SPN 3661 FMI 1: Meaning and Fix
This fault indicates the ECM detected the current for Engine Fuel 1 Injector Cylinder 3 Actuator 2 is below the normal operational range. Commonly seen after a failed injector replacement or loose connector, the ECM logs this when the actuator driver circuit impedance is too high. In practice, technicians often encounter this after an engine harness chafes against a bracket or after a DPF regeneration that caused excessive heat cycling on the injector wiring.
Common Symptoms
- Rough idle: Cylinder 3 misfires at idle due to insufficient fuel actuator current.
- Reduced power: ECM limits torque to protect the injector driver, causing noticeable power loss.
- Check engine light: Solid amber MIL on dash, often accompanied by a derate message.
- Hard starting: Extended crank time as ECM fails to properly actuate cylinder 3 injector.
Probable Causes
- Open circuit: Broken wire or corroded pin in the injector harness connector at cylinder 3.
- High resistance: Damaged injector actuator coil measuring >2.5 ohms above factory spec.
- ECM driver fault: Internal short or open in the ECM output stage for actuator 2.
- Connector fretting: Vibration-induced micro-motion on pin 2 causing intermittent low current.
Advanced Technical Analysis
The ECM monitors the actuator driver current via a low-side sense resistor. When the measured current falls below the calibrated threshold (typically <0.8 A for a 1.2 A nominal actuator), the controller flags FMI 1. This condition triggers immediately after a debounce timer of 50 ms to avoid false positives from electrical noise. Real-world: a technician found a pin pushed back in the connector after a valve adjustment.
Electrical analysis shows that a resistance increase of just 0.3 ohms in the high-side path can drop actuator current below the valid range. The ECM’s diagnostic routine compares instantaneous current against a dynamic threshold derived from battery voltage. In one documented case on a MAN D2676, a corroded terminal at the cylinder 3 injector caused intermittent FMI 1 only when the engine reached operating temperature.
Upon activation, the ECM disables fuel delivery to cylinder 3 and applies a 25% torque derate to protect the injector driver. The engine management system also increments an internal counter; after three occurrences within one drive cycle, the fault becomes active and latched. This safety fallback is consistent with Bosch EDC17 control logic for injector actuator 2 faults.
Long-term prevention requires verifying injector coil resistance (1.0–2.0 ohms at 20°C) and inspecting the harness for chafing near the rocker cover. A common workshop scenario: after replacing an ECM, the fault reappeared because the injector harness pin was not fully seated. Dielectric grease on the connector and a torque check on the injector hold-down bolt (35 Nm) are recommended per Deutz factory manual.
Step-by-Step Troubleshooting Guide
- Visual harness check: Inspect cylinder 3 injector harness for chafing, burns, or loose connector lock.
- Resistance measurement: Measure pin-to-pin resistance at actuator 2; should be 1.0–2.0 ohms.
- Connector pin pull test: Gently pull each wire at the ECM connector to confirm terminal retention.
- ECM output test: Use a breakout box to verify ECM driver current; replace ECM if open.