SPN 110 FMI 2: Meaning and Fix
SPN 110 FMI 2 indicates the Engine Coolant Temperature (ECT) sensor signal is erratic, intermittent, or incorrect per SAE J1939. The ECM detects voltage or rate-of-change values outside expected thresholds. Technicians frequently encounter this after a recent coolant flush, when air pockets cause rapid temperature swings, or after ECM replacement if the sensor calibration is mismatched.
Common Symptoms
- Erratic Temp Gauge: Dashboard coolant temperature needle fluctuates wildly or jumps to extremes without engine load change.
- Cold Start Issues: Engine cranks longer than normal or stalls because ECM receives incorrect temperature for fuel timing.
- Fan Clutch Cycling: Engine cooling fan engages and disengages randomly, causing audible cycling and power loss.
- Derate Activation: ECM reduces engine torque by 25–40% to protect against perceived overheating, often under load.
Probable Causes
- Wiring Harness Damage: Chafed or pinched wires near the thermostat housing causing intermittent short or open circuit.
- Connector Corrosion: Moisture ingress at the two-pin ECT connector, creating intermittent resistance and signal dropouts.
- Sensor Internal Failure: NTC thermistor element cracked or delaminated, producing non-linear resistance under temperature cycling.
- ECM Ground Offset: Poor sensor ground reference due to corroded ECM chassis ground, causing floating voltage readings.
Advanced Technical Analysis
The ECM monitors the ECT sensor via a pull-up resistor to 5V. The signal voltage is sampled at 100 Hz and filtered through a low-pass RC network. If the voltage changes by more than 10°C per 100 ms, the ECM sets FMI 2. This debouncing logic prevents false flags but can be triggered by loose connectors vibrating at engine idle.
Electrical analysis of the sensor circuit shows a 2.5V reference at 25°C (NTC 10kΩ). Intermittent faults often stem from micro-fractures in the sensor’s ceramic substrate. Thermal cycling between -40°C and 120°C expands these cracks, causing sudden resistance jumps that the ECM interprets as erratic data.
Upon detecting erratic data, the ECM activates a safety fallback: it freezes the last plausible coolant temperature and uses intake air temperature as a substitute for fuel mapping. Simultaneously, it commands a torque derate and forces the cooling fan to run continuously. This protects the engine but reduces drivability until the fault clears.
Long-term prevention requires inspecting the sensor harness for chafing at the thermostat housing bracket—a common issue on Deutz TCD engines. Real-world experience shows that replacing the sensor without testing the wiring often leads to repeat failures. Always perform a resistance sweep while flexing the harness to reproduce the intermittent fault.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect the ECT wiring from sensor to ECM for chafing, burns, or rub-through near the fan shroud.
- Connector Pin Tension: Disconnect sensor, verify female terminals have adequate grip; replace if loose to prevent intermittent contact.
- Resistance Sweep Test: Measure sensor resistance while heating with a heat gun; should decrease smoothly from 10kΩ at 25°C.
- ECM Ground Check: Measure voltage between sensor ground pin and battery negative; should be below 50 mV with engine running.
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