SPN 1078 FMI 6: Meaning and Fix
SPN 1078 FMI 6 indicates the Engine Fuel Injection Pump 1 Speed/Position Sensor circuit has current above normal or a short to ground. This typically occurs after a sensor wire chafes against the injection pump housing or during a high-pressure fuel line replacement when the connector is accidentally pinched. The ECM detects the ground fault and logs the code immediately, often preventing engine start or causing an immediate shutdown.
Common Symptoms
- No Engine Start: ECM inhibits cranking or fuel delivery when sensor signal is shorted to ground.
- Intermittent Stalling: Engine runs then dies abruptly as wire harness vibration causes intermittent short circuit.
- Erratic Tachometer: Engine speed gauge fluctuates wildly or drops to zero due to corrupted sensor signal.
- Reduced Power Mode: ECM forces torque derate to protect the injection pump from uncontrolled operation.
Probable Causes
- Wiring Chafing: Sensor harness rubs against pump housing or engine bracket, exposing copper wire.
- Connector Corrosion: Moisture ingress at the 3-pin connector creates a conductive path to ground.
- Sensor Internal Short: Failed Hall-effect sensor element has a direct short between signal and ground pins.
- ECM Pin Damage: Bent or pushed-back terminal at ECM connector J1 causes signal wire to ground.
Advanced Technical Analysis
The ECM monitors the injection pump speed sensor via a pull-up resistor to 5V. Under normal operation, the sensor pulls the signal line low in a square-wave pattern. When FMI 6 sets, the ECM detects the signal voltage is below 0.5V for longer than the 200ms debounce filter, indicating a hard short to ground. This logic is consistent with Bosch EDC7 and EDC17 control units used on MAN D20 and Deutz TCD engines.
Electrical analysis shows that a short-to-ground fault creates a current path far exceeding the 10mA nominal sensor draw. The ECM’s current-limiting driver activates, but the persistent overcurrent triggers the fault. In Mercedes-Benz OM471 engines, the sensor supply is protected by a 50mA polyswitch; a sustained short will trip this device, causing a loss of both sensor and engine speed signal.
Upon detecting the short, the ECM enters a safety fallback mode. It disables the injection pump actuator and sets the engine to idle or zero fuel. In MAN D2676 engines, this triggers a 100% torque derate with a 1500 RPM hard limit. The driver sees a warning lamp and reduced power, preventing overspeed or pump damage from uncontrolled fuel delivery.
Long-term prevention includes inspecting the sensor harness routing at every major service. Technicians frequently encounter this fault after replacing the injection pump and failing to secure the bracket correctly. In Deutz TCD 2015 engines, the sensor wire often chafes against the high-pressure pipe clamp. Using spiral wrap and dielectric grease on the connector reduces recurrence significantly.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect sensor wiring from connector to ECM for chafing, burns, or pinched areas near pump.
- Resistance Measurement: Disconnect sensor and ECM. Measure resistance between signal pin and ground; should be OL.
- Connector Pin Pull Test: Gently tug each wire at the sensor and ECM connector to verify terminals are locked.
- Sensor Substitution Test: If wiring passes, replace the speed sensor with a known-good unit and clear codes.