SPN 807 FMI 31: Meaning and Fix
This fault indicates the ECM has detected a continuous condition on the ASR (Automatic Slip Regulation) valve circuit for differential 2. The valve controls air pressure to the differential lock actuator. Technicians often encounter SPN 807 FMI 31 after a forced DPF regeneration where high exhaust heat damages nearby wiring harnesses, or following an ECM replacement without proper valve calibration.
Common Symptoms
- ASR Warning Lamp: Steady or flashing amber ASR lamp on dash, indicating a system fault is present.
- Traction Loss: Reduced wheel traction control; differential lock may fail to engage or disengage.
- No Differential Lock: Operator reports inability to lock or unlock the rear differential via switch.
- ECM Logged Fault: Fault code SPN 807 FMI 31 stored in ECM memory, often accompanied by derate.
Probable Causes
- Shorted Valve Coil: Internal short in the ASR valve solenoid causing continuous current draw.
- Harness Chafing: Wiring insulation worn through near frame rail or exhaust, creating a ground fault.
- ECM Output Driver: Failed high-side driver inside ECM, unable to switch valve correctly.
- Valve Stuck Open: Mechanical debris or corrosion holding ASR valve in open position.
Advanced Technical Analysis
The ECM monitors the ASR valve output circuit using a feedback voltage divider. When the valve is commanded off, the circuit should float near battery voltage. A continuous condition (FMI 31) means the ECM detects voltage below 2.5V for longer than the debounce timer (typically 500ms). This triggers an immediate fault and disables the ASR system.
Electrical analysis should begin with measuring resistance across the valve solenoid (expected 15-25 ohms). A shorted coil will show <5 ohms. The ECM's internal pull-up resistor (10k ohm) will be overwhelmed, pulling the sense line low. Use an oscilloscope to capture the valve command signal; a flat line near 0V confirms a hard short.
When this fault is active, the ECM sets a torque derate of 25% and disables cruise control. The ASR system enters a safe mode, preventing differential lock engagement to avoid driveline damage. The fault remains latched until the ignition cycle is cleared and the circuit passes a self-test with the valve in the off state.
Prevention strategies include inspecting the valve harness near the rear axle during every major service. Technicians report this code frequently after replacing the ECM without performing a valve calibration routine. Always verify valve plunger free movement with 12V applied; a stuck valve can mimic an electrical fault. Use Deutsch DT connector repair kits for permanent fixes.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check ASR valve wiring for chafing, corrosion, or loose connectors near the differential.
- Resistance Check: Measure valve coil resistance between pins; 15-25 ohms is normal. Short indicates fault.
- Voltage at Valve: Ignition on, engine off: measure voltage at valve connector. Should be 0V when not commanded.
- ECM Output Test: Command valve on via diagnostic tool; listen for click and verify voltage drop to near 0V.