SPN 723 FMI 10: Meaning and Fix
This fault indicates the second engine speed sensor’s signal changed faster than physically possible, typically exceeding 8000 rpm/s. Technicians frequently encounter this after a forced DPF regeneration, where extreme vibrations or a failing bearing cause the reluctor ring to momentarily contact the sensor, generating a spurious signal spike.
Common Symptoms
- Erratic Tachometer: The cab tachometer needle may flutter or drop to zero intermittently, even though the engine runs smoothly.
- Unrequested Derate: Engine power is limited to 1200 rpm, preventing highway speeds and triggering a amber warning lamp on the dash.
- Failed Regen Cycle: The DPF regeneration process aborts immediately with this code stored, due to the ECM losing confidence in engine speed data.
- Hard Start Condition: Cranking is prolonged because the ECM uses sensor 2 for synchronization; a corrupted signal delays injection timing.
Probable Causes
- Shielded Cable Damage: Chafed wiring in the engine harness exposes the twisted pair, causing electromagnetic interference from the alternator to distort the signal.
- Sensor Gap Misadjusted: An air gap exceeding 1.5 mm between the sensor tip and the tone wheel weakens the magnetic field, leading to erratic pulses.
- Tone Wheel Damage: A chipped or bent reluctor ring tooth from debris creates a sudden magnetic flux change, mimicking an abnormal rate of change.
- ECM Internal Fault: A failing signal conditioning circuit inside the ECU occasionally produces false rate-of-change values, often after a firmware update.
Advanced Technical Analysis
The ECM’s microcontroller monitors the time interval between rising edges of the digital square wave from sensor 2. It calculates instantaneous speed and compares it to the previous 10 ms window. If the delta exceeds 8000 rpm/s, the diagnostic sets FMI 10 immediately, without waiting for the debounce timer, to prevent catastrophic overspeed.
Electrical analysis reveals that a 5V pull-up resistor inside the ECM typically biases the sensor signal. A short to ground on the signal line pulls this voltage low, causing the comparator to interpret a high-frequency oscillation. This triggers the rate-of-change fault only when the sensor output passes through the zero-crossing point repeatedly within one engine revolution.
Upon activation, the ECM enters a safety fallback mode. It switches to SPN 190 from sensor 1, but limits torque by 40% and sets a 2500 rpm hard limit. This protects the turbocharger from over-speed if the false signal indicated a sudden deceleration, which could cause the wastegate to close unexpectedly.
Long-term diagnosis often requires a lab scope to capture the signal during a road test. In workshops, this code appears after a technician replaces the ECM but forgets to transfer the engine calibration file, leading to mismatched sensor scaling. Always verify the reluctor ring’s runout using a dial indicator, as a 0.2 mm runout can cause this fault at high RPM.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect the engine harness from the sensor connector to the ECM bulkhead for chafing, especially near the exhaust manifold, and repair any damaged shielding.
- Measure Sensor Resistance: Disconnect sensor 2 and measure resistance between signal and ground; expect 800-1200 ohms. A zero reading indicates a shorted coil.
- Verify Air Gap: Remove the sensor and measure the depth; compare to the factory spec of 0.8-1.2 mm. Adjust using shims if the gap is excessive.
- Scope Signal Capture: Connect a diagnostic oscilloscope to pin A of the sensor connector, crank the engine, and verify a clean 0-5V square wave without glitches.