SPN 520 FMI 2 — Retarder / Engine Brake Data Erratic
Erratic retarder/engine brake data is usually an intermittent connection. Correlate command and status while wiggling the harness.
Fault Overview
SPN 520 relates to retarder status or control. FMI 2 indicates erratic, intermittent, or incorrect data for the retarder/engine brake system.
Engine & Platform Coverage
Symptoms
- •Retarder data erratic
- •Engine brake status intermittent
- •Jake data fault
- •Retarder signal dropout
Possible Causes
- 1.Intermittent solenoid or control circuit connection
- 2.Network glitch affecting status broadcast
- 3.Failing feedback sensor
- 4.Loose connector at the brake assembly
- 5.EMI from aftermarket devices
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 — Monitor Command vs Status for Dropouts: Monitor retarder command and status simultaneously in live data. Document the exact conditions under which the erratic data occurs.
- 2Step 2 — Perform Wiggle Test on All Related Connectors: Perform a wiggle test on all connectors at the brake solenoids and any retarder control modules while monitoring live data. Dropouts during wiggling confirm an intermittent connection.
- 3Step 3 — Check Voltage Stability Under Load and Vibration: Measure voltage stability on control circuits under load and vibration. Intermittent connections at the valve cover or retarder unit create status flicker.
- 4Step 4 — Inspect Network Health: Inspect network health if the retarder status is broadcast on J1939. Network issues can cause erratic status data even when the retarder circuit is good.
- 5Step 5 — Isolate Aftermarket Devices: Disconnect any aftermarket devices that may be introducing EMI or pulling on the retarder circuit.
- 6Step 6 — Verify Solenoid Resistance and Connections: Verify solenoid resistance and connection integrity after any intermittent connection is repaired.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair intermittent connection.
- 2Replace failing connector or solenoid.
- 3Address network issues.
- 4Remove interfering aftermarket devices.
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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Related Resources
Related Fault Codes
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Summary
Erratic retarder/engine brake data is usually an intermittent connection. Correlate command and status while wiggling the harness.