Transmission Shift Position Indicator Lamp: Description and Operation

How the Circuit Works
With the ignition switch in ON (II) or START (III), voltage is applied to the A/T gear position indicator through the gauge relay. The A/T gear position switch provides a ground for each position. As an input is grounded, its indicator light comes on. If you select R, for example, ground will be applied to the input of the A/T gear position indicator, and the R indicator will come on.

With the headlight switch in PARK or HEAD, voltage is applied to the gauge assembly through the BLK/WHT wire and dash lights brightness controller wire terminal. This changes the indicator panel illumination from fixed to controlled by the dash lights dimmer input through the LT GRN wire.

When the powertrain control module (PCM) detects an abnormality in the automatic transmission control system, or when you request diagnostic trouble codes through the service check connector, the PCM will make the D4 indicator light blink.