function. This test can be used to detect low compression in a cylinder as well as any leaks that may not be picked up by a leak down test. Procedure is as follows:

• Hook up the 500A inductive clamp to the starter cable
• Change the time scale to 0.1 s/div.
• Remove the ECM power relay and make sure the battery is fully charged
• Operate the starter to capture the wave form

  Analysis: What you see is a series of peaks representing the starter current draw. Each peak is the starter amperage increasing in magnitude as the piston comes up on the compression stroke.

  More Amperage = More Effort.

  What you want to see on a good engine is even and uniform peaks. If there is a leak on a cylinder there will be a dropout of one of the peaks. If all the peaks are missing, suspect all bad cylinders or check the DSO cables. To find the offending cylinder, remove glow plug or sparkplug in one cylinder and repeat the test. Remember that the peaks appear on the DSO in the firing order of the engine.