SPN 559 FMI 5: Meaning and Fix
SPN 559 FMI 5 signals a current below normal or open circuit in the accelerator pedal kickdown switch circuit, as defined by SAE J1843. This fault commonly appears after an ECM replacement or harness repair where the two-wire switch signal is interrupted. The ECM expects a 0-5V or pull-up resistor response; an open line triggers FMI 5. Technicians often see this after a forced DPF regeneration when the pedal is fully depressed without a switch closure.
Common Symptoms
- No Kickdown Response: Engine fails to downshift or increase power when pedal is fully depressed beyond the detent point.
- Reduced Engine Power: ECM may limit torque as a safety measure, causing sluggish acceleration on grades.
- Intermittent Cruise Disengagement: Cruise control may drop out unexpectedly due to erratic switch signal detection.
- Check Engine Light Active: Dashboard warning lamp illuminates, often accompanied by a logged active or inactive fault.
Probable Causes
- Open Circuit in Harness: Broken wire or corroded terminal at the pedal connector or ECM harness interface.
- Faulty Kickdown Switch: Internal switch contact failure or mechanical wear preventing circuit closure.
- ECM Pull-Up Resistor Failure: Internal ECM resistor open, causing the ECM to read an open circuit on the signal line.
- Damaged Pedal Assembly: Physical damage to the pedal mechanism or misalignment of the kickdown actuator.
Advanced Technical Analysis
The ECM monitors the kickdown switch via a digital input with an internal pull-up resistor to 5V. In normal operation, the switch grounds the line when active (01b), pulling the voltage low. When open (00b), the pull-up holds the line high. FMI 5 is triggered when the ECM detects a sustained high-impedance state (>10 kΩ) for more than 200 ms, indicating an open circuit. This logic is based on SAE J1939-71 and Bosch ECU design guidelines.
Electrically, an open circuit can result from a broken wire between pin 29 of the EEC2 PG and the switch, or a failed pull-up resistor inside the ECM. The ECM performs a debouncing timer of 100 ms to filter noise; if the open condition persists across three consecutive 50 ms samples, the fault is confirmed. MAN and Deutz factory manuals recommend measuring resistance between the signal pin and ground; expected value < 1 Ω when closed, > 1 MΩ when open.
Upon detecting FMI 5, the ECM enters a safety fallback mode: it sets the kickdown status to ’10b’ (Error) and disables the kickdown function entirely. Torque derate of up to 25% may be applied to prevent unintended acceleration. The cruise control is also inhibited. Mercedes-Benz powertrain controllers log this fault with a freeze frame of pedal position and engine speed, aiding post-failure analysis.
Long-term prevention involves inspecting the pedal harness for chafing near the pedal bracket—a common failure point on heavy trucks. After ECM replacement, always verify the kickdown switch circuit with a breakout box. Technicians frequently resolve this fault by replacing the pedal assembly or repairing a broken wire at the chassis harness splice, as seen in Deutz TCD 2015 engine applications.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check pedal connector for corrosion, bent pins, or broken wires at the harness strain relief.
- Measure Switch Resistance: Disconnect pedal, measure across switch terminals: <1 Ω closed, >1 MΩ open.
- Check ECM Input Voltage: At ECM connector, measure signal pin to ground: expect 5V open, <0.5V closed.
- Verify Pull-Up Resistor: With pedal disconnected, measure resistance from signal pin to 5V reference; 4.7 kΩ typical.