SPN 177 FMI 3DrivetrainMedium
Transmission Oil Temperature — Voltage Above Normal
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 177 / FMI 3
Category
Drivetrain
Fault Type
sensor
FMI Category
voltage high
Severity
Medium
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Transmission oil temperature sensor — on transmission housing
Common Symptoms
- Transmission oil temperature sensor reading high
- Transmission temperature monitoring affected
Root Causes
- Sensor shorted high
- Connector damage
- Wiring short to power
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Sensor failure common at 200k–400k miles.
System Interactions
SPN 177 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Allison TSB 2019-013 — Transmission Oil Temperature Sensor Short High
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.
OEM Notes
Allison
Check connector for fluid contamination.
Eaton
Check wiring near transmission.
Meritor
Sensor located on transmission housing.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 177 FMI 3 — 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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