SPN 1067 FMI 6: Meaning and Fix
SPN 1067 FMI 6 indicates the brake signal sensor 1 circuit has detected current above normal or a short to ground. This often occurs after a brake pedal assembly replacement or wiring repair near the frame rail, where insulation chafing causes a direct ground path. Technicians commonly see this fault when the sensor harness rubs against the transmission bell housing.
Common Symptoms
- Reduced engine torque: ECM limits torque output to 50% as a safety response to the invalid brake signal.
- Brake light always on: The dash brake indicator remains illuminated even with the pedal fully released.
- Cruise control inactive: Cruise control disengages and will not re-engage until the fault is cleared.
- J1939 diagnostic lamp: Red stop lamp and amber warning lamp activate simultaneously on the CAN bus.
Probable Causes
- Sensor short to ground: Internal short in the sensor or wiring to chassis ground causing excessive current draw.
- Chafed harness: Insulation wear on the sensor harness near the engine mount or frame rail.
- Failed sensor element: Hall-effect or resistive element inside the sensor has failed closed-circuit.
- ECM driver fault: Internal ECM pull-up resistor or current monitor circuit damaged from a previous short.
Advanced Technical Analysis
The ECM monitors the brake signal sensor 1 circuit using a current-sense resistor and a comparator threshold. When the measured current exceeds the calibrated limit (typically 100 mA for a 5V pull-up), the ECM sets FMI 6 immediately. This logic is derived from Bosch ECU application notes for off-highway vehicles.
A short to ground in the sensor line causes the ECM’s internal pull-up to source maximum current, often triggering a debouncing timer of 200 ms to confirm the fault before logging. This prevents false triggers from transient spikes, as specified in SAE J1939-71 diagnostics.
Upon activation, the ECM enters a safety fallback: it sets the brake signal to ‘brake applied’ permanently, disables cruise control, and requests a torque derate to 50% via the TSC1 message. This aligns with MAN and Deutz factory safety strategies for redundant brake monitoring.
Long-term prevention includes inspecting the harness routing at the bell housing and frame crossmembers, where chafing is common. A real-world case: a Mercedes-Benz Actros repeatedly logged this code until the harness was rerouted away from the exhaust bracket. Using Deutsch DT connectors with proper strain relief reduces recurrence.
Step-by-Step Troubleshooting Guide
- Visual harness inspection: Check the sensor harness from the ECM to the brake pedal for chafing or melted insulation.
- Measure resistance to ground: Disconnect sensor; measure resistance between signal pin and chassis ground; should be >1 MΩ.
- Sensor output test: Apply 5V supply to sensor; verify output voltage changes from 0.5V to 4.5V with pedal travel.
- ECM pin voltage check: With sensor disconnected, measure ECM pin voltage; should be 5V ±0.2V. If lower, ECM may be damaged.