What is a NSBU switch? - acronym for "Neutral Start Back Up". Sometimes also referred to as a PNP ("Park Neutral Position") switch.
Where is the NSBU switch located? - driver's side of the Allison transmission.
Why does the NSBU need to be replaced? - due to road salt/water/debris getting into the switch via the switch hub and contaminates and corrodes the internals. The crap gets kicked up onto the switch on all left hand turns which affects the operation of the transmission because transmission shifts and functions are controlled by this switch.
What are some of the symptoms that the NSBU switch is going bad?
- following DIC codes MAY suggest the NSBU as the culprit: P0700, P0708, P0847, P0872, P0875, P1711 and P1713.
- PRNDL display may not show up or only a partial display
- no engagement even though your shifter is in D or R (like you were in neutral)...this was my situation (and I received a SEL (service engine light) code P0700).
- you will only have forward motion in 3rd gear or reverse.
- SEL (service engine light) message that reads something like "shift range inhibited".
Is there more than one model of NSBU switch? There are 2 switches that I'm aware of.
- 2001-2004 Allison NSBU Switch (LB7). Part no. 29540479. Has 2 electrical connectors on the switch.
- 2004 - 2005 Allison NSBU Switch (LLY). Part no. 29541852. Has 1 electrical connection on the switch.
Why should I replace the switch as opposed to taking it to my dealership?
- because it's a "one beer" job that will take less than an hour.
- did I mention I purchased the NSBU switch at a local Allison dealership for $73.50 while the dealership quoted me $233.23? My dealership parts insider could only give me a reduced rate of $180.00. I believe Merchant Automotive sells the NSBU for $65. That's just the part...labor is EXTRA.
So what improvement is there with the new NSBU switch?
- as far as I can tell, the a white foam "rear shield" has been added to better seal the NSBU housing against the Allison transmission so water and crap does not get in to contaminate. Again, see the pics below for the white foam rear shield on the backside of the switch. The white rear splash shield and front black splash shield on the upgraded NSBU switch are designed to minimize infiltration.
What's the little connection on the new NSBU switch that looks like a vacuum hose connection between the wiring harness connections?
- one way valve to drain any water that may get into the switch.
How do I replace the NSBU switch? - instructions as follows...
1. My original switch is of the black variety. It does not have the white foam rear shield on the backside like the replacement NSBU switch (new one is tan in color).
The new switch also 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.
2. Gather the following tools:
13mm deepwall 6 pt socket
15mm 6pt wrench (mine's the "gear wrench" variety)
6 inch extension
3/8" ratchet
small file
2. Put the transmission in neutral.
3. Disconnect NSBU wiring harness (2 in my case - I have a 2003 LB7...LLY only has 1 electrical connection).
This probably caused me the most grief as the top harness would not come out so I had to break off some NSBU switch plastic around the connection. Finally got it out and it looked like the ends were glued on...it has been suggested to take a hair dryer to warm up the connection to warm them up.
4. Disconnect the shift linkage/cable from the shift lever at the transmission. With a wrench keeping the shifter from rotating (out of neutral...you did put the transmission in neutral right?), 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. I had quite a bit of crap behind the switch (small rocks, dirt, etc.)
6. If for some reason that the transmission is not in neutral, use a wrench on the selector shaft flats and rotate the shaft to its furthest clockwise position and then rotate counter-clockwise two detents.
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. For the geeks out there, the bolts should be tightened to 18-21 lb ft. Now it's ok to remove the metal clip from the NSBU housing.
9. Slid the black 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, I used an adjustable wrench on the shift lever while tightening the nut on the end of the selector shaft. Geek specs for the nut is 15-20 lb ft.
11. Connect the wiring harness(es) 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.
12. Crack open a beer and tell yourself you just saved paying at least $300 to "the man".
