SPN 5927 FMI 31: Meaning and Fix
SPN 5927 FMI 31 signals that the engine coolant pump circuit has detected a continuous fault condition, such as a short to battery or an open ground. This code commonly appears after a forced DPF regeneration when the pump runs at high speed and a weak connector fails. Technicians frequently encounter this fault after replacing the ECM if the pump harness is not properly reseated, leading to intermittent pump shutdown and rapid coolant temperature rise under load.
Common Symptoms
- Overheating: Engine coolant temperature rises above 105°C under moderate load due to reduced pump flow.
- Intermittent Pump Stop: Coolant pump ceases operation for 2-5 seconds during high-speed operation, then restarts.
- Cabin Heater Failure: No warm air from HVAC at idle because coolant circulation through heater core is insufficient.
- DTC Recurrence: Fault returns after ignition cycle even with cleared codes, indicating a persistent electrical issue.
Probable Causes
- Wiring Harness Damage: Chafed or pinched wires at the pump connector or along the frame rail near the radiator.
- PWM Signal Loss: ECM PWM output open circuit or short due to corroded pins in the 2-pin connector.
- Pump Motor Locked: Mechanical seizure of the pump impeller from debris or worn bearings causing high current draw.
- ECM Internal Fault: Faulty driver transistor in the ECM output stage for the pump control circuit.
Advanced Technical Analysis
The ECM monitors the coolant pump feedback line using a high-side driver with current sense. When the commanded PWM duty cycle does not match the actual current draw within a 500ms debounce window, the ECM sets SPN 5927 FMI 31. The trigger threshold is typically a current deviation above 2.5A or below 0.2A for three consecutive samples, per Bosch MD1 ECU logic.
Electrical analysis reveals that a short to battery on the pump control line causes the ECM to see a continuous high voltage (>13V) instead of the expected PWM waveform. The ECM’s debouncing timer (1.2 seconds) must elapse before logging the fault. This prevents false triggers from transient spikes during starter engagement, as documented in MAN D2866 factory wiring diagrams.
Upon activation, the ECM enters a safety fallback: it commands the pump to run at 100% duty cycle (if possible) to maintain minimum flow. If no current is detected, the ECM triggers a torque derate of 25% and illuminates the MIL. The engine may also command the fan clutch to full lock to compensate for reduced coolant circulation, per Deutz common rail strategies.
Long-term prevention requires verifying the pump’s PWM signal with an oscilloscope at the ECM connector. In workshops, this fault often recurs after a coolant change if air is trapped in the pump cavity, causing momentary dry running and high current spikes. A proper vacuum-fill procedure and dielectric grease on pins reduce recurrence. Always check the pump ground stud on the engine block for corrosion.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect pump connector and harness for corrosion, bent pins, or chafing near the fan shroud.
- Measure Supply Voltage: Check for 12V at pump pin 1 relative to ground with ignition on; if missing, repair circuit.
- PWM Signal Test: Use oscilloscope on pin 2; expect a 100Hz square wave with 10-90% duty cycle at idle.
- Current Draw Check: Clamp ammeter on pump supply wire; normal draw is 1.5-2.5A; higher indicates mechanical bind.