Sometimes my car will not shift into overdrive and then sometimes it does, any idea what could be causing this?

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I have noticed while driving my tercel that sometimes it will shift into overdrive and othertimes it will not. There is no check engine light on in the car and the transmission shifts great in 1, 2, 3. I have no idea what could be causing this problem or where to start.

My car has 238000 miles.
My car has an automatic transmission.

This symptom is the TCC (Torque Converter Clutch) is slipping in and out while driving. To check this condition, scan the computer to see whether the TCC action is being commanded by the computer. There could be a problem with the throttle position sensor (TPS) on the throttle body causing the engine to rev up and down to make the TCC disengage and engage. If there is no issue with the TPS, then the TCC valve in the valve body (brain of the transmission) inside the transmission has made its bore larger than the valve and causing an internal leak making the valve buzz back and forth. This action will cause the TCC to engage and disengage multiple times as long as the TCC is to be engaged. I recommend seeking out a professional, such as one from Your Mechanic, to help you diagnose your transmission.

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