A bad transmission speed sensor usually causes harsh or erratic shifting, a transmission that will not upshift or drops into limp mode, an erratic or dead speedometer, cruise control that stops working, and a check engine light. Most automatics use an input (turbine) speed sensor and an output shaft speed sensor; the computer compares them to work out gear ratio, slip and when to shift, so a bad signal from either throws off shifting.
Input vs output speed sensor
| Input (turbine) speed sensor | Output speed sensor | |
|---|---|---|
| Measures | Input shaft speed after the torque converter | Output shaft speed, which tracks road speed |
| Typical symptoms | Harsh or erratic shifts, converter clutch problems, limp mode | Erratic speedometer, no cruise control, harsh or wrong shifts, limp mode |
| Common codes | P0715, P0716, P0717 | P0720, P0721, P0722 |
What happens when the output speed sensor goes bad?
On many vehicles the output sensor also supplies vehicle speed, so the speedometer may jump or drop to zero, cruise control may stop working, and the transmission may shift at the wrong times or refuse to shift. Some vehicles also use wheel speed sensors for speed; see wheel speed sensors.
How to check a speed sensor
- Read the codes and freeze-frame data.
- Inspect the connector and wiring for damage, oil soaking and corrosion.
- Watch input and output speed on a scan tool while driving; a sensor that drops out or reads erratically is suspect.
- Check the sensor’s resistance or signal against the service manual specification.
- Look for metal debris on the sensor tip if it is magnetic; heavy debris points to internal wear.
Is it the sensor or the transmission?
A speed sensor fault code can also come from slipping clutches, which make the input and output speeds disagree. If the sensor and wiring test good and the codes return, the transmission may be slipping internally; see transmission slipping.
Replacing a speed sensor
Many speed sensors are held by a single bolt on the outside of the case and are simple to replace; others sit inside the transmission or on the valve body and need the pan removed. Use a new O-ring, clean the mounting area, keep debris out of the bore, and check the fluid level afterwards. Clear the codes and confirm on a scan tool that both speeds now read smoothly.
Can you drive with a bad speed sensor?
Short distances, carefully. Harsh shifts, limp mode and a wrong speedometer reading make it unsafe for long trips, and the shifting can wear the transmission.
If the transmission itself is worn out, our listed units include a torque converter; see transmissions by code and transmission replacement cost.
More guides: bad TCM symptoms.
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