Oscilloscope Waveform Library (1)

Oscilloscope Waveform Library Table 1: Relative Compression — Diesel Engines Table 2: Relative Compression — Gasoline Engines Description The figures in the tables below are reference waveforms collected from known-good vehicles and faulted vehicles. Each figure is accompanied by a description. Relative Compression — Diesel Engines Normal Abnormal

Oscilloscope Waveform Library (1) — figure 1

Oscilloscope Waveform Library (1) — figure 2

1. 3 Cylinder Engine — Low Compression
2. Low Compression — The waveform repeats every 3 compression strokes, indicating a current and repeatable compression fault.
3. 8 Cylinder Engine — High Starter Load
4. 8 Cylinder Engine
5. Exhaust Valve = Stuck Closed — The
6. 4 Cylinder Engine waveform repeats every 8 compression strokes, indicating a current and repeatable compression fault. When one cylinder displays double the relative compression, this is an indication of a stuck closed exhaust valve on the companion cylinder.
7. 6 Cylinder Engine

Normal

Abnormal

An engine with little to no compression variance between cylinders will have waveform valleys that are even. If the engine compression is equally low across all cylinders, the oscilloscope pattern will appear good. The total amplitude (range from highest to lowest point of the waveform) is NOT an indicator of good compression. Operating Conditions: 1. Fuel System » Disabled 2. { Automatic Transmission } Apply & Hold » Brake Pedal { Manual Transmission } Apply & Hold » Clutch Pedal 3. Engine » Cranking — For 5–10 s

Relative Compression — Gasoline Engines

	Normal	Abnormal
	1. 8 Cylinder Engine 2. 6 Cylinder Engine 3. 4 Cylinder Engine 4. 3 Cylinder Engine	1. 3 Cylinder Engine — Low Compression 2. Low Compression — The waveform repeats every 3 compression strokes, indicating a current and repeatable compression fault. 3. 8 Cylinder Engine — High Starter Load 4. Exhaust Valve = Stuck Closed — The waveform repeats every 8 compression strokes, indicating a current and repeatable compression fault. When one
	© 2023 General Mo	cylinder displays double the relative tors. All rights reserved.

Oscilloscope Waveform Library (1) — figure 3

Normal	Abnormal
	compression, this is an indication of a stuck closed exhaust valve on the companion cylinder.
Note: An engine with little to no compression variance between cylinders will have waveform valleys that are even. If the engine compression is equally low across all cylinders, the oscilloscope pattern will appear good. The total amplitude (range from highest to lowest point of the waveform) is NOT an indicator of good compression. Operating Conditions: 1. Fully Applied » Accelerator Pedal @ Wide Open Throttle 2. { Automatic Transmission } Apply & Hold » Brake Pedal { Manual Transmission } Apply & Hold » Clutch Pedal 3. Engine » Cranking — For 5–10 s