SPN 790 FMI 13: Meaning and Fix
SPN 790 FMI 13 indicates the wheel speed sensor for axle 1 right (ABS) has an output signal outside the expected calibration range, often due to incorrect air gap or sensor wear. Technicians frequently encounter this after replacing a wheel end or bearing without re-calibrating the sensor clearance according to manufacturer specifications.
Common Symptoms
- ABS Warning Light: Steady or flashing amber ABS lamp on the dash, indicating a sensor calibration fault.
- Erratic Speed Reading: Instrument cluster shows intermittent or zero speed reading for the right front wheel.
- Brake Imbalance: Vehicle pulls slightly to one side during hard braking due to incorrect wheel speed data.
- Reduced Traction Control: TCS and stability systems may deactivate or intervene incorrectly at low speeds.
Probable Causes
- Incorrect Air Gap: Sensor tip not within 0.3–1.5 mm from the tone ring, causing signal amplitude deviation.
- Damaged Tone Ring: Missing or chipped teeth on the ABS tone ring generate irregular pulses.
- Sensor Corrosion: Rust or debris on the sensor face alters the magnetic field, shifting calibration.
- ECM Software Mismatch: After ECM replacement, the calibration parameters for the right sensor were not updated.
Advanced Technical Analysis
The ABS ECU continuously monitors the rising and falling edges of the wheel speed signal. When the pulse count per revolution deviates by more than 5% from the stored calibration table, FMI 13 sets. The ECU compares the right front sensor against the left front and rear axle sensors to confirm the fault is not a wiring issue.
Electrically, the sensor outputs a sinusoidal AC voltage whose frequency and amplitude vary with speed. The ECU’s debouncing timer expects a minimum amplitude of 200 mV at 0.5 km/h. If the amplitude falls below this threshold for 10 consecutive wheel rotations, the calibration fault is logged.
Once FMI 13 is active, the ECU disables ABS control for the right front wheel and defaults to a ‘drag compensation’ mode, reducing brake pressure on that corner. The vehicle remains operable but loses stability assist. No torque derate is applied, but the driver will notice increased stopping distances.
Long-term prevention requires verifying sensor gap after any hub or bearing service. In workshops, this fault often appears after a bearing replacement where the technician reused the old sensor without checking clearance. Using a non-magnetic feeler gauge to set the gap to 0.8 mm eliminates recurrence.
Step-by-Step Troubleshooting Guide
- Inspect Air Gap: Remove sensor and measure gap to tone ring with a brass feeler gauge; adjust to 0.8 mm.
- Check Tone Ring: Rotate wheel by hand and inspect for missing or deformed teeth; replace if damaged.
- Verify Signal Output: Use oscilloscope at sensor connector; expect clean sine wave of 0.5–2 V at 1 km/h.
- Re-calibrate Sensor: Perform ABS calibration routine via diagnostic tool per manufacturer service procedure.