Replacing NSBU Switch
NSBU: "Neutral Start Back Up" switch. AKA PNP ("Park
Neutral Position") switch.
switch is located on driver's side of the Allison transmission.
Symptoms of a bad
- 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.
13mm Deep 6 pt
6 inch extension
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
1. Make sure you block the wheels and set the
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.
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.
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
12. If you have any DIC codes as a result of a bad
NSBU switch, you'll have to clear them.