SPN 3661 FMI 4: Meaning and Fix
SPN 3661 FMI 4 indicates the ECM detected voltage below normal or a shorted-low condition on the second actuator circuit for cylinder 3 fuel injector. This fault commonly appears after a wiring harness chafes against a valve cover or during injector replacement when the connector is not fully seated. The ECM monitors driver feedback and will set the code when the actuator voltage remains below the calibrated threshold for more than 0.5 seconds.
Common Symptoms
- Misfire Cylinder 3: Engine runs rough with a persistent misfire on cylinder 3, noticeable at idle and low load.
- Reduced Power Output: ECM activates torque limitation, reducing engine power by up to 40% to protect the injector.
- Hard Start Condition: Extended cranking time due to incomplete fuel delivery from the shorted actuator circuit.
- Illuminated MIL: Malfunction Indicator Lamp and Amber Warning Lamp are active on the dash panel.
Probable Causes
- Shorted Actuator Wiring: Insulation breakdown causing the actuator signal wire to short to engine ground or chassis.
- Failed Injector Actuator: Internal coil short-circuit in the cylinder 3 actuator 2 solenoid, drawing excessive current.
- ECM Driver Damage: Overcurrent event has permanently damaged the low-side driver output for that actuator channel.
- Corroded Connector Pins: Moisture ingress at the injector harness connector causing pin-to-pin short or high resistance.
Advanced Technical Analysis
The ECM uses a low-side driver with a pull-up voltage of 5V to monitor the actuator circuit integrity. When the driver is off, the ECM expects to see battery voltage on the line. A voltage below 2.5V for more than 200ms triggers FMI 4. The debouncing timer prevents false flags from transient noise but confirms a sustained short.
A short-to-ground creates a current path that bypasses the actuator coil. The ECM’s overcurrent detection (typically 10A threshold) will shut down the driver within 1ms to prevent thermal damage. This is followed by a diagnostic trouble code latched after three consecutive failed actuation attempts during a key cycle.
Once the fault is active, the ECM disables fuel injection to cylinder 3 and enters a torque derate strategy. The injection timing for remaining cylinders is re-optimized to reduce vibration. On MAN D26 engines, this derate limits torque to 60% of nominal until the fault is cleared.
Long-term prevention requires dielectric grease on all injector connectors and routing harnesses away from sharp valve cover edges. Technicians often encounter this fault after a forced DPF regeneration because thermal expansion loosens connector terminals. Always replace the entire injector harness if any pin corrosion is found.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect the injector harness from ECM to cylinder 3 for chafing, burns, or pinched wires near the valve cover.
- Resistance Measurement: Measure actuator resistance between pin A and B at the injector connector. Spec is 0.5-2.0 ohms.
- Short-to-Ground Test: With key off, measure resistance from actuator signal pin to engine ground. Must be >10k ohms.
- ECM Output Test: Use a breakout box to command actuator on. Verify 12V at the ECM connector pin with a test light.