SPN 1639 FMI 5Cooling
Engine Fan Clutch Output - Current Below Normal
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1639 / FMI 5
Category
Cooling
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: The engine fan clutch is mounted on the engine front cover, driven by the crankshaft through a belt or gear system.
Common Symptoms
- Engine fan does not engage or runs intermittently
- Engine overheating or high coolant temperature warnings
- Check engine light illuminated with active fault code
- Fan clutch remains disengaged during high load conditions
- Loss of engine power due to temperature derate
Root Causes
- Open circuit in fan clutch wiring harness Faulty fan clutch solenoid or actuator Damaged or corroded electrical connector at fan clutch Faulty ECM fan clutch driver circuit Broken or chafed wiring between ECM and fan clutch
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Derate Behavior
Power Derate
No
System Interactions
SPN 1639 FMI 5 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 5 — 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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