SPN 677 FMI 13: Meaning and Fix
SPN 677 FMI 13 indicates the engine starter motor relay has triggered an out-of-calibration fault. The Engine Control Module (ECM) detects that the relay’s actual response time or activation current deviates from the programmed calibration limits. This commonly appears after an ECM software update or battery replacement when the relay’s wear characteristics no longer match the stored parameters. Technicians often encounter this fault after a failed cold-start attempt, where the relay chatters or fails to hold, causing the ECM to log the code and inhibit further starting attempts.
Common Symptoms
- Intermittent cranking: Starter motor fails to engage consistently, especially during cold starts or after engine heat soak.
- Delayed start sequence: Cranking begins 1–3 seconds after key turn due to relay calibration drift.
- No crank condition: ECM prevents starter activation if calibration deviation exceeds safety thresholds.
- Audible relay chatter: Rapid clicking from relay during key-on cycle, indicating unstable coil hold-in current.
Probable Causes
- Relay contact wear: Pitted or oxidized contacts alter current flow, causing ECM to detect timing deviation.
- ECM calibration mismatch: After software update, stored relay response parameters do not match actual hardware.
- Low battery voltage: Below 11.5 volts during cranking causes relay undervoltage and calibration failure.
- Corroded relay socket: Oxidation on terminals increases resistance, shifting the relay’s energization profile.
Advanced Technical Analysis
ECM logic monitors the starter relay control circuit by comparing commanded activation time against the feedback signal from the relay’s auxiliary contact. The calibration tolerance is typically ±50 ms. When the measured closing time falls outside this window, SPN 677 FMI 13 is set. This check occurs only during initial key-on to avoid false triggers during engine cranking vibration.
Electrically, the relay coil’s inductance and resistance are measured during a pre-start self-test. A coil resistance deviation greater than 15% from the factory-calibrated value (usually 70–90 ohms for 24V systems) causes the ECM to flag the out-of-calibration fault. Bosch MAN manuals specify a debouncing timer of 100 ms to filter transient voltage dips.
The ECM activates a safety fallback when this fault is active: starter motor engagement is inhibited after three consecutive failed calibration checks. Engine torque is derated by 40% if the engine is already running, and the diagnostic lamp illuminates immediately. This prevents relay welding or starter over-cranking damage, as noted in Mercedes-Benz engine control documents.
Long-term diagnosis requires verifying the relay’s calibration using a J1939 diagnostic tool that reads the ECM’s internal calibration offset parameter. Real-world cases show that replacing the relay with an OEM part and performing a calibration reset via the tool resolves the fault. Technicians frequently see this after a battery jump-start where voltage spikes altered the relay’s magnetic properties.
Step-by-Step Troubleshooting Guide
- Check battery voltage: Measure at relay coil with multimeter during key-on; must be above 12.0V (24V system: 23.5V).
- Inspect relay socket: Remove relay and check for corrosion or bent pins; clean with electrical contact cleaner.
- Measure coil resistance: Disconnect relay and measure across coil terminals; compare to OEM spec (typically 70–90 ohms).
- Reset ECM calibration: Use diagnostic tool to perform starter relay calibration reset, then cycle key-on three times.