SPN 1112 FMI 5 — Engine Compression Brake Output Circuit Open / Current Below Normal
Open circuit on the compression brake output is a solenoid, wire, or fuse problem until proven otherwise. Measure resistance first.
Fault Overview
FMI 5 indicates an open circuit or current below the expected minimum on the compression brake output. The ECM detects that the driver current does not rise when the brake is commanded.
Engine & Platform Coverage
Symptoms
- •Jake brake open circuit
- •Compression brake no current
- •Engine brake inoperative
- •Solenoid open
Possible Causes
- 1.Open solenoid coil
- 2.Broken control or power wire
- 3.Unplugged connector
- 4.Blown fuse on the brake circuit
- 5.High-resistance connection limiting current
- 6.Failed open ECM driver
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Step 1 — Measure Solenoid Resistance: Measure solenoid resistance. An open reading (infinite or very high resistance) confirms the coil has failed. Compare to OEM specification.
- 2Step 2 — Check Continuity of Control and Power Wires: Perform continuity tests on the control and power wires from the solenoid connector to the ECM and power source. Identify any open segments.
- 3Step 3 — Verify Fuse Integrity: Check all fuses in the compression brake circuit. A blown fuse will cause an open-circuit condition that appears identical to a failed solenoid or wire.
- 4Step 4 — Command Brake and Confirm Current Draw: Command the brake with a scan tool and confirm current draw with an inductive clamp if available. No current confirms the open is still present.
- 5Step 5 — Inspect Connector Pin Fit: Inspect connector pin tension and seating. Backed-out or low-tension pins create intermittent opens that become permanent over time.
- 6Step 6 — ECM Output Capability (Last): Confirm ECM output capability only after the load circuit is proven open or good. ECM driver failure is the least common cause.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Replace open solenoid.
- 2Repair open wire.
- 3Replace blown fuse and identify root cause.
- 4Restore connector pin tension or replace connector.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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Summary
Open circuit on the compression brake output is a solenoid, wire, or fuse problem until proven otherwise. Measure resistance first.