NSBU: "Neutral Start Back Up" switch. AKA PNP ("Park Neutral Position") switch.

Location: The switch is located on driver's side of the Allison transmission. 

Symptoms of a bad NSBU switch

 - No Start condition
- Sets DIC codes MAY suggest the NSBU as the cause: P0700, P0708, P0847, P0872, P0875, P1711 and P1713.
- PRNDL display may not show up or flash.
- Delayed or No Engagement of the transmission.
- SEL (service engine light) message that reads something like "shift range inhibited".

There are 2 switches for the 5 Speed Allison Transmissions:
- 2001-2004 Allison NSBU Switch (LB7). Part no. 29540479. Has 2 electrical connectors.
- 2004-2005 Allison NSBU Switch (LLY). Part no. 29541852. Has 1 electrical connector.

Tools Needed:
13mm Deep 6 pt socket
15mm Wrench
6 inch extension
3/8" ratchet
Small file (Optional)

Here is what we used.

The new switch has a thin metal clip called a Neutral Assurance Bracket to ensure that the NSBU remains in the neutral position. DO NOT REMOVE UNTIL THE NEW SWITCH HAS BEEN INSTALLED.

1. Make sure you block the wheels and set the parking brake.

2. Put the transmission in neutral.

3. Disconnect NSBU wiring harness. This one has one connector. LB7s 2001-2004 have 2 plugs. Locate the retainer (blue) and pull it out. Now gently pry the lock up. Wiggle as you pry helps removal.

4. Disconnect the shift linkage/cable from the shift lever at the transmission. With a wrench hold the shifter from rotating out of neutral. Remove the nut from the end of the selector shaft. Carefully remove the selector lever from the selector shaft.

5. Remove the 2 bolts that attach the NSBU switch (the front driveshaft will get in the way a bit but you should be able to access the bolt with the socket) and pull off the old NSBU switch. There'll be some resistance when you pull it off.

 

6. If for some reason the transmission is not in neutral, use a wrench on the selector shaft flats and rotates the shaft to its furthest clockwise position and then rotates counter-clockwise two detents. Or align the flats with the “N” on the transmission.

 

7. Take a file and touch up the selector shaft at the end near the flats...there may be some burrs that could create some slop if the new NSBU switch is slid over the shaft without filing.

8. Slide the new NSBU switch over the selector shaft. The metal clip (neutral assurance bracket) on the switch helps to ensure the switch is properly aligned to the shaft. The bolts holding the NSBU to transmission should be tightened to 18-21 lb ft. Now it's ok to remove the metal clip from the NSBU housing.

9. Slide the grey splash shield onto the NSBU switch so that it fits securely against the switch. Do this BEFORE re-installing the shift lever.

10. Re-install the shift level. Again use an adjustable wrench on the shift lever while tightening the nut on the end of the selector shaft. OEM specs for the nut is 15-20 lb ft.

11. Connect the wiring harness back onto the NSBU switch.

12. If you have any DIC codes as a result of a bad NSBU switch, you'll have to clear them.