SPN 791 FMI 31: Meaning and Fix
SPN 791 FMI 31 indicates the ABS wheel speed sensor on axle 2 left has a continuous fault condition, meaning the ECU detects an invalid or stuck signal for a calibrated duration. This code commonly appears after a forced DPF regeneration when excessive heat damages the sensor harness near the exhaust. Technicians frequently encounter this fault after replacing the ECM without reprogramming the wheel configuration.
Common Symptoms
- ABS Warning Lamp: Steady or flashing ABS dash lamp; traction control and stability systems are disabled.
- Erratic Speed Signal: J1939 data shows intermittent or missing wheel speed pulses on axle 2 left.
- Retarder Limitation: Engine retarder or exhaust brake may be deactivated due to loss of wheel speed reference.
- Cruise Control Off: Cruise control will not engage; system requires valid speed from all wheel sensors.
Probable Causes
- Sensor Air Gap: Excessive or zero air gap between sensor tip and tone ring; typical spec is 0.3–1.2 mm.
- Harness Damage: Chafed or melted wiring near exhaust or moving suspension components causing intermittent short.
- Tone Ring Damage: Missing or broken teeth on tone ring due to corrosion or mechanical impact.
- ECM Input Fault: Internal ECM pull-up resistor failure or corrupted calibration for wheel speed parameters.
Advanced Technical Analysis
The ECM monitors the wheel speed sensor signal via a dedicated digital input channel with a Schmitt trigger circuit. When the sensor output remains above or below the calibrated threshold (typically 0.8–4.5 V) for more than 2.5 seconds, the ECU sets FMI 31. This indicates a persistent condition, not a transient glitch. The diagnostic timer resets only when the signal returns to a valid frequency range for 10 consecutive seconds.
Electrical analysis reveals that the sensor is a passive variable-reluctance type generating an AC sine wave proportional to wheel speed. The ECM detects the zero-crossing events to compute RPM. A stuck high or low condition (FMI 31) often results from a short to battery voltage or ground in the harness. Debouncing timers in the ECM ignore pulses shorter than 50 microseconds to filter noise.
Upon detecting this fault, the ABS controller sets a diagnostic trouble code and disables all traction and stability functions. The ECM also inhibits engine torque derate strategies that rely on wheel speed, such as overspeed protection. In severe cases, the transmission may lock out automated shifting modes to prevent driveline damage from erroneous speed data.
Long-term prevention requires verifying sensor gap during brake relines and inspecting harness routing after exhaust work. In one workshop case, a technician found that a loose tone ring on a MAN TGS 26.480 caused intermittent FMI 31 after every left turn. Replacing the tone ring and recalibrating the sensor gap resolved the issue permanently. Always perform a road test after repairs to confirm diagnostic reset.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor wiring for chafing, melting, or corrosion near axle 2 left; repair or replace as needed.
- Measure Air Gap: Use a non-magnetic feeler gauge; adjust sensor bracket to achieve 0.3–1.2 mm clearance.
- Tone Ring Check: Rotate wheel and inspect tone ring for missing or damaged teeth; replace if defective.
- ECM Signal Test: Backprobe sensor signal pin at ECM; oscilloscope should show clean sine wave at low wheel speed.