SPN 677 FMI 3 — Engine Starter Relay Circuit Voltage Above Normal
Starter relay high-voltage codes are control-circuit open diagnostics first. No-crank after body or alarm installs that open the starter control circuit is the most documented pattern. The FSM separates interlock, control, and battery-power paths before starter replacement.
Fault Overview
SPN 677 FMI 3 indicates starter relay control circuit voltage above the valid maximum — typically an open or high-signal class on the control side. Open control wire, failed relay coil open, unplugged relay, blown control fuse, and poor ignition feed to the control side are the primary causes. No-crank after body or alarm installs that open the starter control circuit is a documented pattern. Replacing the starter motor before proving the control path is the most documented misdiagnosis pattern.
Engine & Platform Coverage
Symptoms
- •Starter relay high voltage
- •Starter control open
- •No crank relay circuit
- •ECM starter enable fault
- •No-crank after body or alarm install
Possible Causes
- 1.Open control wire — open control wire producing high-voltage reading on control side
- 2.Failed relay coil open — failed relay coil open producing high-voltage reading
- 3.Unplugged relay — unplugged relay producing open circuit
- 4.Blown control fuse — blown control fuse producing open circuit
- 5.Poor ignition feed — poor ignition feed to control side producing open circuit
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 — Confirm Start Request and Interlocks: Confirm a start request is present and all interlocks are satisfied. Verify interlock status PIDs before condemning the relay. No-crank after body or alarm installs that open the starter control circuit is a documented pattern — always inspect for aftermarket starter interrupt devices.
- 2Step 2 — Check Control Power at Relay: Check control power at the relay. Verify ignition feed voltage is present at the relay base control terminal. A blown control fuse or poor ignition feed to the control side produces an open circuit. Inspect the relay base terminals and control-side wiring.
- 3Step 3 — Continuity on Control Wire: Check continuity on the control wire. Verify continuity through the control wire from the ECM or ignition feed to the relay base. An open control wire producing a high-voltage reading confirms a harness fault.
- 4Step 4 — Relay Coil Test: Test the relay coil. Measure coil resistance versus OEM specification. A failed relay coil open producing a high-voltage reading confirms a relay fault. Replace the relay if coil resistance is out of specification.
- 5Step 5 — Starter Power-Side Tests Only After Control Path Is Proven: Perform starter power-side tests only after the control path is proven. Confirm interlock, control power, control wire continuity, and relay coil integrity before any starter power-side assessment. A starter replaced before proving the control path will not resolve a control circuit fault.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Remove faulty aftermarket starter interrupt if confirmed.
- 2Replace blown control fuse if confirmed.
- 3Repair open control wire if confirmed.
- 4Replace relay if coil failure is confirmed ($20–$80).
- 5Repair ignition feed if poor feed is confirmed.
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
Starter relay high-voltage codes are control-circuit open diagnostics first. The AI Diesel Tech FSM separates interlock, control, and battery-power paths before starter replacement.