'94 Torque Converter Lock-up

The overdrive (torque converter lock-up) is not working on my '94. I can bypass the computer by grounding the TCC solenoid and it works but the computer will not do the function on its own automatically. No lights, no fault codes. Someone said it could be the throttle position sensor so I replaced that, no go. Is there a switch on the brake pedal? What are all the inputs to the computer that control lock-up that I could check? Common failures?

Thanks!