Circuit/System Description

Backup Lamps (without T90) 

With the engine running and the transmission in the reverse position, the transmission control module (TCM) sends a serial data message to the body control module (BCM). The message indicates that the gear selector is in the reverse position. The BCM applies battery voltage to the backup lamps control circuit illuminating the backup lamps. Once the driver moves the gear selector out of the reverse position, a message is sent by the TCM via serial data requesting the BCM to remove battery voltage from the backup lamps control circuit. The engine must be running for the backup lamps to operate.

Backup Lamp (with T90) 

With the engine running and the transmission in the reverse position, the transmission control module (TCM) sends a serial data message to the body control module (BCM). The message indicates that the gear selector is in the reverse position. The BCM responds to the backup lamp request by applying ground to the backup lamp relay control circuit which energizes the backup lamp relay. With the backup lamp relay energized, the switch contacts close allowing battery voltage to flow through the backup lamp fuse. Battery voltage is then applied from the fuse through the backup lamp control circuit illuminating the backup lamp. Once the driver moves the gear selector out of the reverse position, a message is sent by the ECM via serial data requesting the BCM to remove battery voltage from the backup lamp relay control circuit turning off the backup lamp. The engine must be running for the backup lamps to operate.