SPN 973 FMI 2: Meaning and Fix
SPN 973 FMI 2 indicates the Engine Retarder Selection signal is erratic, intermittent, or incorrect. The ECM detects a voltage or frequency outside the valid range for the selector position. This code commonly appears after a forced DPF regeneration when heat damages the selector potentiometer wiring. Technicians frequently encounter this fault after replacing the ECM without recalibrating the retarder lever.
Common Symptoms
- No Retarder Response: Operator selects braking but no engine retarder torque is applied due to invalid signal.
- Intermittent Braking: Retarder engages and disengages randomly while driving at highway speeds.
- Warning Lamp On: Amber or red engine warning lamp illuminates with active SPN 973 FMI 2.
- Torque Derate: ECM reduces engine power to prevent uncontrolled braking events.
Probable Causes
- Faulty Selector Potentiometer: Worn resistive track or wiper causing erratic voltage output to ECM.
- Open or Shorted Wiring: Chafed harness between selector and ECM causing intermittent signal loss.
- ECM Ground Offset: High resistance in ECM ground circuit shifts signal reference voltage.
- Corroded Connector Pins: Oxidation on J1939 or selector connector pins causes intermittent contact.
Advanced Technical Analysis
The ECM monitors the selector signal at a 100 Hz sampling rate. A valid signal must remain between 0.5V (0%) and 4.5V (100%) with less than 200 mV noise. FMI 2 sets when the ECM detects signal transitions exceeding 500 mV within 10 ms, indicating a mechanical or electrical bounce. The ECM then freezes the last valid value and triggers the fault.
Electrical analysis of the debouncing timer shows the ECM uses a 50 ms filter window. If the signal crosses the valid threshold more than three times within that window, the fault is logged. This often results from a failing potentiometer wiper that creates momentary open circuits. The ECM then flags the data as erratic rather than out-of-range.
When FMI 2 is active, the ECM enters a safety fallback mode. It sets the retarder request to 0% and disables the engine brake. Simultaneously, a torque derate of 25% is applied to prevent unintended braking. The derate remains until the fault becomes inactive and a key cycle is performed. This protects the driveline from sudden load changes.
Long-term diagnosis requires oscilloscope capture of the selector signal during engine vibration. Technicians should replicate real-world conditions by running the engine at 1500 RPM and wiggling the harness. A common workshop scenario is finding a broken wire inside the selector stalk insulation after a steering column repair. Replacing the stalk assembly and recalibrating the retarder via diagnostic tool resolves the fault.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check selector harness for chafing, pin corrosion, and loose connectors at ECM and dash.
- Signal Voltage Test: Measure selector output at ECM pin; should sweep 0.5-4.5V with smooth transitions.
- Oscilloscope Capture: Monitor signal during engine vibration; look for spikes or dropouts >200 mV noise.
- Calibration Check: Perform retarder lever recalibration using OEM diagnostic tool to match ECM range.