Allison 29540479 - NSBU Switch - Neutral Start Back Up, 2 Connector, 2001-2004

One of the most common problems with the Allison transmission is failure of the Neutral Start Back Up switch.  

Some of the symptoms of failure include but are not limited to:

• No start condition
• Gear shift indicator on dash not following properly
• Trans stuck in limp mode or fail safe
• DTC's set indicating pressure switch codes 

This switch easily installs with just common hand tools. 

How does it work and why does it cause my transmission not to shift?

The vehicle is equipped with a column type shift selector. In addition to the column shifter, another component associated with the shift selector is the Neutral Start Back Up (NSBU) switch mounted on the selector shaft of the transmission. The NSBU switch transmits selector position information to the TCM. The NSBU switch mounts directly onto the transmission housing, from the outside, and detects the angular position of the shift selector shaft. This position is communicated to the TCM so that certain vehicle control functions can be coordinated with the position of the shift controls. The neutral signal output of the NSBU switch is typically used as confirmation that the transmission is in NEUTRALor PARK before the engine starter is engaged.