VMI3 Basic Usage AC Battery Ripple Measurement (Cylinder Compression Pressure Balance Check)

Step 1: Oscilloscope Settings and Switching to AC Mode

Adjust the oscilloscope settings to observe minute fluctuations (ripple) in voltage.

• Test Vehicle: Seltos (SP2) G2.0 MPI
• VMI3 Settings:
  • Voltage Range (Volt/Div): 0.4V (to capture very small voltage changes)
  • Time Base (Time/Div): 50.0 mS
  • Mode Setting: [Important] Be sure to change from DC mode to 'AC mode'.

Step 2: Probe Connection and Synchronization

• Channel A (CH.A) - AC Ripple Voltage Measurement:
  • (+) Probe: Connect directly to the battery (+) terminal
  • (-) Probe: Connect to the battery (-) terminal
• Channel B (CH.B) - Cylinder Synchronization Waveform Measurement (CKPS): 
  To identify which cylinder corresponds to which part of the waveform, connect to the crank angle sensor signal.
  • (+) Probe: Connect to pin 2 (signal line) of the CKPS (Crankshaft Position Sensor)
  • (-) Probe: Connect to the battery (-) terminal

Step 3: Cranking Measurement and Balance Analysis

With the engine start restricted (fuel cut or ignition cut), operate only the starter motor.

• Waveform Analysis Principle: When the piston reaches top dead center of compression, rotational resistance is at its maximum and the starter motor draws the most current from the battery. At this point, the battery voltage drops to its lowest point.
• Normal Judgment: The depth (amplitude) of the lower troughs of the ripple waveform shown on the screen (CH.A) should be uniform for all cylinders for normal operation.
• Abnormal Detection: If the ripple depth is noticeably shallower in a certain section, it can be inferred that the corresponding cylinder is leaking compression pressure (mechanical failure) at that time.