SPN 5927 FMI 5: Meaning and Fix
SPN 5927 FMI 5 indicates the engine coolant pump control circuit has detected current below normal or an open circuit. The ECM monitors the pump driver feedback and sets this code when the expected current draw is not present. Technicians frequently encounter this fault after replacing the ECM without reprogramming the pump parameters, or after a wiring harness chafes against the engine block near the pump connector, breaking the signal wire.
Common Symptoms
- Coolant Overheating: Engine coolant temperature rises above normal range due to reduced or no coolant circulation.
- Check Engine Lamp: Red or amber malfunction indicator lamp illuminates on the dash, often accompanied by a derate.
- No Pump Operation: Coolant pump does not run during key-on or engine operation; no audible or tactile vibration.
- Intermittent Pump Run: Pump starts and stops erratically, especially under vibration or thermal cycling, indicating loose connection.
Probable Causes
- Open Circuit Wiring: Broken wire in the pump power or ground circuit, often at connector pins or harness chafe points.
- Failed Pump Motor: Internal open winding or brush failure in the electric coolant pump motor assembly.
- ECM Driver Fault: Internal open circuit in the ECM low-side driver or damaged output stage from overcurrent.
- Corroded Connector: Oxidation or fretting corrosion on pump connector terminals causing high resistance or open circuit.
Advanced Technical Analysis
The ECM continuously monitors the pump current via a precision shunt resistor. When the measured current falls below the calibrated threshold (typically 0.1 A for more than 2 seconds), the microcontroller sets FMI 5. The diagnostic debounce timer prevents false triggers from transient loads, but a sustained open circuit immediately latches the fault.
Electrical analysis reveals that an open circuit increases the voltage at the ECM driver pin to near battery potential due to pull-up resistors. The ECM compares this against expected voltage drop across the pump winding. If the pump resistance exceeds 10 kΩ, the driver detects no current flow and flags the fault. Common causes include broken wires at the 90-degree bend near the pump housing.
Upon detecting SPN 5927 FMI 5, the ECM initiates a progressive torque derate strategy, reducing engine power by 25% after 30 seconds. If the coolant temperature exceeds 105°C, the ECM commands an engine shutdown to prevent thermal damage. The pump is depowered, and the fan clutch may engage at maximum speed as a fallback.
Long-term diagnosis should include measuring pump winding resistance (typically 0.5–2.0 Ω at 20°C) and checking for intermittent opens by flexing the harness. Real-world workshops report this code often appears after a DPF regeneration event, when thermal stress causes a marginal connection to fail. Using Deutsch DT connectors with dielectric grease prevents recurrence.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect pump connector and wiring harness for chafing, corrosion, or broken wires near the pump.
- Resistance Measurement: Disconnect pump, measure resistance across pump terminals. Spec: 0.5–2.0 Ω. Open indicates failed pump.
- Voltage Drop Test: With pump connected, measure voltage at pump connector. Expect battery voltage. Low indicates open circuit.
- ECM Driver Check: Probe ECM pin for pump control. Key-on, engine-off: expect 12V. No voltage indicates ECM driver fault.