I remember chasing a Chrysler minivan that had a elusive miss when it was at idle. When the engine was idling, I could barely notice it. But as soon as a throttle opened up, everything went back to normal. After checking the fuel and ignition systems, I began to examine the engine mechanically. You can get the best automotive lab scope in this sites.
The vacuum gauge showed nothing so I decided to do a compression test on the transverse V6. It was confirmed that the front rear bank cylinder had a low reading. Next, I performed a leakdown test on the cylinder to confirm that it was indeed leaking.
There were two problems. It was a two-fold problem. It took me a long time to perform the leakdown test and the compression test. This is because many transverse engine types are difficult to access. This vehicle had low mileage but was still under warranty. So the customer did not have to pay.
Was he going to do? Would I be able to justify 2 hours and more to run these tests, if the results were good? It was only after I found out that I could easily perform a relative compress test using a digital oscilloscope, and this in just minutes.
When I diagnose a driveability issue, this test is among the first ones that I run. To eliminate quickly the possibility of an engne failure, I will perform this test.
Starting Draw with Relative compression
My first ever method was starter draw. Consider this for a second…it requires current to rotate the starter. The more resistance that the starter must overcome to move, the higher its current draw. When we were young, technicians learned how to calculate starter draws when testing a system. However, when the test is run using a Scope, which can graph current as well as adjust time base, the results are more accurate.
For this test, place your high-amp clamp on the positive battery wire leading to the starter. Start your scope and set the time base at 100ms. If you want to make the pattern more clear, you can fine-tune your adjustments. As you would when performing a normal compression test, make sure that your battery is good and you disable the fuel or ignition system.