SPN 975 FMI 6 — Engine Fan Drive Circuit Current Above Normal
Fan overcurrent codes need short versus mechanical-bind classification. Drivers and relays damaged by unresolved fan circuit overcurrent is the most documented consequence of skipping this step.
Fault Overview
SPN 975 FMI 6 indicates the fan drive circuit current is above normal — the overload class. Shorted coil, binding fan drive drawing excess current, harness partial short, and seized actuator mechanism are the primary causes. Electrical-versus-mechanical overcurrent classification is required before any fan drive replacement.
Engine & Platform Coverage
Symptoms
- •Fan fuse or circuit protection trips repeatedly
- •High current draw in the fan control circuit
- •Fan drive replacement did not resolve the fault
- •Harness damage visible near the fan assembly
- •Fan drive binding mechanically under engagement
Possible Causes
- 1.Shorted coil in the fan drive assembly
- 2.Binding fan drive drawing excess current under engagement
- 3.Harness partial short increasing circuit current
- 4.Seized actuator mechanism in the fan drive
- 5.Collateral driver or relay damage from unresolved overcurrent
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Disconnect and Measure Resistance: Disconnect the fan drive assembly and measure coil or actuator resistance. Low resistance below OEM specification supports an internal electrical short in the fan drive.
- 2Step 2 — Short-Check the Harness: With the fan drive disconnected, check the harness for partial shorts. A harness partial short can increase circuit current without causing a complete short-to-ground fault.
- 3Step 3 — Inspect Mechanical Freedom of the Fan Drive: Inspect the fan drive for mechanical binding. A seized or binding fan drive can draw excess current under engagement even with a normal electrical circuit.
- 4Step 4 — Classify Electrical Versus Mechanical Overcurrent: High current with low resistance supports an electrical short. High current with mechanical binding under engagement supports a mechanical fault. This classification determines the correct repair path.
- 5Step 5 — Repair Circuit or Replace Fan After Evidence Split: Repair the harness if a partial short is found. Replace the fan drive if an internal short or mechanical bind is confirmed. Check for collateral relay or driver damage if overcurrent was sustained.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Repair harness partial short at identified location.
- 2Replace fan drive assembly after internal short is confirmed.
- 3Replace fan drive after mechanical bind is confirmed.
- 4Replace relay or driver if collateral damage from overcurrent is found.
- 5Reroute harness away from abrasion points after repair.
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.
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Summary
SPN 975 FMI 6 is a fan circuit overcurrent fault. Classify the source as electrical short or mechanical bind before any replacement to protect drivers and prevent repeat failures.