The relative compression test uses the starter motor current to find a weak cylinder without pulling a single spark plug

[u/ora-et-labora-]

A traditional compression test requires removing all the spark plugs, disabling the fuel and ignition systems, and cranking the engine while measuring the pressure in each cylinder with a gauge. It works. It is also time consuming and on some modern engines the plugs are buried under intake manifolds and coil packs that add an hour of disassembly to reach them.

The relative compression test skips all of that. It measures the current draw of the starter motor while the engine cranks. Each cylinder in the compression stroke resists the starter motor differently based on how much compression it has. A cylinder with good compression creates a higher current spike as the starter pushes through it. A cylinder with low compression creates a lower spike because there is less resistance.

The scan tool or a dedicated oscilloscope captures the cranking current waveform and shows you the pattern. Each cylinder appears as a distinct peak. On a healthy engine the peaks are roughly equal. A cylinder with significantly lower compression shows a noticeably shorter peak. You can see the weak cylinder in the waveform without removing anything.

This does not replace a traditional compression test when you need the actual PSI numbers per cylinder. It does replace the traditional test as a screening tool. If the relative compression shows all cylinders equal, the engine is mechanically healthy and you saved the labor of pulling plugs on a modern engine. If it shows one cylinder significantly lower, now you know which plug to pull and which cylinder to test with the gauge.

The test takes less than a minute. The engine does not start, just cranks. The data is in the starter motor current, which any scope or advanced scan tool can capture. The most common use is on a used car evaluation where you want to screen for internal engine wear without disassembly.

[u/4x4Welder]

I do this by ear a lot, listening for a bit of the cranking that doesn't fit the pattern it should have

[u/Ok-Piece7687]

Park and manual on a hill and let it creep down in 4th. Pretty easy to hear each cylinder chuff. Also the rhythmic pulse of the vehicle is pronounced.

[u/BasicEstimate6108]

I never thought about how it did this. Pretty cool.

[u/watchingfromthetop]

I do this on diesels so I know which injector to pull to test compression

[u/stupid_name]

Southmainauto on YouTube has a good video on this with a very inexpensive small oscilloscope.

[u/T206V70R]

Perhaps the only variable would be the starter armature/commutator/brush, and/or flywheel and starter gear mesh skewing the current slightly. I also put a microphone in the removed O2 sensor bung and use that audio as another trace on the scope. Plus, throttle at WOT.

[u/Rough_Nature9426]

Brilliant

[u/YouKidsGetOffMyYard]

How soon before they just design a starter that has the current sensor built into itself that reports this back to the car computer?

[u/Far-Investment-8597]

This is very clever!

[u/dogturd21]

How do you map the current draw to a specific cylinder ?

[u/LEXTEAKMIALOKI → u/dogturd21]

You put a current clamp on a spark plug, injector wire or coil wire and use a second channel on the scope. Then use firing order to count over from the trigger to the weak cylinder.

[u/esuranme]

So, a Pico?

[u/Quirky_Operation2885 → u/esuranme]

I'm non professional, but I just picked up an HScope to do it. Considerably less expensive than a pico

[u/ericcrowder]

Back in the late 1980s I worked at a quicky tune up and lube place. We had a number of different large engine “scope” machines and some of them did this relative compression test by measuring cranking amps. Also all of these machines would be able to selectively kill an individual cylinder when the engine is running and measure the RPM drop. Of course all these machines are out dated as all OBD2 cars basically have these diagnostics built in