SPN 678 FMI 31: Meaning and Fix
SPN 678 FMI 31 signals that the ECU has detected a persistent condition on its internal 8-volt DC supply rail, which powers sensors like the accelerator pedal and rail pressure sensor. This code commonly appears after a forced DPF regeneration when thermal stress degrades a connector, or when a technician inadvertently back-probes the sensor supply line, causing a latent short. The ECM monitors this rail using a window comparator; if the voltage remains outside 7.3–8.7 V for more than ten seconds, this fault activates.
Common Symptoms
- Erratic Sensor Data: Accelerator pedal and rail pressure readings jump or peg to default values.
- Reduced Engine Power: ECM enters torque derate, limiting speed to 1400 rpm for limp-home.
- Stalling at Idle: Engine may stall when returning to idle due to unstable sensor reference.
- No Start Condition: If 8V rail collapses below 5V, ECM refuses to enable fuel injection.
Probable Causes
- Shorted Sensor Supply: Internal short in a 5V or 8V sensor, backfeeding or pulling rail low.
- ECM Internal Failure: Failed voltage regulator or decoupling capacitor inside the ECM housing.
- Wiring Harness Chafe: Insulation worn through on engine harness, contacting frame ground or 24V.
- Corroded Connector Pins: Oxidation at ECM main connector pins C and D, increasing contact resistance.
Advanced Technical Analysis
The ECM microcontroller runs a background diagnostic task that samples the 8V rail via an analog-to-digital converter at 100 Hz. If the sampled voltage falls below 7.3V or exceeds 8.7V for a debounce time of 2.5 seconds, a pending code is set. After ten consecutive seconds of violation, FMI 31 becomes active. This logic prevents false triggers from transient loads like injector solenoids firing.
Electrical analysis of the 8V rail shows it is derived from the main 12V or 24V supply via a linear regulator with a 500 mA current limit. A short to ground on any sensor line can pull the rail below threshold. The ECM includes a current-sense resistor; if current exceeds 600 mA, the regulator folds back, causing a brownout. Technicians should measure resistance between each sensor supply pin and ground—values below 10 kΩ indicate a problem.
When the 8V rail fails, the ECM activates a safety fallback: it sets the default value for all dependent sensors to a predetermined safe state (e.g., accelerator pedal to 0%, rail pressure to 200 bar). Torque is derated linearly to 30% of maximum, and vehicle speed is limited to 20 km/h. This prevents runaway conditions but can strand the vehicle if the operator ignores the warning lamp.
Long-term prevention requires verifying that all sensor connectors have dielectric grease applied and that harness clips are intact to prevent chafing. In real-world workshops, this fault is often misdiagnosed as a failed pedal sensor; always disconnect all sensors on the 8V rail first. If the rail recovers, reconnect one sensor at a time to identify the culprit. A known case: a MAN TGS saw this code after a DPF burn-off melted the injector harness near the turbocharger.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect engine harness from ECM to sensors for chafing, burns, or melted insulation.
- Disconnect All Sensors: Unplug all 8V sensors; if voltage returns to 8.0V, reconnect one by one.
- Pin Resistance Test: Measure resistance from ECM pin to ground; less than 10 kΩ indicates short.
- ECM Supply Check: Back-probe ECM power pins; if 8V absent with all sensors off, replace ECM.