SPN 190 FMI 31: Meaning and Fix
SPN 190 FMI 31 indicates the Engine Control Module (ECM) has detected a continuous abnormal condition on the engine speed signal path, typically a sensor output stuck at an implausible value or a persistent electrical fault. Technicians frequently encounter this fault after a forced DPF regeneration when the engine speed fluctuates wildly, or after replacing the ECM without recalibrating the crankshaft position sensor. This code is often logged alongside a no-start or severe power loss complaint.
Common Symptoms
- Erratic Idle: Engine speed hunts or surges at idle, failing to maintain a stable rpm setpoint.
- No Start Condition: ECM refuses to enable fuel injection due to missing or invalid speed reference signal.
- Torque Derate: Engine power is limited to limp-home mode, typically below 1500 rpm to protect components.
- Check Engine Lamp: Red or amber MIL illuminated continuously, often with a flash code on the dash display.
Probable Causes
- Sensor Wiring Fault: Open, short, or high resistance in the magnetic pickup circuit between sensor and ECM pin.
- Crankshaft Sensor Gap: Air gap exceeding 1.5mm due to bracket damage or wear reduces signal amplitude below threshold.
- Tone Wheel Damage: Missing, chipped, or magnetized teeth on the 60-2 wheel cause irregular pulse train.
- ECM Internal Failure: Condition exists flag set if the ECM’s input comparator detects a stuck signal for 10+ seconds.
Advanced Technical Analysis
The ECM microcontroller monitors the variable reluctance sensor’s AC waveform across a 720° crankshaft rotation. The signal is filtered through a Schmitt trigger to produce a digital square wave. If the pulse count deviates from the expected 58 teeth per revolution minus the sync gap, the ECM sets FMI 31 as a condition exists fault, indicating the signal is persistently invalid.
Electrical analysis reveals that the sensor’s internal impedance (typically 200-900 ohms) must match factory specs. A debouncing timer in the ECM firmware waits 5 consecutive invalid cycles before logging the fault. This prevents transient noise from triggering false codes, but a continuous offset (e.g., from a shorted shield) will trip the condition exists flag after the timer expires.
Upon activation, the ECM engages a safety fallback: it disables cylinder cut-out tests, locks the injection timing to a default map, and reduces maximum torque by 40%. In MAN D2676 engines, the ECM also forces the exhaust gas recirculation valve to a closed position to prevent uncontrolled combustion at unknown speed.
Long-term prevention requires verifying the sensor air gap with a brass feeler gauge (0.8-1.2 mm per Bosch specs). In real workshops, this fault often reappears after engine rebuilds if the tone wheel is not replaced or if the sensor bracket is torqued incorrectly. Deutz factory manuals recommend checking the sensor cable routing to avoid chafing against the bellhousing.
Step-by-Step Troubleshooting Guide
- Inspect Connectors: Check sensor and ECM connectors for corrosion, bent pins, or loose terminal retention.
- Measure Sensor Resistance: Disconnect sensor, measure across pins A and B: should be 200-900 ohms per factory data.
- Check Air Gap: Use a non-ferrous feeler gauge between sensor tip and tone wheel; spec is 0.8-1.2 mm.
- Oscilloscope Waveform: Crank engine and capture AC sine wave; peak voltage must exceed 1V at cranking speed.