What this test proves

Screen for unequal cylinder loading during cranking. This compares cylinders; it does not measure absolute compression or prove all cylinders are healthy.

Tools

  • High-current clamp and automotive scope
  • Optional approved cylinder synchronization pickup
  • Battery support if specified

Preparation and precautions

  • Use only a conventional low-voltage starter application supported by the procedure.
  • Disable fuel/ignition by the OEM method, secure the vehicle and observe starter cool-down limits.

Connections

  • Clamp the specified starter or battery cable, documenting other loads. Add a safe synchronization channel only when needed to identify cylinder order.

Equipment settings

  • DC current; range above inrush. Capture several complete engine cycles after speed stabilizes. Use sufficient sampling to separate individual compression humps.

Test sequence

  1. Verify battery condition and stable cranking speed.
  2. Crank for the specified interval.
  3. Ignore the initial inrush and compare repeated peaks.
  4. Use an appropriate synchronization reference before assigning a weak peak to a cylinder.
  5. Confirm a suspect cylinder with compression, leak-down or another mechanical test.

Expected signal / readings

Repeated compression-related current humps should be reasonably even under stable conditions. A consistently reduced hump can indicate reduced compression loading.

Interpreting results

  • All cylinders may be equally low and still look balanced.
  • Battery condition, starter problems and changing cranking speed can distort the comparison.

Common mistakes

  • Reporting relative percentages as pressure
  • Guessing cylinder identity without synchronization
  • Applying this setup to an HV motor-generator