SPN 559 FMI 2: Meaning and Fix
The accelerator pedal kickdown switch signal is erratic, intermittent, or incorrect per SAE J1939. The ECM detects invalid state transitions (e.g., 10b error) or signal dropout. This fault commonly appears after a forced DPF regeneration when the pedal is fully depressed, causing switch contact bounce or wiring chafing. Technicians frequently encounter this after replacing the ECM without recalibrating the pedal assembly.
Common Symptoms
- No kickdown response: Full throttle depression fails to trigger downshift or power boost; transmission stays in current gear.
- Intermittent power loss: Engine torque derates sporadically during highway driving, especially after pedal floor contact.
- False kickdown activation: Transmission downshifts without intentional full throttle, causing surging on slight inclines.
- Stored diagnostic codes: SPN 559 FMI 2 logs alongside SPD or transmission controller faults for cross-system validation.
Probable Causes
- Faulty kickdown switch: Internal contact wear or corrosion causes erratic open/close signals during pedal travel.
- Wiring harness damage: Chafed or broken wires near pedal pivot or chassis ground cause intermittent signal loss.
- Poor ECM ground: High resistance at ECM ground pin disrupts switch pull-up voltage, creating erratic readings.
- ECM software mismatch: Incorrect calibration after ECM replacement fails to recognize switch debounce timing parameters.
Advanced Technical Analysis
The ECM monitors the kickdown switch via a digital input with a pull-up resistor. The signal must transition cleanly between 00b (passive) and 01b (active) within a defined debounce window (typically 20-50 ms). FMI 2 sets when the ECM detects a 10b error state or multiple rapid transitions exceeding the debounce timer, indicating contact bounce or intermittent open circuit. This logic is based on SAE J1939-71 and Bosch EDT control strategies.
Electrically, the kickdown switch is a normally open contact that closes to ground when the pedal reaches 95-100% travel. A 10b error occurs if the ECM sees both high and low signals simultaneously due to a short to battery or ground. German OEM manuals (Mercedes-Benz, MAN) specify a maximum contact resistance of 5 ohms; higher values cause erratic data. Intermittent faults often stem from moisture ingress in the pedal connector.
When FMI 2 activates, the ECM defaults to a safe mode: it ignores the kickdown signal and uses accelerator pedal position alone for torque calculation. Engine torque is derated by 25-40% to prevent unintended acceleration. The transmission controller receives a ‘no kickdown’ request via J1939, disabling downshifts. This fallback ensures vehicle safety but reduces performance, especially during overtaking or hill climbing.
Long-term diagnosis requires oscilloscope capture of the switch signal during pedal sweep. A stable 12V pull-up dropping to 0V at kickdown indicates proper operation. Real-world workshops report that replacing the pedal assembly (including the switch) resolves 80% of cases; the remaining 20% involve ECM connector pin fretting. Preventive maintenance includes dielectric grease on contacts and verifying pedal floor stop clearance per OEM specs.
Step-by-Step Troubleshooting Guide
- Verify switch operation: Use multimeter at pedal connector; continuity should appear only at full pedal depression.
- Inspect wiring continuity: Check resistance between switch and ECM pins; must be below 2 ohms per MAN factory spec.
- Check ECM ground: Measure voltage drop between ECM ground and battery negative; should be under 50 mV.
- Recalibrate pedal assembly: Perform ECM pedal learn procedure per manufacturer; clear codes and road test for kickdown response.