If the ECM lacks a speed or timing reference, the engine will...

• A. Exceed governed speed
• B. Hunt at governed speed
• C. Idle and start
• D. Cranks but does not start

Answer: (D)

The engine management system must know the engine’s position and speed to time fuel delivery and ignition. If the ECM doesn’t receive a speed or timing reference (for example, from the crankshaft or cam sensors), it has no way to determine when to fire the spark plugs or when to inject fuel. Without that timing, combustion can’t be initiated, even though the starter can still crank the engine. So the engine spins but never starts.