SPN 676 FMI 18: Meaning and Fix
SPN 676 FMI 18 indicates the Engine Glow Plug Relay circuit is reporting voltage below the normal operating range, but the data is valid. This typically occurs when the relay coil resistance increases due to thermal stress or when the ECM detects a voltage drop across the relay control circuit. Technicians frequently encounter this fault after replacing the ECM without performing a proper relay actuation test, or after a cold-start event where the battery voltage sagged below 11.5V during preheat.
Common Symptoms
- Hard Cold Start: Engine cranks longer than normal below 0°C due to insufficient glow plug heating.
- White Exhaust Smoke: Unburned fuel from cold cylinders produces white smoke on initial startup.
- Glow Plug Lamp Flicker: Dashboard indicator may flash intermittently or stay on longer than usual.
- ECM Logged Event: Fault is stored as active or inactive, often accompanied by SPN 677.
Probable Causes
- High Relay Resistance: Corroded relay contacts or coil winding degradation increases circuit resistance.
- Low Battery Voltage: Battery below 12.2V during preheat causes ECM to see below-range data.
- Wiring Harness Damage: Chafed or loose connector at relay socket introduces voltage drop.
- ECM Driver Fault: Internal ECM low-side driver may be partially shorted or thermally stressed.
Advanced Technical Analysis
The ECM continuously monitors the glow plug relay control line using a pull-up resistor network. When the relay is de-energized, the circuit should float near battery voltage. FMI 18 triggers when the sampled voltage is valid (not open or shorted) but falls below the calibrated threshold, typically 8.5V for a 24V system. This indicates the relay coil is drawing current but not enough to fully close the contacts, often due to increased series resistance from thermal aging.
Electrical analysis of the relay drive circuit reveals that a debouncing timer of 100 ms is used to filter transient voltages. If the voltage remains below the normal window for more than 500 ms, the fault is confirmed. In practice, this occurs when a relay with a coil resistance above 150 ohms (24V system) is installed, causing the ECM driver to see a voltage drop across the internal sense resistor that mimics a partially engaged relay.
When SPN 676 FMI 18 is active, the ECM may disable the glow plug relay entirely to protect the driver circuit. This prevents preheat even if the relay is physically functional. The engine management system enters a cold-start torque derate strategy, limiting fuel injection until coolant temperature rises above 40°C. Real-world cases show this often follows a battery jump-start where voltage spikes damaged the relay coil.
Long-term diagnostic strategy includes verifying relay coil resistance with a milliohm meter and performing a voltage drop test across the relay control circuit during a commanded preheat cycle. In workshops, this fault is frequently misdiagnosed as a bad glow plug module. However, factory data from MAN and Mercedes-Benz indicates that replacing the relay with an OEM-specified component (e.g., Bosch 0 332 019 150) resolves 90% of cases.
Step-by-Step Troubleshooting Guide
- Measure Relay Coil: Disconnect relay and measure coil resistance; should be 30-70 ohms for 24V.
- Check Battery Voltage: Verify battery is above 12.5V (12V system) or 24.5V (24V) at rest.
- Inspect Connector Pins: Look for corrosion or bent pins at relay socket and ECM connector.
- Command Relay On: Use diagnostic tool to actuate relay; observe voltage drop at coil.