Bo’s Blog – Favorite Diagnostic Tool

January 11, 2016

One of my favorite diagnostic tools to ever be built into an engine management system would have to be the Cylinder Contribution test (power balance for gas engines) used by Ford. If you have spent some time on forums, or in a shop, you may have seen it. A graph with a bar for each cylinder, you fire up the engine and see the line start moving up and down across the cylinders. But what does it mean? To the trained eye, this means you just saved a good hours work trying to isolate a problem cylinder. The graph shows you, right there which cylinder(s) is/are not contributing. Before the computer based cylinder contribution test, you would fire up the engine, and either ground out a spark plug, or unplug an injector; one at a time, and measure the RPM drop for each cylinder you disabled. Manually doing this on a v8 engine is extremely tedious, and can be inconclusive. I guess it is all relative, but I would much rather plug in an IDS or Autoenginuity, and within 5 minutes be ready to hook up a compression gauge to do to my next test.

Now that we know why we use the contribution test, let’s talk about how it does it. The PCM is always looking at the signal from the crankshaft position sensor (CKP), or camshaft position sensor (CMP). This is a permanent magnet-type sensor, meaning it actually creates an a/c voltage that the computer uses to interpret crankshaft position, crank speed, and crank acceleration. It is the crankshaft acceleration that makes the cylinder contribution test possible. When the PCM is reading the signal from the CKP, it can look at the amplitude of the signal from each individual cylinder. When the amplitude drops on an individual cylinder, the computer can assign a percent value to that and spit it out on the graph on our laptop screen. When you have a cylinder that is not firing due to an injector problem, this will show on the graph as little-to-no contribution. It is important to realize that the contribution test only tells us if each cylinder is firing. It does not tell us why it is not firing! You still have to figure out if you are losing fuel or compression in the cylinder.