SPN 1112 FMI 4 — Engine Compression Brake Output Circuit Voltage Below Normal
Low voltage on the compression brake circuit points to a short-to-ground or open power path until wiring and solenoid resistance are proven.
Fault Overview
FMI 4 sets when the ECM measures voltage below the low threshold on the compression brake control circuit. Common with low-side drivers when the circuit is shorted to ground or the driver has failed low.
Engine & Platform Coverage
Symptoms
- •Jake brake low voltage
- •Compression brake circuit short
- •Engine brake won't engage
- •Retarder control low voltage
Possible Causes
- 1.Control wire shorted to ground
- 2.Open power feed to the solenoid
- 3.Failed open solenoid coil
- 4.Poor ground on the driver circuit
- 5.ECM driver failed low
- 6.Oil-soaked connector creating low-resistance path to ground
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 Control Voltage in Both States: Measure control pin voltage in the off state and during commanded brake. Document the voltage and compare to expected values for the driver type.
- 2Step 2 — Check Solenoid Resistance: Measure solenoid resistance and compare to OEM specification. An open coil will prevent current from flowing and can cause low-voltage readings on some driver types.
- 3Step 3 — Short-to-Ground Test on Control Wire: Disconnect the solenoid and measure resistance from the control wire to chassis ground. A low reading confirms a short to ground in the harness.
- 4Step 4 — Verify Power Feed and Ground: Measure voltage at the solenoid power feed and voltage drop on the ground circuit. An open power feed or high-resistance ground can cause low-voltage faults.
- 5Step 5 — Command Brake and Monitor Response: Command the brake and monitor voltage and current response. Confirm the circuit behaves as expected after any repairs.
- 6Step 6 — Inspect Connectors for Contamination: Inspect solenoid connectors for oil or coolant intrusion. Contamination creates low-resistance paths that mimic short-to-ground conditions.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair shorted or open wire.
- 2Replace oil-contaminated connector and solenoid.
- 3Clean or reseal valve cover area to prevent oil intrusion.
- 4Replace failed solenoid coil.
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
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Summary
Low voltage on the compression brake circuit points to a short-to-ground or open power path until wiring and solenoid resistance are proven.