Transmission Input Speed Sensor Circuit Voltage Below Normal / Erratic
FMI 4 (short to ground) or FMI 2 (erratic/intermittent) on the input speed sensor causes the TCM to lose reliable turbine speed. Live monitoring under vibration and load is required to isolate intermittent faults that static tests miss.
Fault Overview
Signal wire shorted to ground produces a persistent low-voltage fault. Intermittent connector contact, damaged tone wheel, or excessive air gap produces erratic dropouts. EMI from aftermarket devices and shared ground issues can also cause erratic readings.
Engine & Platform Coverage
Symptoms
- •input speed sensor low voltage
- •ISS erratic
- •turbine speed intermittent
- •transmission speed sensor short
- •shift problems speed sensor
Possible Causes
- 1.Signal wire shorted to ground
- 2.Failed sensor
- 3.Intermittent connector contact
- 4.Damaged tone wheel
- 5.Excessive air gap
- 6.EMI from aftermarket devices
- 7.Shared ground issues
Wiring & Circuit Notes
Inspect for pinched wires, chafing against the transmission case, and oil-soaked connectors. Wiggle-test while monitoring live speed data. Intermittent connectors and tone-wheel damage after clutch or torque-converter work are frequent.
DVOM Specs & Sensor Tests
Signal should produce clean, consistent pulses or voltage proportional to speed. Erratic dropouts under vibration indicate connector or tone-wheel problems. Continuity and short-to-ground tests with the sensor disconnected.
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 — Monitor Input Speed Data for Dropouts at Idle, During Acceleration, and Under Vibration: Monitor live input speed data. Note whether dropouts occur at idle, under acceleration, or only under vibration or rough road conditions.
- 2Step 2 — Wiggle-Test the Connector and Harness: With the engine running or shaft rotating, wiggle the sensor connector and harness while watching live speed data. A dropout during wiggle confirms a connector or wiring fault.
- 3Step 3 — Measure Signal Integrity with a Scope or Frequency Meter if Available: Scope the signal for clean, consistent pulses. Erratic or missing pulses under vibration indicate connector or tone-wheel problems.
- 4Step 4 — Check for Short to Ground: With the sensor disconnected, measure resistance from the signal wire to ground. Any measurable resistance indicates a short.
- 5Step 5 — Inspect Tone Wheel and Air Gap: Inspect the tone wheel for missing or damaged teeth. Measure air gap and compare to OEM specification.
- 6Step 6 — Verify Ground Integrity: Measure voltage drop on the sensor ground circuit under operating conditions. High ground resistance can cause erratic readings.
- 7Step 7 — Replace Sensor or Repair Wiring After Isolation: Replace the sensor or repair wiring only after the fault is isolated to a specific component or circuit.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Repair shorted wiring $80–200; service connector $50–150; replace sensor $100–300; repair or replace damaged tone wheel $150–450.
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.
Related Systems
Related Fault Codes
Related Sensor Locations
Common Symptoms
Related Articles
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician and engineered for professional shop use. Trusted by technicians nationwide — used in shops, fleets, and training programs.
Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.