Relative Compression Test with an Oscilloscope — Step-by-Step Guide

What relative compression actually measures

When the starter cranks the engine, each piston coming up on compression pushes back against the starter motor. That resistance shows up as a momentary increase in starter current. A healthy cylinder produces a tall, sharp hump. A cylinder with leaking rings, burned valves or a blown head gasket lets some of that pressure escape — the starter doesn't have to work as hard — and you see a shorter hump in exactly the same place every revolution.

The test is called "relative" because it doesn't give you a PSI number — it compares cylinders to each other. That's usually all you need: if one of four humps is 30% shorter, you've found your dead cylinder in under five minutes without pulling a single spark plug.

What you'll need

• Lab scope with at least one channel (any 2-channel automotive scope works)
• High-amp current clamp — 600 A or 1000 A range (low-amp clamps will clip on cranking inrush)
• A way to disable spark and fuel (typically the ignition and injector fuses)
• A fully charged battery — a weak battery skews every hump and ruins the test

Step-by-step procedure

1. Charge the battery to at least 12.4 V resting. Borderline batteries pull the starter down so far that all cylinders look equally weak.
2. Disable spark and fuel. Pull the ignition coil fuse (or COP relay) and the injector fuse. On diesels, pull the fuel-pump relay or shutoff solenoid.
3. Clamp the current probe around the main battery-to-starter cable (or the negative battery cable). One conductor only. Zero the clamp before you start.
4. Set the scope:
   • Timebase: 200–500 ms/div (you want 3–5 seconds of cranking on screen)
   • Voltage scale: match your clamp (e.g. 100 mV/A clamp at 50 A/div = 5 V/div)
   • Coupling: DC
   • Trigger: single-shot, rising edge at ~100 A so the capture starts on cranking inrush
5. Crank for 3–5 seconds. Ignore the giant inrush spike at the start — that's just the starter armature getting moving. Look at the steady humps that follow.
6. Count and compare. A 4-cylinder engine produces 4 humps every 2 revolutions (1 hump per cylinder, in firing order). A 6-cylinder gives 6 humps, V8 gives 8, and so on.

Reading the waveform

Healthy pattern

All humps within ~10% of each other, evenly spaced, with a consistent baseline between them. The starter current settles to roughly 100–200 A on a gasoline 4-cylinder, 150–250 A on a V6, and 200–350 A on a V8 once cranking stabilizes.

One short hump

That cylinder is leaking. The piston rises but pressure escapes past the rings, past a burned valve, or through a head-gasket breach. Identify the cylinder by counting humps in firing order from a known reference — easiest if you trigger a second scope channel on the #1 ignition or injector pulse before disabling the system, or by manually rotating the engine to TDC compression on #1 first.

One tall hump

Usually means the cylinder before it is weak — the starter slowed down on the weak cylinder, then had to accelerate harder against the next good compression event. Always look for the dip, not the peak.

Two adjacent weak humps

Classic head-gasket failure between two cylinders. Confirm with a cooling-system pressure test or a chemical block tester.

All humps gradually rising or falling

The battery is draining during the test. Stop, charge the battery, and try again. This is the most common source of false positives.

Worked example: 4-cylinder with cylinder 3 dead

Firing order 1-3-4-2. On the trace you'll see four humps per two crank revolutions. The third hump in firing-order sequence is ~35% shorter than the other three. That points at cylinder 3 — confirm with a cylinder leakdown test before tearing into the head. Common findings on a single weak cylinder: a burnt exhaust valve (~80% of cases), broken ring land, or carbon-stuck valve from long idling.

Common mistakes

• Reading the inrush spike instead of the steady humps after it
• Forgetting to disable fuel — the engine starts and the test is invalid
• Using a weak battery; humps appear equal because everything is starved
• Clamping over a wire bundle instead of a single conductor (field cancels, reads zero)
• Confusing the tall hump for the bad cylinder — look at the dip
• Cranking for only 1–2 seconds; you need 3+ seconds of steady cranking after inrush

When to follow up with a leakdown test

Relative compression tells you which cylinder is weak. A cylinder leakdown test tells youwhy: bubbles in the radiator (head gasket), air out the exhaust pipe (exhaust valve), air out the intake (intake valve), or air out the oil filler / crankcase (rings). Always do both before quoting head or engine work.

Frequently asked questions

Related guides

• Using current clamps for engine diagnostics →
• Setting triggers & timebase →
• Fuel injector waveform deep dive →
• Intermittent fault capture →
• Knock vs bearing noise → — when the compression story matches an audible knock
• Engine sound diagnostics overview →