SPN 677 FMI 8: Meaning and Fix
SPN 677 FMI 8 indicates the Engine Starter Motor Relay circuit is sending an abnormal frequency, pulse width, or period signal to the ECM. This typically occurs when the relay coil’s PWM duty cycle is corrupted by voltage spikes or a failing driver. Technicians often encounter this after a jump-start with reversed polarity or a failed aftermarket alarm installation that taps into the starter circuit.
Common Symptoms
- No Crank Condition: Engine fails to crank intermittently despite adequate battery voltage and a healthy starter motor.
- Erratic Starting: Starter engages and disengages rapidly during key-on, producing a chattering sound from the relay.
- ECM Derate: Engine enters a reduced power mode, limiting RPM to protect the starter circuit from further damage.
- False Active Codes: Intermittent spurious faults logged in the ECM, often linked to electromagnetic interference from high-current wiring.
Probable Causes
- Shorted Relay Coil: Internal winding short circuit alters the relay’s inductive load, causing abnormal frequency readings at the ECM input.
- Wiring Harness Damage: Chafed or corroded wires between the relay and ECM change the signal pulse width due to resistance fluctuations.
- Faulty ECM Driver: The internal low-side driver in the ECM is failing, producing an unstable PWM signal to the relay coil.
- Aftermarket Modifications: Non-factory remote start systems or security modules inject unintended pulses into the starter relay control line.
Advanced Technical Analysis
The ECM monitors the starter relay command signal through a dedicated feedback pin. It compares the commanded duty cycle against the actual measured frequency. When the feedback deviates by more than 15% from the expected 100 Hz baseline for more than 200 ms, FMI 8 is latched. This logic prevents misdiagnosis of transient noise.
Debouncing timers in the ECM firmware filter out short glitches, but a relay with a worn mechanical contact can generate arcing spikes. These spikes shift the measured period, triggering the fault. In Deutz and MAN ECUs, the threshold is set to 120 Hz ± 10 Hz, requiring an oscilloscope to verify the actual signal integrity.
Upon detecting this fault, the ECM activates a safety fallback by disabling the starter command entirely. It also sets a torque derate to 40% if the vehicle is in motion, preventing unexpected starter engagement. Mercedes-Benz powertrains additionally trigger a warning lamp and store a freeze frame with battery voltage and engine speed.
Long-term prevention involves verifying the relay’s flyback diode is intact; a failed diode allows voltage spikes to corrupt the signal. In workshop practice, this code appears after replacing the ECM without reprogramming the starter control parameters. Always perform a resistance check and use a current clamp to measure coil draw, which should be 2-3 amps.
Step-by-Step Troubleshooting Guide
- Inspect Wiring: Visually check the relay-to-ECM harness for chafing, corrosion, or loose pins, especially near the battery tray.
- Measure PWM Signal: Use an oscilloscope to capture the relay control wire frequency; verify it matches 100 Hz ± 10 Hz during cranking.
- Test Relay Coil: Measure coil resistance; it should be 60-80 ohms. A shorted coil reads below 20 ohms, causing abnormal frequency.
- Check ECM Grounds: Verify the ECM ground studs are clean and torqued to spec, as poor grounds induce noise that alters pulse width.