Relative compression test is one of the fastest ways to check engine compression without removing spark plugs or using a compression tester.
Instead of measuring absolute pressure, this method compares cylinder performance using battery voltage during cranking.
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What is relative compression test?
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Relative compression test is a diagnostic method that uses an oscilloscope connected to the battery to analyze voltage drops while the engine is cranking.
Each voltage drop represents one cylinder’s compression stroke.
✔ Equal drops = good compression
✔ Missing or weak drop = compression problem
Used tool (budget oscilloscope)
1-channel scope is enough for this test.
✅ How to read the results
Each dip in voltage = one cylinder
Normal result:
dip dip dip dip
All cylinders have similar drops → compression OK
Bad compression:
dip dip dip ___
Missing dip → cylinder with no compression
Real-world note (important insight)
Sometimes you may see slightly deeper dips on some cylinders.
✔ This can still be normal
✔ Always confirm with compression gauge if unsure
✅ When to use this test
- quick engine health check
- misfire diagnosis
- used car inspection
- before doing full compression test
- when you don’t want to remove spark plugs
Hi, I am Juraj Lukacko. I got frustrated by unhelpful and scammy mechanics, so I decided to learn everything about car diagnostics myself. I test dozens of new car diagnostic tools every month along with learning new strategies to fix and customize cars.
