If your automatic transmission quits shifting, it could point to a variety of problems. The solutions range from the simple to the more complicated, with professional assistance required with the latter. We have identified the usual transmission problems and have also listed a few not-so-apparent issues that may surprise you.
Bad Transmission Control Module (TCM)
- Modern transmissions use an electronic TCM.
- The TCM processes sensor data and controls shifting.
- Exposure to extreme temperatures, vibrations, and fluid leaks can damage the module.
- A faulty TCM may trigger the check engine light.
- Diagnosis requires a professional scan tool.
Broken Shift Cable
- The shift cable connects the shifter to the transmission.
- Over time, plastic components and bushings may wear out.
- The metal cable itself is durable but its connectors can fail.
- If the shifter moves but the transmission does not respond, the cable may need replacement.
Failed Throttle Sensor
- The throttle sensor tells the transmission how much throttle input the driver is applying.
- If the transmission doesn’t receive this signal, it may not shift gears.
- Older transmissions use a mechanical cable for throttle input.
- A faulty sensor or misalignment can cause severe transmission damage.
Faulty Shift Solenoid
- Automatic transmissions rely on pressurized fluid for operation.
- Shift solenoids direct fluid to shift actuators.
- A failed solenoid prevents fluid from reaching the correct actuator, stopping gear shifts.
- In older models, shift valves serve this function.
- Fixing a solenoid issue often requires removing the transmission fluid pan.
Low Transmission Fluid
- Automatic transmissions use hydraulic pressure to function.
- Fluid is drawn from the pan to operate components.
- Low fluid levels prevent proper fluid circulation and can introduce air into the system.
- Air in the system can cause component damage due to lack of lubrication.
- Some vehicles have a dipstick for fluid level checks; others require a specialized process.
Low Transmission Fluid Pressure
- Even with the right fluid level, pressure problems can occur.
- Causes include a worn-out pump, clogged passages, or a dirty fluid filter.
- Low pressure can result in failure to shift gears.
- A professional inspection is needed to determine the root cause.
- If the pump is failing, a transmission rebuild may be necessary.
Shift Lock Engaged
- Modern vehicles include a shift lock safety feature.
- The brake pedal must be pressed before shifting out of park.
- A faulty brake pedal sensor or a break in the interlock circuit can prevent shifting.
- In emergencies, most cars have a shift lock bypass.
Worn-Out Bands
- Transmission bands hold internal components in place during shifts.
- Like brake pads, they wear out over time.
- Worn bands can prevent the transmission from engaging certain gears.
- Debris from worn bands can clog fluid passages.
- A full transmission rebuild is often required.
- If your automatic car won’t shift, diagnosing the problem early can prevent costly repairs.
Automatic Transmission Problem Solutions
Some transmission issues, such as low fluid levels or a stuck shift lock, can be resolved with simple fixes. However, more complex problems, such as a faulty solenoid or a failing transmission control module (TCM), require professional expertise. Ignoring these signs can lead to expensive repairs or complete transmission failure. To maintain your vehicle’s performance, address any transmission concerns promptly. If you are uncertain about the severity of the issue, consult a mechanic for an accurate diagnosis. It is advisable to act quickly to prevent minor issues from escalating into significant repairs.
See Also — Transmission Care & Troubleshooting
