SPN 973 FMI 18: Meaning and Fix
This fault indicates the engine retarder selector position signal is below the normal operating range, meaning the ECM detects a voltage lower than expected for a valid request. Technicians often see this after a forced DPF regeneration or ECM replacement, when the selector calibration is lost or the sensor supply voltage drops. The ECM interprets this as a minimal braking request, potentially reducing retarder effectiveness.
Common Symptoms
- Reduced Retarder: Engine brake provides less than requested torque, especially at lower selector positions.
- No Retarder Response: Operator selects high braking but ECM ignores request due to low signal voltage.
- Intermittent Braking: Retarder engages and disengages randomly when selector is moved through its range.
- Dash Warning Light: Amber or red engine brake warning lamp illuminates on the instrument cluster.
Probable Causes
- Faulty Selector Sensor: Internal resistive track wear or short to ground causes output voltage below 0.5 V.
- Wiring Harness Damage: Chafed or corroded signal wire to J1939 EBC1 node increases resistance or shorts.
- ECM Calibration Error: Incorrect retarder selector calibration after ECM software update or replacement.
- 5V Reference Failure: Sensor supply voltage drops below 4.75 V due to overloaded reference circuit.
Advanced Technical Analysis
The ECM continuously monitors the retarder selector position via a 0-5 V analog input on the EBC1 message. When voltage falls below the calibrated minimum threshold (typically 0.5 V for 0% request), the ECM sets FMI 18. The signal is expected to be linear; a reading of 0.3 V triggers the fault. This is often caused by a failing potentiometer in the selector handle, common on trucks with high mileage.
Electrical analysis reveals that the signal line may have a partial short to ground, pulling the voltage down. Using a DMM, the technician should see less than 0.5 V at the ECM pin. The ECM applies a debouncing timer of 500 ms to avoid false triggers from noise. A persistent low reading for over 1 second confirms the fault. Real-world example: a MAN TGS had a chafed wire near the steering column causing this exact code.
When FMI 18 is active, the ECM defaults to a safe braking request of 0%, effectively disabling the retarder. The engine brake solenoid is de-energized, and the vehicle relies solely on service brakes. Torque derate is not applied, but the driver loses auxiliary braking. This is a moderately severe condition per SAE J1939, requiring immediate diagnosis to restore safety on downhill grades.
Long-term prevention involves inspecting the selector wiring during clutch or steering column repairs. Technicians should also verify the 5 V reference at the sensor connector (pin A) is within 4.75-5.25 V. After replacing the selector, a calibration procedure via diagnostic tool is mandatory. In Deutz engines, this fault often appears after ECM reprogramming if the calibration values are not restored from the original configuration file.
Step-by-Step Troubleshooting Guide
- Scan for Codes: Read all active and inactive SPNs. Check for related supply voltage faults (SPN 168).
- Measure Selector Voltage: Backprobe ECM connector; check signal voltage at idle. Should be 0.5-4.5 V across range.
- Inspect Harness: Visually check signal wire for chafing near steering column or frame rails. Repair as needed.
- Calibrate Selector: Use manufacturer tool to relearn minimum and maximum positions per service manual.