Shift Light: Description and Operation

Upshift Indicator Lamp

Vehicles equipped with a manual transmission may have an optional upshift indicator lamp.

One side of the indicator lamp bulb receives ignition-switched battery feed.
The ground feed for the other side of the lamp bulb is switched by the Powertrain Control Module (PCM).

The lamp lights to indicate when the driver should shift to the next highest gear for best fuel economy.
The PCM will turn the lamp OFF after 3 to 5 seconds if the upshift is not performed.
The lamp will remain OFF until the vehicle stops accelerating and is brought back to the range of lamp operation, or until the transmission is shifted into another gear.

The indicator lamp is normally ON when the ignition switch is in the ON position, and is turned OFF when the engine is started.
The lamp will be turned ON and OFF during vehicle operation, according to the engine speed and load conditions that are being monitored by the PCM.