Cummins · Transmission Output SpeedMedium

Cummins SPN 723 FMI 2: Transmission Output Speed Erratic

SPN 723 FMI 2 (FMI 2) — Transmission Output Shaft Speed - Erratic, Intermittent, or Incorrect

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Quick Answer

SPN 723 FMI 2 means the transmission output speed sensor signal is erratic. Inspect sensor air gap, reluctor ring, and wiring before replacing the sensor.

Overview: Cummins SPN 723 FMI 2

The output shaft speed sensor provides shift timing and vehicle speed data to the transmission controller. When the signal becomes erratic, Cummins and transmission modules set SPN 723 FMI 2.

Repair Snapshot

Time

2 - 4 Hours

Difficulty

Advanced

Parts Cost

$80 - $400

Dealer Labor

$450+

Tools Required

  • Cummins INSITE
  • Noregon JPRO
  • Multimeter
  • Feeler Gauge

What SPN 723 FMI 2 Means

SPN 723 FMI 2 on Cummins J1939 networks means the transmission output speed sensor signal is erratic or implausible compared to turbine and engine speed. Failed tone ring, damaged sensor, or wiring fault is common. Inspect sensor air gap, reluctor ring teeth, and harness continuity before replacing the sensor.

Symptoms Drivers Notice

  • Harsh or erratic shifts
  • Transmission warning lamp
  • Output speed reading jumps in live data
  • Cruise control or ABS may disable from erratic speed signal
  • Fault may appear only under vibration or specific gear ranges

Most Likely Root Causes

Failed output speed sensor

Common

Internal sensor element degradation produces intermittent or noisy speed signals.

Damaged reluctor ring or incorrect air gap

Common

Missing tone ring teeth or excessive gap prevents clean speed pulses.

Wiring harness chafe at transmission mount

Occasional

Frame flex damages sensor wires causing intermittent open circuits.

Common Misdiagnoses (Don't Waste Parts)

Rebuilding transmission for an erratic speed sensor signal

Why it happens: Harsh shifts feel like internal trans failure.

Do this instead: Parts-cannon trap — verify sensor signal and air gap before authorizing transmission overhaul.

Replacing TCM when sensor or wiring is at fault

Why it happens: Electronic faults default to module replacement.

Do this instead: Scope or AC-voltage test the sensor output during shaft rotation first.

Master Tech Commentary

Mechanic's Notes

On Eaton and Allison behind Cummins, a cracked output speed sensor bracket causes gap changes under torque — inspect bracket before sensor replacement.

Step-by-Step Diagnostic Procedure

  1. Step 1: Monitor output shaft speed, input/turbine speed, and engine RPM in Cummins INSITE or Noregon JPRO — output speed should correlate with road speed.
  2. Step 2: Inspect output speed sensor mounting and air gap — target 0.5–1.5 mm per OEM spec.
  3. Step 3: Check reluctor ring on output shaft for missing teeth, metal debris, or contamination.
  4. Step 4: Wiggle-test sensor harness at transmission case connector — look for resistance changes above 2 ohms.
  5. Step 5: Measure sensor AC voltage while rotating output shaft by hand — expect measurable AC signal above 0.5 VAC.

Professional service / OEM-level scan tools are recommended before clearing this fault for good.

Live Data & Freeze-Frame Tips

Log output shaft speed vs vehicle speed and turbine speed during a test drive. Erratic spikes in output speed with smooth turbine speed confirm sensor or tone ring faults.

Safety Warnings

  • Chock wheels and use transmission jack supports before under-vehicle work.
  • Hot transmission fluid can cause burns during sensor removal.

When to Limp In vs Tow

Tow if transmission is slipping badly or speed signal loss prevents safe shifting in traffic.

Prevention Tips for Fleets & Owner-Operators

  • Inspect speed sensor harness routing at every transmission service.
  • Keep metal debris away from reluctor ring during driveline work.
  • Torque sensor mounting bolts to OEM spec to maintain air gap.

Related Fault Codes

SPN 191 FMI 2

Transmission input speed erratic — compare both speed sensors.

SPN 84 FMI 2 — Wheel-Based Vehicle Speed Sensor

Wheel speed erratic — may affect shift strategy on integrated systems.

Frequently Asked Questions

What is the correct speed sensor air gap?

Typically 0.5–1.5 mm — verify against transmission OEM specification.

Can a bad output speed sensor cause no shift?

Yes — transmission may hold a default gear or limp mode without valid speed input.

Is this a Cummins or transmission code?

SPN 723 is on the J1939 network — reported by transmission controller but readable in INSITE.

How do I test the sensor?

Measure AC voltage while rotating the output shaft — expect increasing voltage with speed.

More Cummins codes on this system

All Cummins fault codes

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