SPN 1639 FMI 2Cooling
Engine Fan Speed - Data Erratic, Intermittent or Incorrect
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Overview
Fault Code
SPN 1639 / FMI 2
Category
Cooling
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Engine fan speed sensor is typically mounted on the fan shroud or fan clutch assembly near the cooling fan.
Common Symptoms
- Engine fan cycles on and off erratically
- Engine fan runs continuously at high speed
- Engine fan does not respond to coolant temperature changes
- Intermittent engine overheat conditions
- Check engine light illuminated
- Engine protection derate may occur
Root Causes
- Faulty engine fan speed sensor Damaged or corroded wiring harness to fan speed sensor Intermittent connection at fan speed sensor connector Failed fan clutch or fan drive unit ECM communication issue with fan control module
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Derate Behavior
Power Derate
No
System Interactions
SPN 1639 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.
Common Diagnostic Mistakes
Replacing the sensor without verifying wiring continuity and supply voltage first.
Clearing codes without identifying and resolving the root cause — the fault will return.
Ignoring related active faults — cascading faults often share a root cause.
Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.
Condemning the ECM before exhausting all wiring and component tests.
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OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1639 FMI 2 — Full FSM Sequence & Repair Steps — includes wiring notes, sensor specs, and gated repair procedures.
Authority References
Diagnostic procedures on this page are informed by OEM service literature and industry standards.
Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.
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