SPN 1112 FMI 3 — Engine Compression Brake Output Circuit Voltage Above Normal
High voltage on the compression brake output circuit is a short-to-power or failed solenoid until proven otherwise. Confirm driver type and resistance first.
Fault Overview
SPN 1112 monitors the engine compression brake (Jake brake) output circuit. FMI 3 indicates the ECM detects voltage above the calibrated high threshold on the control or driver circuit when the brake is commanded or during circuit integrity checks. Detection occurs when the driver is expected to be low or open but measures high voltage consistent with a short to power.
Engine & Platform Coverage
Symptoms
- •Jake brake not working
- •Compression brake high voltage
- •Engine brake circuit fault
- •Retarder control high voltage
- •Jake stays on or won't engage
Possible Causes
- 1.Control wire shorted to battery voltage
- 2.Failed brake solenoid or actuator shorted internally
- 3.Incorrect relay or driver substitution
- 4.Harness chafing near valve cover or turbo heat shields
- 5.ECM driver internal fault (less common)
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Step 1 — Identify Driver Type: Identify whether the circuit uses a high-side or low-side driver. This determines the expected voltage states and the correct interpretation of the fault.
- 2Step 2 — Measure Control Voltage in All States: Measure control voltage key-off, key-on engine off, and during brake command. Document the voltage in each state and compare to expected values for the driver type.
- 3Step 3 — Measure Solenoid Resistance: Measure solenoid resistance cold and compare to OEM specification (typically 10–30 Ω). A near-zero reading indicates an internal short.
- 4Step 4 — Check for Short to Power on Control Wire: Disconnect the solenoid and measure voltage on the control wire. Voltage present with the solenoid disconnected confirms a short to power in the harness.
- 5Step 5 — Voltage-Drop Test on Power and Ground Feeds: Perform a voltage-drop test on the power and ground feeds to the brake circuit under commanded load. Total drop should be below 0.5 V.
- 6Step 6 — Command Brake and Monitor Response: Command the brake with a scan tool and verify response while monitoring voltage. Confirm the circuit behaves as expected after any repairs.
- 7Step 7 — ECM Driver Evaluation (Last): Only after circuit proof is complete, consider ECM driver evaluation. ECM driver failure is the least common cause of this fault.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair shorted control wire.
- 2Replace failed solenoid with correct resistance unit (verify OEM part number).
- 3Repair or re-route damaged harness near valve cover or heat shields.
- 4Replace oil-soaked 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
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Summary
High voltage on the compression brake output circuit is a short-to-power or failed solenoid until proven otherwise. Confirm driver type and resistance first.