SPN 791 FMI 15: Meaning and Fix
SPN 791 FMI 15 indicates the left wheel speed sensor on axle 2 reports a voltage or frequency above the calibrated upper limit. This fault often appears after a wheel-end service where the sensor gap was incorrectly set or a damaged tone ring produces excessive amplitude. Technicians may see this after a brake job if the sensor was not reinstalled properly.
Common Symptoms
- ABS Warning Lamp: Steady or flashing amber ABS lamp on dash, indicating a wheel speed sensor circuit anomaly.
- Intermittent Traction Control: Erratic engagement of traction control or differential lock due to implausible wheel speed data.
- Retarder Disengagement: Retarder or engine brake may deactivate above 25 km/h to protect driveline from false signals.
- Speedometer Fluctuation: Vehicle speed display may jump or drop to zero momentarily as the ECM rejects the faulty input.
Probable Causes
- Sensor Gap Too Small: Air gap below 0.3 mm causes magnetic saturation, generating an abnormally high voltage signal.
- Damaged Tone Ring: Missing or bent teeth on the tone ring create mechanical over-speed pulses detected by the sensor.
- Shorted Sensor Wiring: Internal short between signal and supply lines raises the voltage above the ECM’s valid range.
- ECM Ground Offset: High resistance in sensor return path shifts the reference ground, causing false high readings.
Advanced Technical Analysis
The ECM monitors the wheel speed sensor signal against a calibrated frequency-to-voltage map. For axle 2 left, the expected peak-to-peak voltage at 50 km/h is 1.5–4.0 V. FMI 15 sets when the ECM detects a sustained voltage >4.5 V for 2 seconds, indicating the signal is valid but exceeds the normal operating range. This triggers a diagnostic trouble code without immediate derate.
Electrically, the sensor is a variable-reluctance or Hall-effect type. A Hall sensor outputs a digital square wave; a voltage above 5.5 V suggests a short to battery or a pull-up resistor fault. The ECM debounces the signal for 200 ms before confirming the fault. In practice, this often occurs after a wheel-end replacement where the sensor was forced into contact with the tone ring.
Upon detecting FMI 15, the ECM sets the ABS warning lamp and disables traction control for that wheel. The vehicle remains fully drivable, but the retarder may be inhibited above 30 km/h to prevent driveline damage from false wheel slip. The ECM logs the fault with a snapshot of sensor voltage and vehicle speed for later analysis.
Long-term diagnosis should include oscilloscope measurement of the sensor output while rotating the wheel. A clean sine wave with amplitude >5 V indicates a mechanical over-speed condition. Workshop experience shows this code frequently follows a brake drum replacement where the tone ring was installed backwards or damaged by a hammer. Always verify the sensor gap with a brass feeler gauge (0.5–1.5 mm per manufacturer).
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor wiring for chafing, corrosion, or pinching at the axle 2 left location near the brake chamber.
- Measure Sensor Gap: Use a non-ferrous feeler gauge to confirm air gap between sensor tip and tone ring is 0.5–1.5 mm.
- Oscilloscope Test: Jack up wheel, rotate manually, and capture sensor waveform. Peak voltage must not exceed 4.5 V.
- Check Tone Ring: Rotate wheel and inspect tone ring for missing, bent, or magnetized teeth using a dial indicator.