FIRST LOOK / RCOMP

RELATIVE COMPRESSION

Compare the starter-current load created as each cylinder reaches compression without removing spark plugs.

ESTIMATED SETUP2–5 min1–2 channels
WHY BEFORE DISASSEMBLY

CAPTURE THE SYSTEM
AS IT FAILED.

It screens every cylinder in one capture while the engine remains assembled. A balanced pattern can prevent unnecessary mechanical teardown; an uneven pattern gives you a focused reason to continue with mechanical compression, leak-down or in-cylinder pressure testing.

Reading the signal

Good waveform

With stable cranking speed and a capable battery, starter-current compression peaks repeat with similar height and spacing. Identify cylinder position with a separate synchronization signal before assigning a low event to a cylinder.

Faulty waveform

A consistently lower compression-related current peak can identify a cylinder that loads the starter less. Uneven spacing or a changing overall level can instead reflect cranking-speed or battery changes. Confirm a repeatable imbalance with the appropriate mechanical test; this does not measure absolute compression pressure.

Common mistakes

AVOID THE
FALSE CLUE.

PREPARE THE TEST

✓ Battery must be adequately charged✓ Disable fuel delivery using the approved procedure✓ Prevent unintended engine start✓ Zero the current clamp and verify direction

STEP-BY-STEP PROCEDURE

  1. Connect the high-current clamp around one battery cable. Use only one conductor and keep the clamp clear of moving parts.
  2. Set a timebase long enough to display 8–12 compression events. Begin around 200 ms/div and select a range suitable for starter current.
  3. If cylinder identification is required, connect a second channel to a safe ignition-primary or injector control signal for cylinder 1.
  4. Crank the engine for approximately five seconds at a steady speed without allowing it to start.
  5. Stop cranking and compare the repeating current peaks. Use the sync event and firing order to identify a consistently low cylinder.
  6. Confirm an indicated cylinder with an absolute compression, leak-down or in-cylinder pressure test before disassembly.
WHAT THIS TEST CANNOT PROVE

This is a comparison test, not a PSI measurement. Starter condition, battery voltage, variable valve timing and cranking speed can change the pattern.

⚠ SAFETY / CONNECTION CAUTION

Disable fuel or engine operation using the exact vehicle procedure. Avoid extended cranking and protect the catalyst from unburned fuel.

STOP BEFORE CONDEMNING THE PART

Repeat the capture, verify the circuit under load and use vehicle-specific specifications. Follow with the most direct confirmation test indicated by the evidence.

NEXT QUICK TEST / CRANKCranking Voltage + Current→