SPN 790 FMI 31: Meaning and Fix
SPN 790 FMI 31 indicates the wheel speed sensor for the right front axle (Axle 1 Right) has a condition exists fault. The ABS ECU detects a continuous signal that is either stuck high, stuck low, or out of valid range. Technicians frequently encounter this after a wheel-end service or brake job where the sensor was disturbed, or after a forced DPF regeneration that caused high exhaust heat to damage nearby wiring. The fault may not illuminate a dash lamp immediately but will log in the ABS ECU.
Common Symptoms
- ABS Warning Lamp: Steady or intermittent amber ABS warning lamp on the dashboard during key-on or driving.
- No Wheel Speed Data: Missing right front wheel speed on J1939 data link; other wheels report normally.
- Erratic Cruise Control: Cruise control disengages or refuses to set due to missing or unstable wheel speed input.
- Retarder Limitation: Retarder or exhaust brake may deactivate or refuse to engage to prevent wheel lock.
Probable Causes
- Sensor Air Gap: Excessive air gap between sensor tip and tone ring due to worn bearing or loose mounting.
- Wiring Chafing: Chafed or shorted sensor wires near the brake caliper or chassis frame causing intermittent signal.
- Tone Ring Damage: Missing, cracked, or magnetized tone ring teeth preventing proper magnetic flux generation.
- ECM Power Loss: Loss of 5V sensor supply from ABS ECU due to internal fault or pin corrosion at connector.
Advanced Technical Analysis
The ABS ECU monitors the wheel speed sensor continuously by evaluating the frequency and duty cycle of the AC signal. When the signal remains at a fixed voltage (typically above 4.8V or below 0.2V) for more than 500 ms, the ECU sets FMI 31. This logic prevents false triggering during transient noise but confirms a persistent condition. Real-world cases often involve a sensor physically broken inside the bore after a brake pad replacement.
Electrical analysis using an oscilloscope is critical. A healthy sensor produces a clean sine wave (0.5V to 5V amplitude) proportional to wheel speed. A stuck signal appears as a flat line at supply voltage or ground. The ECU debouncing timer (typically 800 ms) must expire before the fault is logged. Technicians frequently see this after a wheel bearing failure that pushed the tone ring out of alignment.
Upon detecting a sustained invalid signal, the ABS ECU disables traction control and stability functions for that wheel. Torque derate may be applied if the vehicle is equipped with an engine retarder that relies on wheel speed data. The J1939 message (EBC1 or EBC2) will contain a ‘wheel speed not available’ flag. This can cause the transmission to inhibit automatic shifting patterns.
Long-term prevention includes verifying sensor air gap with a non-ferrous feeler gauge (0.3-1.2 mm per MAN TGA manual). During wheel-end service, always inspect the tone ring for cracks or missing teeth. In Deutz and Mercedes-Benz applications, this fault often reappears after a bearing replacement if the tone ring was not replaced simultaneously. Use OEM-specified anti-corrosion compound on sensor threads.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor wiring harness for chafing, cuts, or corrosion near the brake caliper and frame rail.
- Measure Air Gap: Insert a non-ferrous feeler gauge between sensor and tone ring; spec is 0.3–1.2 mm.
- Oscilloscope Test: Spin the wheel manually and observe AC sine wave on scope; flat line confirms sensor or wiring fault.
- ECU Supply Check: Measure 5V reference at sensor connector with key on; if missing, inspect ABS ECU pins and power.