I've seen relative compression testing using voltage in trade magazines for awhile, but was reminded of it recently when Andy MacFadyen brought it up in another thread. I normally go straight to the amp probe for this test, but I can think of a few times when getting the probe in place was problematic. Why not experiment a bit with a new technique?
2000 Honda CR-V comes in with a flashing MIL and a noticeable miss at idle. Cranking the engine produces that noticeable 'skip' in the cranking cadence. Ruh roh, this won't end well.
Andy describes using voltage drop across the starter positive cable in his method, which is totally fine. In this example, I opted to try connecting the scope to the battery directly, then switching to AC coupling. I've seen enough starter voltage waveforms to know that I should see something here. Throw in an ignition trace on #1, crank it, and we get this:
Kinda a mess, I know, and I should have rescaled the ignition trace. Let's zoom in a bit:
Okay, now I can make something of this. The firing of #1 lines up too perfectly with the first positive voltage tower, so I'm associating higher voltage with more compression. Then we have a short tower, a strong one, and a meh one before starting over. I'm ready to say we have at least one weak cylinder.
The firing order is 1-3-4-2, which makes the weak cylinder #3. Going in cylinder backs up the relative compression results. Yay!
Nooooope, 70ish PSI does not cut it. Here's the same cylinder running:
The asymetrical tower, the low vacuum in the decompression pocket, and the exhaust valve opening happening too soon says leaking exhaust valve.
The static compression numbers were #1 at 140, #2 at 105, #3 at 75, and #4 at 140. All in all, the compression towers in the relative compression waveform corresponded nicely.
Thoughts/comments/complaints? Post 'em!
[Last edited Apr 11, 2016 01:00:09]