What it means
The two input or turbine shaft speed sensors A and B do not agree within tolerance. Either sensor may be drifting or missing pulses.
At a glance
- Usually lights the check-engine light
How it works
Input or turbine speed sensors A and B report how fast the transmission input shaft is turning, using pulses from a rotating tone wheel. P2784 sets when the control module finds that the two speed signals do not track each other within the expected relationship, which can happen if a tone wheel is damaged or a sensor signal weakens or drops out.
Common causes
Most likely first.
- Damaged tone wheel teeth
- One sensor drifts in air gap or amplitude
Symptoms
- Check engine or transmission warning light may be on
- Erratic or harsh shifting
- Reduced performance or transmission limp mode
How to diagnose it
- Save freeze-frame data and graph both input/turbine speed A and B while reproducing the fault to see whether one signal drifts or loses pulses.
- Inspect both sensor connectors and wiring for looseness, corrosion, fluid intrusion, chafing, or damage near the transmission.
- Check the sensors’ mounting and air gaps against the manufacturer's specification, since incorrect spacing can weaken or distort one signal.
- If accessible, inspect the tone wheels for damaged, missing, or contaminated teeth that could interrupt pulses.
- Test each sensor circuit and signal at the transmission control module connector per the manufacturer's procedure to isolate a sensor, wiring, or module-side fault.
Common fixes
- Repair damaged or poorly seated sensor wiring and connectors.
- Correct a sensor mounting or air-gap issue to the manufacturer's specification.
- Replace the faulty input/turbine speed sensor A or B after confirming which signal is incorrect.
- Repair or replace a damaged tone wheel.
Can I keep driving?
Drive only as needed to reach a safe place or repair facility, and avoid hard acceleration if shifts are erratic. A lost speed signal can cause harsh shifts or reduced performance and may leave the transmission in a protective mode.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.