1996 Ford Thunderbird Owner's Manual - Page 171

Transmission Control Indicator Light (O/D OFF) is located on the instrument panel. You can put the gearshift lever in any of the several positions. The OVERDRIVE OFF light is off (not illuminated) during normal vehicle operation. This allows the transmission to upshift and downshift from 1st through 4th gears. When the TCS is pressed, the Transmission Control Indicator Light will illuminate (O/D OFF) on the instrument panel. When this O/D OFF is illuminated the transmission will upshift and downshift from 1st through 3rd gears. If the O/D OFF light does not come "on" when the TCS is depressed or if the light "FLASHES" when you are driving, have your vehicle serviced at the first opportunity. If this condition persists, damage could occur to the transmission. To return the transmission to normal operation (1st through 4th gear) depress the TCS again. This switch may be used to cancel overdrive any time the vehicle is being driven. Each time your vehicle is started, the overdrive system will automatically be in the normal overdrive mode and the light will not be illuminated. When to use D (Overdrive) Overdrive is the usual driving position for an automatic overdrive transmission. It works the same way Drive works, automatically upshifting or downshifting as the vehicle speeds or slows. However, Overdrive also shifts into a fourth gear - an overdrive gear - when your vehicle cruises at an appropriate consistent speed for any length of time. This fourth gear will increase your fuel economy when you travel at cruising speeds. 168