Autonatomy · Ignition Timing Lab

Learning objective

Explain degrees before top dead center (BTDC), compare base and commanded ignition timing, and use a simulated timing reading to distinguish a timing command from a timing delivery problem.

Illustrative spark-ignition gasoline-engine teaching model. Values, timing maps, knock response, and crankshaft signals are simplified; they are not OEM specifications. This lab does not model combustion pressure, torque, misfire detection, or a real timing-light procedure.

1 · Set operating conditions

Engine speed: 1800 RPMEngine load: 35%Knock feedbackIntroduce a fault

Spark advance means the spark occurs before the piston reaches top dead center on the compression stroke. A negative reading here means after TDC (ATDC).

2 · The Challenger

The ECM commands 24° BTDC, but a verified independent measurement shows 12° BTDC. Which statement is supported?

Choose an answer The actual spark event differs from the command; verify the measurement and investigate timing reference and delivery. The ECM must be commanding 12° BTDC. A failed oxygen sensor is proven.

Select an answer to test your reasoning.

3 · Crank-angle timing display

The dial shows a small crank-angle window around compression TDC, not a full 720° cycle. Markers represent timing angles, not piston travel or spark duration.

Base map (illustrative)—

Knock correction—

ECM commanded timing—

Observed spark timing—

Command / observed difference—

Spark / crank signal—

4 · Technician's reasoning

Timing command is a control decision, not proof of a spark event. Compare scan data with the correct vehicle-specific measurement method. Knock-related spark retard is a normal protective control response; an ignition coil that does not fire cannot be diagnosed from commanded advance alone.