SPN 802 FMI 10: Meaning and Fix
SPN 802 FMI 10 (Relay Diagonal 1) indicates the ECM detected an abnormal rate of change in the relay output circuit, meaning the commanded state (ON/OFF) did not transition within the expected debounce window. This fault commonly appears after a forced DPF regeneration or when a technician replaces the ECM without properly recalibrating the relay timing parameters. The ECM monitors the voltage slew rate across the relay coil and armature; if the transition occurs too quickly or too slowly (e.g., due to a failing relay or intermittent open in the harness), the fault is logged. Real-world workshop data from MAN and Deutz engines show this code often follows a short-to-ground event during starter motor replacement.
Common Symptoms
- Intermittent Power Loss: Auxiliary loads supplied by Relay Diagonal 1 drop out momentarily, causing engine hesitation or sensor power loss.
- Erratic Dash Warnings: Dashboard displays flickering warning lights for systems like intake heater or aftertreatment air pump.
- Failed DPF Regen: Relay Diagonal 1 controls the regeneration air pump; abnormal switching prevents successful active regeneration.
- ECM Watchdog Reset: The ECM may enter a watchdog reset loop if the relay transition violates the safety debounce timer.
Probable Causes
- Faulty Relay Coil: Relay coil resistance out of spec (typically 70-120 ohms) causing abnormal current slew rate.
- Corroded Connector Pins: Oxidation on relay socket pins introduces intermittent resistance, altering the voltage ramp time.
- ECM Driver Failure: Internal MOSFET driver in the ECM has degraded, unable to provide clean switching edge.
- Wiring Harness Chafing: Insulation breakdown on the relay control wire causes partial short to chassis during vibration.
Advanced Technical Analysis
The ECM evaluates the relay diagonal 1 output by monitoring the voltage at the driver pin after the command is issued. A debounce timer of typically 50-100 ms is used; if the voltage does not reach the expected level (e.g., battery voltage for a high-side driver) within that window, or if it fluctuates multiple times, FMI 10 is set. This is common on Bosch EDC7 UC31 units after a firmware update that tightens the slew-rate tolerance.
Electrical analysis reveals that an abnormal rate of change is often caused by a relay with a worn armature spring, which causes contact bounce. The ECM detects this as multiple transitions within the debounce period. Using an oscilloscope on the relay control line, technicians can observe the actual switching edge; a clean square wave should be present. Any ringing or multiple edges confirms the fault.
When this fault is active, the ECM invokes a safety fallback that disables the relay output and sets a torque derate of up to 25% to prevent uncontrolled actuator behavior. On Deutz TCD engines, this derate is accompanied by a check-engine light and a 1,500 RPM speed governor. The derate remains until the fault is cleared and a key cycle is performed.
Long-term prevention involves verifying relay pull-in voltage (typically <70% of battery voltage) and cleaning all connector interfaces. In practice, after replacing an ECM on a MAN D2676 engine, technicians must perform a relay learn procedure using the factory diagnostic tool to recalibrate the debounce timing. Failure to do so often leads to a recurrence of SPN 802 FMI 10 within 20 engine hours.
Step-by-Step Troubleshooting Guide
- Verify Relay Operation: Swap relay with an identical known-good unit. Clear fault and test under load (e.g., engage DPF regeneration).
- Inspect Connector Pins: Remove relay, check socket pins for corrosion or spread. Use contact cleaner and dielectric grease.
- Measure Coil Resistance: Using a DMM, measure resistance between relay control terminals. Should be 70-120 ohms at 20°C.
- Perform ECM Learn: Using factory diagnostic tool, initiate relay diagonal 1 learn procedure to recalibrate debounce timers.