Allison Transmission Reverse Backup Light Hydraulic Actuated Switch used on most models. This has a 1/8" NPT male pipe thread.   When you put the truck in reverse, hydraulic pressure (60 to 90psi) is sent to the reverse clutches and the switch mechanically closes to turn on the backup lights or backup alarm. This switch mounts to the side of the transmission.   Any electrical devices ran through this switch must be run through a standard 12 volt relay, or premature switch failure will occur from overloading the switch.