SPN 802 FMI 21: Meaning and Fix
SPN 802 FMI 21 indicates the Relay Diagonal 1 output signal has drifted below the expected voltage threshold. This fault commonly appears after a forced DPF regeneration when the relay driver overheats or after ECM replacement if the relay harness is not properly reseated. The ECM monitors the feedback voltage and triggers the code when the signal remains low for a calibrated debounce period, typically 2-3 seconds.
Common Symptoms
- Starter Lockout: ECM inhibits starter engagement when relay feedback confirms low voltage during crank inhibit logic.
- Dash Warning Lamp: Amber warning lamp activates immediately; red stop lamp may follow if derate threshold is exceeded.
- No A/C Clutch: Air conditioning compressor clutch fails to engage because relay diagonal 1 supplies its control power.
- Intermittent Power Loss: Engine torque derate of 25-40% occurs as ECM enters safe mode due to missing relay feedback.
Probable Causes
- Open Relay Coil: Internal relay coil winding open circuit prevents magnetic pull-in, causing feedback to float low.
- Shorted Flyback Diode: Failed suppression diode across relay coil shorts to ground, pulling the feedback signal below threshold.
- Corroded Connector Pin: Pin 5 on the ECM J2 connector corrodes, increasing resistance and causing voltage drop during load.
- ECM Driver Failure: Internal low-side driver MOSFET in the ECM fails shorted to ground, permanently holding signal low.
Advanced Technical Analysis
The ECM microcontroller executes a cyclic monitoring routine every 10 ms. It compares the Relay Diagonal 1 feedback pin voltage against a calibrated threshold of 2.5 V. If the feedback stays below this threshold for 200 consecutive samples (2 seconds), the diagnostic sets FMI 21. This logic prevents false triggers from transient noise but catches persistent drift caused by resistive shorts.
Electrical analysis reveals the relay coil inductance creates a back-EMF pulse when de-energized. The ECM uses an internal clamping circuit to limit this spike to 36 V. If the flyback diode fails open, the voltage spike can exceed 60 V and damage the driver, causing the feedback to drift low over time. This is a known failure mode on Bosch MS6.2 ECUs.
When the fault is active, the ECM immediately disables the relay output and sets a torque derate of 25% after 5 seconds. After 30 seconds, if the fault persists, the derate increases to 50%. The engine idle speed is also raised to 900 RPM to ensure adequate alternator output for safety-critical loads. This fallback strategy is documented in Deutz 2015 TCD engine manuals.
Technicians frequently encounter this fault after replacing the ECM without transferring the relay module. The new ECM calibrations may have different debounce timings, causing intermittent low drifts. Long-term prevention includes verifying relay coil resistance (typically 70-100 ohms) and installing a transient voltage suppressor across the relay coil. Real-world data from MAN TGS trucks shows 80% of cases are resolved by replacing the relay module and cleaning the ECM connector.
Step-by-Step Troubleshooting Guide
- Scan Tool Check: Read active and inactive faults. Verify SPN 802 FMI 21 is current and note freeze frame data for voltage.
- Visual Inspection: Inspect relay diagonal 1 connector for corrosion, bent pins, or melted plastic at ECM J2 position 5.
- Voltage Measurement: Measure feedback pin voltage with ignition on, engine off. Should be > 2.5 V. If low, isolate relay.
- Relay Bypass Test: Remove relay and jumper socket pins 30 and 87a. If fault clears, replace relay module.