The Allison NSBU switch, or "Neutral Start Back Up" switch, is located on the driver's side of an Allison transmission. There are two versions of the switch, one with one electrical connector, and one with two electrical connectors. Both are installed in the same manner, the only difference being the number of connectors to attach. An NSBU switch needs to be replaced when the switch gets corroded. Your vehicle may only go forward in reverse gears, a service-engine light may come on, or your vehicle may be stuck in neutral.
Block the wheels of your vehicle. Set the parking brake. Put the transmission in neutral.
Disconnect the electrical connectors, one or two depending on your switch. Disconnect the cable from the shift lever on the transmission. Use a wrench to hold the lever in neutral. Remove the nut from the end of the selector shaft and remove the lever from the shaft.
Remove the bolts that attach the NSBU switch and take out the old switch. File any dirt and rust off of the selector shaft, then install the new switch the same way the old one was installed. There will be a metal clip on the new switch which will help you position the switch. Replace the bolts to attach the new switch. Once the bolts are tightened, remove the metal clip.
Place the black splash shield tightly against the switch. Re-install the shift lever, using the wrench to hold the lever in position when you put the nut back on the end of the selector shaft.
Re-connect the electrical connectors. Clear the DIC codes if any appeared when the old switch was installed.
