This guide explains how to use the VMI3 to measure AC voltage ripple at the battery terminals and how to quickly check the mechanical compression pressure balance of each engine cylinder without disassembling the ignition device.
Guide Video (English)
Step 1: Oscilloscope Settings and Switching to AC Mode
Adjust the oscilloscope settings to observe fine 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 Synchronisation
- 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 Synchronisation Waveform Measurement (CKPS):
To identify which part of the waveform corresponds to which cylinder, receive 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), only operate the starter motor.
- Principle of waveform analysis: When the piston reaches top dead centre of compression, rotational resistance is at its maximum and the starter motor draws the most current from the battery. At this moment, the battery voltage drops (drop) to its lowest point.
- Normal judgement: The sine wave-shaped ripple waveform trough depths (amplitude) displayed on the screen (CH.A) should be uniform for all cylinders for normal operation.
- Fault detection: If the ripple depth in a particular section is noticeably shallower, it can be assumed that the corresponding cylinder at that moment has a compression pressure leak (mechanical fault).
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