SPN 789 FMI 3Chassis
Retarder Control - 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 789 / FMI 3
Category
Chassis
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Retarder control solenoid is typically mounted on the engine cylinder head or intake manifold near the exhaust brake or compression brake assembly.
Common Symptoms
- Check engine light or warning lamp illuminated
- Engine retarder inoperative or disabled
- Loss of engine brake function
- Retarder does not activate when switch engaged
Root Causes
- Shorted retarder control circuit to power source Faulty retarder control solenoid or valve Damaged wiring harness or connector pins Failed ECM internal driver circuit Incorrect ECM software or calibration
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Derate Behavior
Power Derate
No
System Interactions
SPN 789 FMI 3 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 789 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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