SPN 2629 FMI 13: Meaning and Fix
The Engine Turbocharger 1 Compressor Outlet Temperature sensor (SPN 2629) reports a value outside the calibrated range (FMI 13). The ECM compares the sensor reading against a model-based expected temperature derived from intake pressure and engine load. This fault commonly appears after a turbocharger replacement or ECM software update when the sensor offset is not re-calibrated, causing the ECM to flag an out-of-calibration condition even if the sensor hardware is intact.
Common Symptoms
- Derated Power: ECM reduces engine torque by up to 40% to protect the turbocharger from potential overheating.
- MIL Illuminated: Red or amber malfunction indicator lamp on the dashboard activates immediately after fault detection.
- Poor Acceleration: Driver notices sluggish response under load, especially during hill climbs or highway merging.
- No Boost Deviation: Actual boost pressure remains normal, but compressor outlet temperature reading is implausibly high or low.
Probable Causes
- Sensor Drift: Thermistor element aged or contaminated, causing resistance to deviate from factory calibration curve.
- ECM Calibration Mismatch: After a software flash, the sensor calibration coefficients stored in the ECM do not match the sensor.
- Wiring Harness Corrosion: Moisture ingress in the connector increases contact resistance, shifting the voltage signal seen by the ECM.
- Incorrect Sensor Installed: A replacement sensor with a different temperature-resistance profile (e.g., 2.2kΩ vs 1kΩ at 25°C) causes out-of-calibration.
Advanced Technical Analysis
The ECM monitors the compressor outlet temperature via a 5V reference circuit with a pull-up resistor inside the ECU. The sensor is a negative temperature coefficient (NTC) thermistor. The out-of-calibration fault (FMI 13) is triggered when the measured voltage deviates more than ±15% from the modeled value calculated from intake air mass flow, ambient temperature, and turbocharger speed. The model is derived from Bosch and MAN factory mapping tables.
Electrical analysis requires measuring the sensor resistance at known temperatures using a thermal bath. At 25°C, a typical NTC sensor should read between 2.2kΩ and 2.6kΩ. At 100°C, the resistance drops to approximately 200Ω. If the sensor reads 3.5kΩ at 25°C, the ECM interprets this as a calibration shift. The debouncing timer is typically 5 seconds; the fault must be present for this duration before it becomes active.
When SPN 2629 FMI 13 is active, the ECM enters a safety fallback mode: it substitutes a default temperature value of 120°C and activates a torque derate of 25% to 40%, depending on the engine platform (e.g., Deutz TCD 2015 or MAN D2676). The ECM also disables closed-loop boost control to prevent overboost conditions. The fault will not self-clear; manual calibration or sensor replacement is required.
Long-term strategy involves verifying sensor calibration using a diagnostic tool that reads the raw ADC counts. Technicians frequently encounter this fault after replacing the ECM or turbocharger without performing the required ‘sensor learn’ procedure. On Mercedes-Benz OM471 engines, a forced regeneration can also cause thermal shock, temporarily shifting the sensor offset. Prevention includes using only OEM sensors with matching part numbers and performing a calibration reset after ECM software updates.
Step-by-Step Troubleshooting Guide
- Read Sensor Values: Use a J1939 scan tool to compare compressor outlet temperature against intake manifold temperature for plausibility.
- Measure Resistance: Disconnect the sensor and measure resistance at ambient temperature; compare to the manufacturer’s specification table.
- Inspect Connector: Check for bent pins, corrosion, or moisture in the 3-pin Deutsch connector; clean with dielectric contact cleaner.
- Perform Calibration: Use the OEM diagnostic software to recalibrate the sensor offset; on some engines, this requires a learn cycle with known temperature.