Purpose

Mounted on the crankshaft free end. It continuously reports crank angle and engine speed to the ECU, and every timed event on the engine is derived from that one signal.

Types

• Incremental — counts pulses from a reference mark, so it must re-index after a blackout.
• Absolute — every position carries a unique code and is correct the instant power returns. Modern ME installations use absolute.

What it drives

• Fuel injection — start and end of injection, per cylinder
• Exhaust valve — opening and closing angle
• Cylinder lubrication — timing of each lube pulse

If it fails at sea

Two are fitted for redundancy — the system runs on one and cross-checks against the other. On losing both, the ECU falls back to a fixed timing table and power must be reduced. Expect the surveyor to follow up on the fail-safe position.