One of the most common problems with the Allison transmission is failure of the Neutral Safety 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 Nuetral Safety 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.
Common trouble codes associated with NSBU failure include :
DTC P0708, Neutral start/backup (PNP or NSBU) switch
DTC P0847, Pressure Switch Manifold (PSM)
DTC P0872, Pressure Switch Manifold (PSM)
DTC P0875, Pressure Switch Manifold (PSM)
DTC P1711, Pressure Switch Manifold (PSM)
DTC P1713, Pressure Switch Manifold (PSM)
DTC U1000, Serial Data Communications
Blank PRNDL Display
Flashing PRNDL Display
Slow or No Engagement of Automatic Transmission
The reason that PSM codes may set is that the NSBU switch and the internal PSM switch "look at each other" and "compare readings" Basically, the NSBU switch lets the TCM know what gear position that you, the driver, have selected. The PSM has the job of telling the TCM what gear the transmission is in based on hydraulic inputs. If the data received from the PSM and the NSBU do not agree, the above mentioned trouble codes may set. The NSBU is subject to water contamination and will allow the switch to give faulty readings.