Practical Technique

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

Run a relative compression test with a lab scope and a high-amp current clamp. Find a weak cylinder in minutes from starter current, without removing a spark plug.

What relative compression measures

As the starter cranks the engine, each piston rising on its compression stroke pushes back against the starter. That resistance appears as a rise in starter current, one hump per compression event. A cylinder that leaks (rings, valves, head gasket) builds less pressure, resists less, and produces a smaller hump at the same position every cycle.

It's called relative because it gives no PSI value. Each cylinder is compared against the others in the same capture, which share the same battery, starter, temperature, and cranking speed. That shared baseline is what makes the comparison reliable.

Event rate: compression events per second = cylinders × cranking RPM / 120. A V6 cranking at 220 RPM produces 11 events per second.

What you'll need

Lab scope, one channel minimum; a second channel for cylinder identification.
High-amp current clamp (600–1,000 A range, typically 1 mV/A). A low-amp clamp clips on the first hump.
A way to disable fuel and ignition (fuses, relays, or connectors).
A charged battery. Relative compression compares cylinders against each other, but a battery that sags during the test adds a trend that can bury a weak cylinder.

Step-by-step procedure

Verify the battery. Rested voltage at or above 12.4 V; see Battery & charging system health.
Disable fuel and ignition so the engine cranks without starting. On diesels, disable fuel delivery.
Clamp the main starter cable or the battery negative cable (one conductor). Zero the clamp first, with no current flowing.
Add a sync channel (recommended): a low-amp clamp or voltage lead on one known cylinder's coil or injector, captured before disabling it, or a cam sensor signal.
Set the scope:
- Timebase: 500 ms/div (5 s on screen)
- Sample rate: at least 5 kS/s
- Range: at 1 mV/A and 50 A/div, 50 mV/div; DC coupled
- Trigger: single-shot, rising edge at about 100 A
Crank for 3–5 seconds. Ignore the inrush spike at the start; the armature is only getting up to speed. Read the steady humps after it.
Record the clamp's mV/A rating with the upload.

Reading the waveform

Healthy pattern

Evenly spaced humps, one per cylinder per 720° cycle, repeating in firing order.
Hump heights within normal cranking variation of each other. The known-good V6 capture below spreads 14% from strongest to weakest.
A flat baseline between humps, with no rising or falling trend.
Steady cranking speed through the capture.

One small hump, repeating every cycle

That cylinder isn't holding compression: rings, a valve, or a head gasket. Identify it with the sync channel and count forward in firing order.

Locate the fault by the lowest event, not the tallest. Neighbouring events can read above average (in the known-bad capture below, the event immediately before the weak one is the strongest in the cycle), so a tall hump doesn't name a cylinder.

Two weak events

If the two cylinders are physically adjacent, suspect a head gasket breach between them. Adjacent in the block isn't always adjacent in firing order, so check the firing order before calling it.

All humps trending up or down

The battery or starter is changing during the test, not compression. Charge the battery and capture again.

All humps equal, but the engine still misfires

Compression is even. Look at ignition, injection, and fuel trim, or at a fault shared by every cylinder (such as cam timing), which a relative test cannot see.

Worked example: 4-cylinder, firing order 1-3-4-2

The sync channel marks the first hump as cylinder 1. Counting in firing order, the humps are cylinders 1, 3, 4, 2. The third hump in each cycle is about 35% smaller than the others, so the weak cylinder is cylinder 4.

Confirm with a cylinder leakdown test before any teardown.

When to follow up with a leakdown test

Relative compression tells you which cylinder is weak. Leakdown tells you why:

Air escaping at	Leak path
Throttle body / intake	Intake valve
Tailpipe	Exhaust valve
Oil filler / crankcase	Rings
Coolant (bubbles)	Head gasket

Common mistakes

Reading the inrush spike instead of the steady humps.
Leaving fuel enabled: the engine starts and the test is void.
Using a low-amp clamp: the humps clip flat.
Clamping a bundle instead of one conductor.
Naming the cylinder from the tallest hump instead of the lowest.
Cranking too briefly: you need 3+ seconds of steady cranking after inrush.
Guessing the cylinder without a sync reference.

Frequently asked questions

Do I need to disable spark and fuel?

Yes. The engine must crank without starting, at a steady speed. Disabling fuel also avoids washing the cylinders with fuel during the test.

Why is one hump taller than the others?

Neighbouring events can read above average around a weak one. Don't use the tall hump to find the fault; find the lowest event.

Can I do this on a diesel?

Yes, with a charged battery and a clamp rated for the higher cranking current. Disable fuel delivery; there's no spark to disable.

What if all cylinders look equal but the engine misfires?

Compression is even across cylinders. Move on to ignition, injection, and fuel trim, and remember that a fault common to every cylinder, like retarded cam timing, won't show as a difference between humps.

Example: a known-good capture

2015 Nissan Quest 3.5L V6 — even relative compression (known good)

Relative Compression (Starter Current) · V6 · amp clamp on the starter feed.

Ch A · 1.2 s shown.

Measured findings: Cranking speed~220 RPM; Compression events analysed48 over 4.3 s; Firing position 1111% of average; Firing position 2102% of average; Firing position 399% of average; Firing position 497% of average; Firing position 596% of average; Firing position 695% of average; Strongest-to-weakest spread14% (within normal cranking variation)

What to look for

Six evenly spaced humps per 720° cycle, no shallow event.
Every position between 95% and 111% of the average.
Cranking speed steady near 220 RPM.

Example: a known-bad capture

2017 Ford Escape 2.0L — one weak cylinder

Relative Compression (Starter Current) · 4-cylinder · amp clamp on the starter feed.

Ch A · 2.5 s shown.

Measured findings: Cranking speed~240 RPM; Compression events analysed38 over 4.8 s; Firing position 190% of average; Firing position 2108% of average; Firing position 3147% of average; Firing position 455% of average — WEAK; Strongest-to-weakest spread63% (the known-good V6 above spreads 14%)