The Relative Compression Test

The relative compression test is one of the fastest and least invasive ways to evaluate cylinder health across an entire engine — without removing a single spark plug. Using nothing more than a current clamp and an oscilloscope, you can identify a weak cylinder in minutes. It is not a replacement for a traditional compression test, but it is a powerful screening tool that every diagnostic technician should have in their arsenal.

Written by Anthony Calhoun, ASE Master Tech A1-A8

What Is a Relative Compression Test?

A relative compression test measures the starter motor's current draw during cranking. When the starter turns the engine, each cylinder offers resistance as the piston compresses the air-fuel mixture. A cylinder with good compression requires more energy (higher current) to compress than a cylinder with low compression. By capturing the starter current waveform on an oscilloscope, you can see each cylinder's compression event as a current peak — and quickly spot which cylinder, if any, is weak.

The key word is "relative." You are not measuring PSI. You are comparing each cylinder's current draw peak to the others. If five cylinders produce 180-amp peaks and one produces a 130-amp peak, that cylinder has significantly lower compression — regardless of the actual PSI value.

How It Differs from a Traditional Compression Test

Setting Up the Test

Equipment

• Oscilloscope: Any automotive scope works — Pico, Autel, Snap-on, or even a USB scope. You need at least one channel.
• Current clamp: An inductive current clamp (amp clamp) that attaches around the battery cable. Low-amp clamps (0-60A range) work for the signal but may clip the peaks. A 0-400A or 0-600A clamp is ideal for starter current.
• Disable fuel and ignition: You want the engine to crank but not start. Pull the fuel pump fuse and disconnect the ignition coil(s) or pull the CKP sensor connector.

Current Clamp Placement

Clamp the current probe around the battery negative cable (ground cable). The direction of the clamp matters — if the waveform is inverted (peaks going down instead of up), flip the clamp. Make sure the cable is centered in the clamp jaw and not touching either side for an accurate reading.

Scope Settings

• Time base: 200-500 ms/division. You want to see at least two full engine revolutions.
• Voltage scale: Adjust based on your clamp's output ratio. A 1mV/A clamp at 200A will output 200mV, so set the scope to about 50-100 mV/division.
• Trigger: Set to rising edge, auto trigger. You want the scope to start capturing when cranking begins.
• Record length: Capture at least 3-5 seconds of cranking for a good sample.

Reading the Waveform

What You Will See

During cranking, the starter current waveform shows a repeating pattern of peaks and valleys. Each peak corresponds to a cylinder reaching top dead center (TDC) on its compression stroke — this is when the starter works hardest. Each valley corresponds to the piston moving through a non-compression stroke where resistance is lower.

On a 4-cylinder engine, you will see 4 major peaks per two engine revolutions (one complete engine cycle). On a 6-cylinder, 6 peaks. On a V8, 8 peaks. The peaks should be roughly equal in height if all cylinders have equal compression.

Identifying Cylinder Order

The peaks appear in firing order, not cylinder number order. For example, on a GM LS V8 with a firing order of 1-8-7-2-6-5-4-3, the peaks will appear in that sequence. To identify which peak corresponds to which cylinder, you can:

• Use a secondary trigger on the number 1 ignition coil or CKP signal to mark cylinder 1 TDC
• Count peaks from the cylinder 1 marker following the firing order
• Some scope software (like Pico) has automatic cylinder identification when used with the CKP reference signal

Normal vs Abnormal Patterns

Normal Pattern

All compression peaks are within 10-15% of each other in amplitude. Minor variations are normal — no engine is perfectly balanced. The pattern is consistent across multiple engine revolutions.

Weak Cylinder

One peak is noticeably lower than the others — typically 20% or more below the average. This indicates that cylinder has lower compression. The weak cylinder's peak will be consistently low across all captured revolutions, confirming it is a real finding and not a one-time anomaly.

