For years, I was amazed at how few technicians I worked with had ever used a scope, let alone owned one of their own. In every shop I’ve worked in over the last 20 years, I was it. But I’ve been pleased to see that scope use is on the rise, as evidenced by the number of techs who raise their hands in the presentations I make around the country and the growth of online support groups – even on Facebook! And no matter the reason for the growth, I’m glad that more and more of you are seeing just how valuable this diagnostic tool can be. Here are a few of my favorite uses for the scope. I hope you find them helpful.
As a note before we get started, I am going to share the scope settings I use. I admit I am no “guru” in scope use and learn continuously from those that are. I’m sure there are many of you who can offer advice on even more efficient ways of setting up and performing these tests, and I hope you will – in the comments section on MotorAge.com!
The relative compression test
Many years ago, I was tracking down the cause of a slight misfire on a Chrysler minivan V6. After checking fuel and ignition, I found myself having to dive deeper into the mechanical health of the engine. All of you know how time consuming performing a conventional cranking compression test on a transverse V6 can be, and I also checked cylinder leak down on all six at the same time! Needless to say, I spent a few hours on the job that I never did get paid for.
The relative compression test is a way of assessing the overall health of the engine with a lot less trouble and in a matter of minutes, instead of hours. It is one test I perform on every engine that I find myself troubleshooting a drivability concern on.
Channel 1 – High current clamp attached to either negative or positive battery cable. Scale set to read to at least 600 amps to capture initial inrush current expected.
Channel 2 – Standard lead or secondary clamp attached to cylinder ignition source (coil or secondary lead). Choose cylinder that is easiest to access. Adjust voltage scale to match connection and be careful to use attenuator if needed.
Time base – 500 milliseconds per division OR 5 seconds total screen time
Trigger – Single capture with trigger set to rising slope and +1 amp.
These settings allow me to capture all the info I need on one screen, and to do so without worrying about starting or stopping the scope manually. As soon as I turn the key “on,” the scope will begin to capture data. Figure 1 shows an example of what you can expect to see.
The green trace is the current pattern captured. The focus is on the repetitive sawtooth pattern, where each peak represents the amount of current it took to push a cylinder through its compression stroke. Even on my old UEI scope, I could distinguish as little as a 10 percent drop in an individual cylinder’s compression using this method. In this capture, the cylinders are uniform in appearance. But remember, this is a “relative” view, relative to all the other cylinders. If there is a mechanical issue that is causing low compression across the board, you’ll see a very similar picture, with only one noticeable difference – the current levels of the peaks will be lower than you’re used to seeing on a healthy engine. I can also see that ignition timing appears to be OK, since the ignition reference is intersecting the current pattern on, or just before, the peak. Spark occurs just before TDC, right?
Take a look at Figure 2 for an example of a “known bad” capture. Though hiding behind the ignition reference, it is clear that the peak for that cylinder is not reaching the same level as the others. By following the firing order as shown in Figure 3, I know which cylinder I need to take a closer look at. With that information in hand, I have no problem going back to the customer and asking for the additional time I’m going to spend isolating the exact cause.