CC:

Look at this engine. Now look at this one and this one. Think to yourself, how would you get to the spark plugs to do a compression test with a gauge like this? For some vehicles, it’s going to be a nightmare. It’s very labor intensive, but is there a way to get the same information to diagnose an issue with compression on the engine or even a misfire without having to do all that tear down? Well, it’s actually called relative compression testing. You’re taking the current draw from the starter on the battery and turning that into a waveform on a scope. When you turn on this waveform, you see current being drawn into the different cylinders by the starter. If there’s an issue with compression, it’s going to be a lower peak on the waveform. Also, you’ll notice that if one cylinder has a misfire or an issue or a waveform that looks like this, it also affects the previous and sometimes the next cylinder because that momentum of the flywheel has to build up again because the engine’s misfiring on that certain cylinder.

With relative compression testing, you just need a scope, an amp clamp or amp probe, and you can measure this type of waveform. With this, you can compare cylinder to cylinder. You can even sync up the pattern with the number one spark plug. This way, you can know which cylinder has issues. Relative compression testing, it may take only two minutes compared to maybe an hour or two if you use conventional compression gauge. I’m Andrew Markel. Thank you very much.

This video is sponsored by Auto Value and Bumper to Bumper.