Allison LCT1000 PRNDL Inoperative and/or Transmission Related DTC

When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.

Transmission may experience a condition of no or partial PRNDL display, MIL indicator on, erratic shifting and or no movement. DTC's related to the transmission fluid pressure switch manifold, range switch (NSBU), speed signal or ratio may be active or in history. The vehicle may be exposed to wet or muddy conditions during the time condition occurs (i.e. snow plowing, driving on slushy roads or off-road use).

Check for signs of water intrusion in the NSBU switch, wiring and connector.

With the NSBU (PNP Switch) C1 Connector (4 way) disconnected and the ignition switch in the run position, ground two of the signal wires and leave two of the signal wires open. The chart below provides the various combinations of on/off to light the various PRNDL ranges in the IPC. NSBU Connector C2, Terminal D, CKT 451 is the ground for the range Switch Signal circuits. In the open state the Transmission Range Switch Signal Circuits will have 12 volts from the TCM.

on = open circuit

off = grounded circuit

Based on the diagnostics above, if the PRNDL operates properly in each range with the combination of opens and grounds, replace the NSBU switch or repair the Range Switch Signal Circuit(s) if the PRNDL does not operate properly. Secure the connector with a wire/zip tie.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.