SPN 5681 FMI 17: Meaning and Fix
SPN 5681 FMI 17 indicates the driver activation demand for the Advanced Emergency Braking System (AEBS) is below the normal operating range, meaning the signal is valid but represents a deactivation command (00b). This code commonly appears after a technician or driver accidentally toggles the AEBS off via the dashboard switch, or after a failed HMI unit sends a constant low signal. In practice, it is frequently seen on Mercedes-Benz Actros trucks after battery disconnection or software updates, where the default state is misinterpreted.
Common Symptoms
- AEBS Disabled: The AEBS warning lamp illuminates and the system does not intervene, despite the driver not intentionally deactivating it.
- No Alerts: Forward collision warnings are suppressed, and the driver receives no audible or visual alerts when approaching obstacles.
- HMI Malfunction: The dashboard switch or menu may be unresponsive, or the system status display shows ‘Off’ even when toggled on.
- Intermittent Loss: The AEBS may temporarily activate and deactivate, causing inconsistent braking behavior and driver confusion.
Probable Causes
- HMI Output Low: The human-machine interface unit sends a continuous low signal (00b) due to internal failure or stuck switch contact.
- Wiring Short: Short to ground in the signal line between the HMI and AEBS ECU, pulling the voltage below the normal threshold.
- ECU Misinterpretation: The AEBS ECU incorrectly interprets a valid high signal as low due to corrupted calibration or software glitch.
- Power Supply Issue: Insufficient voltage supply to the HMI or AEBS unit, causing the signal to drop below the valid range during operation.
Advanced Technical Analysis
The AEBS ECU monitors the driver activation demand via a dedicated digital input. The signal is expected to be either 00b (deactivation) or 01b (activation). FMI 17 indicates that the received value is valid (no electrical fault) but below the normal operating range—meaning the ECU sees a persistent 00b even when the driver requests activation. The microcontroller compares the input against a debounced threshold; if the signal remains low for more than 500 ms, it sets the fault and disables AEBS functions.
Electrical analysis reveals that the signal line is typically pulled up to 5V via a 10kΩ resistor inside the ECU. A short to ground or an internal HMI driver failure can force the line low. The ECU’s input stage includes a low-pass filter to suppress noise, but a continuous low state bypasses this filter. In workshop scenarios, technicians often find that the HMI switch has worn contacts, causing intermittent low states that eventually become permanent.
For safety, the AEBS ECU enters a fallback mode when FMI 17 is set. It disables all automatic braking interventions and warning functions, reverting to manual braking only. This is mandated by the Vienna Convention, ensuring the driver retains full control. The ECU does not derate engine torque, but it may log the event and illuminate the amber warning lamp. In some vehicles, the system automatically resets after the next ignition cycle if the signal returns to normal.
Long-term prevention involves verifying the HMI software version and ensuring proper grounding of the cab. Real-world examples include a fleet of MAN TGX trucks where repeated FMI 17 occurrences were traced to a faulty dashboard cluster. Replacement of the cluster and reprogramming resolved the issue. Regular checks of the switch contacts and wiring harness, especially near the steering column, reduce recurrence. Technicians should also check for aftermarket accessories that may interfere with the CAN bus or signal line.
Step-by-Step Troubleshooting Guide
- Scan & Verify: Use a diagnostic tool to confirm SPN 5681 FMI 17 is active. Check if the AEBS status changes when toggling the HMI switch.
- Inspect Wiring: Measure voltage at the AEBS ECU pin for the driver demand signal. Ensure it reads near 5V when activated and near 0V when deactivated.
- Test HMI Unit: Operate the HMI switch and monitor the signal. If it does not transition to high, replace the HMI or repair the switch contacts.
- Check Grounds: Inspect the ground connections for the HMI and AEBS units. Poor grounds can cause voltage drops that mimic a low signal.