What does it mean when the OD Off light is on in a 1984 Vovlo Wagon?

The OD Off light in a 1984 Volvo Wagon indicates that the overdrive gear is turned off. Overdrive is a higher-gear setting that allows the engine to run at a lower RPM at higher speeds, which can improve fuel economy. When the OD Off light is on, the transmission will not shift into overdrive gear, so the engine will run at a higher RPM at higher speeds. This can be useful for towing or climbing steep hills, as it provides more power and torque.