Flat Cylinder

One peak is dramatically lower or nearly absent. This indicates a cylinder with very little or no compression — a burnt valve, broken ring, or blown head gasket affecting that cylinder. This is an obvious finding that does not require measurement — you can see it at a glance.

Mechanical Timing Issues

If the peaks are uneven in a pattern that repeats every two engine revolutions (not every revolution), suspect a camshaft timing issue. A jumped timing chain or belt will change which cylinders see full compression and which do not, creating an asymmetric waveform pattern.

When to Use Relative Compression vs Traditional

Use Relative Compression When:

• Spark plugs are hard to access: Modern engines with coil-on-plug buried under intake manifolds make plug removal time-consuming. A relative compression test takes minutes with no disassembly.
• You need a quick screening: Before committing to a full compression test, a relative test tells you if there is even a problem worth investigating.
• The customer has not authorized full diagnosis: A relative compression test during a routine inspection adds value without adding significant labor time.
• You suspect one weak cylinder: A misfire code on a specific cylinder can be confirmed with a 2-minute relative compression test before pulling plugs.
• The engine will not start: On a no-start, a relative compression test during cranking quickly rules out a major mechanical failure (jumped timing, broken belt) without any disassembly.

Use Traditional Compression When:

• You need actual PSI values for comparison to specifications
• You need to perform a wet test (adding oil to differentiate rings from valves)
• You are performing a cylinder leak-down test as a follow-up
• The relative test showed a weak cylinder and you need to quantify how weak
• Warranty or insurance documentation requires specific PSI measurements

Limitations of Relative Compression

Relative compression is a powerful screening tool, but it has limitations you need to understand:

• It cannot detect uniformly low compression: If all cylinders are equally low (worn rings across the board), all peaks will be equal — the test will look normal even though the engine has a compression problem. Only a traditional test with PSI values will catch this.
• Battery condition matters: A weak battery changes the cranking speed and current draw, which can distort the waveform. Make sure the battery is fully charged and in good condition before testing.
• Starter condition matters: A worn starter with inconsistent current draw can create peaks that look like compression differences. If the waveform is noisy or erratic, consider the starter as a variable.
• Engine temperature: Cold engines have different compression characteristics than warm engines. For the most accurate relative comparison, test at a consistent temperature — ideally warm (but remember, you are disabling fuel and ignition, so do not overheat the catalytic converter).

Using Relative Compression on Diesel Engines

Relative compression works exceptionally well on diesel engines because diesel compression ratios are much higher (16:1 to 22:1 vs 10:1 to 13:1 for gasoline). The higher compression means bigger current draw differences between cylinders, making weak cylinders even more obvious on the waveform.

On diesel engines, relative compression can help identify:

• Weak cylinders causing hard starting or rough idle
• Injector issues (a leaking injector hydraulically locks the cylinder, showing a dramatically higher current peak)
• Glow plug or combustion chamber problems on specific cylinders

Starter Current Draw Analysis — Bonus Diagnostic

While you have the current clamp on the battery cable, you get a free starter health check. Normal starter current draw is typically:

Excessive cranking current with normal compression suggests a starter with internal friction (worn bushings, dragging armature). Low cranking current with slow cranking suggests high resistance in the starter circuit (cables, connections, ground straps).

Real-World Example

A 2018 Ford F-150 3.5L EcoBoost comes in with a P0302 — cylinder 2 misfire. The spark plugs on this engine are buried under the turbo plumbing on bank 1 and require significant disassembly to access. Before pulling the intake apart, you clamp the current probe on the battery cable, disable fuel and ignition, and crank the engine for 5 seconds.

The waveform shows five cylinders with peaks at 195-205 amps and cylinder 2 at 155 amps — a 23% difference. That confirms a mechanical compression issue on cylinder 2, not just an ignition problem. Now when you pull the plug on cylinder 2, you know to follow up with a leak-down test rather than just replacing the coil and plug. The relative compression test took 3 minutes and prevented a wasted coil replacement.