How do I do a relative compression test with a high-amp current probe?
Short answer: Disable the engine from starting, clamp the probe around the battery cable, and crank while recording; even current humps mean even compression, and a low hump points to a weak cylinder.
When the engine cranks, the starter has to work harder as each cylinder comes up on compression, so starter current rises with each compression stroke. A high-amp probe on a scope turns that into a row of humps, one per cylinder. If every hump is about the same height, compression is relatively even. A short hump means that cylinder offers less resistance, from a burned valve, ring problem or similar.
Disable fuel or ignition per the service procedure so the engine cranks without starting, and make sure the battery is fully charged. Clamp around all the battery cables in the same direction or the main starter cable, and set the scope to see several engine revolutions.
To know which hump belongs to which cylinder, add a second channel synced to a known cylinder's ignition or injector signal. A relative test only shows differences between cylinders, so confirm a low cylinder with a compression or leak-down test. Keep clear of belts and fans while cranking.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.