Why wont your 91 Toyota tercel shift into overdrive?

There could be several reasons why your 1991 Toyota Tercel won't shift into overdrive. Common issues include low Transmission Fluid levels, a faulty overdrive switch, or problems with the transmission itself, such as worn or damaged components. Additionally, if the vehicle's speed sensor is malfunctioning, it may prevent the transmission from engaging overdrive. It's best to have a qualified mechanic diagnose the issue to pinpoint the exact cause.