In working with a marine surveyor on a boat I'm buying (long story, different thread) I ran into something I've never heard of before that's pretty nifty. He uses an oscilloscope to do Relative Compression Testing. Instead of pulling plugs and hooking up a gauge, he hooks an ammeter around the starter lead, pulls the ignition fuse, and spins the engine over. The starter draw peaks with each piston compression and the resulting graph is analogous to cylinder compression. You don't get actual PSI but you can tell what the cyl-cyl variation % is. Has it's limits but proven to be accurate when back checked with a gauge.
That's pretty cool.
nifty!
Apparently you can do it like Saab too and test resistance across the spark plug gap since the air resistance will change with compression. Seems like that might be pretty hard to control across cylinders though with no 2 spark plugs ever gapped exactly the same.
Opti
Dork
7/23/21 1:19 p.m.
Old timers in the auto industry can tell you all about the cool stuff they could check with scopes back when they were a more commonly used tool. Especially in the tune up business.
Opti said:
Old timers in the auto industry can tell you all about the cool stuff they could check with scopes back when they were a more commonly used tool. Especially in the tune up business.
Funny enough this same approach is now a feature on some diagnostic scan tools.
The first time I saw someone do it during a demo on a launch I was really impressed.
Go find some Bernie Thompson videos.
The Mercedes has a diagnostic port that you can hook up to an oscilloscope to troubleshoot ignition. Think you get a duty cycle for each cylinder firing. The way I understood it was if something is wrong with the mixture or compression it will affect the spark somehow and you can read it using diagnostic port. Never done it myself.
