Home › Codes › SPN 637 › FMI 8

SPN 637 FMI 8

Engine Timing Sensor (Crankshaft) — Abnormal frequency, pulse width, or period.
A frequency- or PWM-type signal is outside the expected range or shape.

What the SPN covers: Crankshaft position/speed signal, the main reference for engine speed and timing. More on SPN 637.

What FMI 8 usually points to: Damaged speed sensor or tone ring, wrong air gap, electrical noise, or a failing PWM sensor. More on FMI 8.

Severity

HIGH — limit use, diagnose now

Can cause stalling or no-start.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Faulty crank sensor
  3. Wrong air gap or debris
  4. Damaged flywheel tone ring
  5. Wiring fault

Checks & fixes (easiest first)

  1. Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins
  2. Check the reference voltage, ground and signal at the component per OEM pin-out
  3. Inspect sensor and connector
  4. Check resistance and AC output while cranking
  5. Scope the crank and cam signals
  6. Check for SPN 636 together

FAQ

What does SPN 637 FMI 8 mean?

Engine Timing Sensor (Crankshaft) — abnormal frequency, pulse width, or period. A frequency- or PWM-type signal is outside the expected range or shape.

Is SPN 637 FMI 8 serious?

Can cause stalling or no-start.

What should I check first for SPN 637 FMI 8?

Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins; Check the reference voltage, ground and signal at the component per OEM pin-out; Inspect sensor and connector.