Park/Nuetral Position Switch

[HoytBows]

Guys,

I pulled a code off my buddy's LB7 last night and got a TCM P0708 Code for the PNP Switch. The tranny kinda lags behind when he shifts in and out of drive and the little orange light also disappears between N and D sometimes. So I guess I've narrowed it down to the Park/Nuetral Position Switch, but i guess my two questions are...

1. Has anybody delt with this before that can give me words of wisdom? and

2. Where the heck is that switch at??

Thanks...

 

[RI Chevy Silveradoman]

There were quite a few issues related to the NSB switch. It should be located on the side of the Allison. The newer ones are more weatherproof than the original ones were. I know Bud had a few issues with the same NSB switch.

Some info from Merchants site:

One of the most common problems with the Allison transmission is failure of the Neutral Start 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 Start 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 recieved 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.

 

[HoytBows]

Awesome. I know exactly where it is now, I pulled it off when I pulled my Tranny in my old LB7. Those symptoms from Eric are exactly whats going on. I love fixing things!

 

[RI Chevy Silveradoman]

Good luck! Take some photos, and keep us posted. :thumbsup: Should be a piece of cake.

 

[BudTX]

If replacing the NSBU switch does not correct the problem, post up here.

My problem ended up being installation errors inside my Merchant built transmission, but the symptoms mimicked a bad NSBU.

 

[THEFERMANATOR]

If replacing the NSBU switch does not correct the problem, post up here.

My problem ended up being installation errors inside my Merchant built transmission, but the symptoms mimicked a bad NSBU.

From the symptoms I'm guessing this is an 01-02 as an 03+ would throw up the shift range inhibited message. The 01-02's are alot dumber as far as the PRNDL indicator goes. On 03+ the ywent to the shift range inhibited which can also include trans problems.

 

[HoytBows]

I never got a chance to replace the switch, we bought one and were planning to put it in, but the thing magically fixed itself after I cleared the codes... The best we can figure is when it gets real slushy and wet the thing acts up. I don't understand how it threw a death code them just went away. My buddy probably thinks I mis diagnosed and wasted his money. Oh, and it is an '03 ferm...

 

[RI Chevy Silveradoman]

Thanks for the update.

 

[THEFERMANATOR]

I never got a chance to replace the switch, we bought one and were planning to put it in, but the thing magically fixed itself after I cleared the codes... The best we can figure is when it gets real slushy and wet the thing acts up. I don't understand how it threw a death code them just went away. My buddy probably thinks I mis diagnosed and wasted his money. Oh, and it is an '03 ferm...

If it has the 2 plug NSBU switch that is black, they were known for having problems in teh rain and slush. Make sure it has the new tan style one on it.

 

[fastjohnny]

I never got a chance to replace the switch, we bought one and were planning to put it in, but the thing magically fixed itself after I cleared the codes... The best we can figure is when it gets real slushy and wet the thing acts up. I don't understand how it threw a death code them just went away. My buddy probably thinks I mis diagnosed and wasted his money. Oh, and it is an '03 ferm...

Mebbe he'll think differently when the problem reoccurs.

 

[mecaniksgrl35]

Where it is!?

Guys,

I pulled a code off my buddy's LB7 last night and got a TCM P0708 Code for the PNP Switch. The tranny kinda lags behind when he shifts in and out of drive and the little orange light also disappears between N and D sometimes. So I guess I've narrowed it down to the Park/Nuetral Position Switch, but i guess my two questions are...

1. Has anybody delt with this before that can give me words of wisdom? and

2. Where the heck is that switch at??

Thanks...

It is on the side of the tranny right above the shifter cable...east to change.