# 2500 HD 8.1 Allison trans codes P0700 and P0734



## Rtlofishing (Mar 23, 2018)

Hello all, great site here. I have a unique lengthy exp of a problem I'm having. 2001 Silverado 4x4 2500 HD 8.1 with Allison. Just bought the truck and driving it home in overdrive on deceleration heard what sounded like sticking a stick in a box fan for a couple seconds while running. Allison had 3rd and reverse only. At low speed tried changing gears and display wouldn't match where the Trans was in relation to the dash. Dis conn battery and reconnected and still had service engine but all trans functions. Drove home out of overdrive and trans never skipped a beat. Previous owner stated he replaced Gear selector trans switch 1.5 yr ago. Connected a reader and got the listed codes and cleared from dash but scanner still showed yellow. I need all the help I can get please guys. Thanks


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## Philbilly2 (Aug 25, 2007)

Change the NSBU switch again.

Limp mode combined with the gear indicator tell me that is what it is.

Take your vin to an Allison dealer to get the new NSBU. It will be far cheaper than getting at GM dealer. Yours will have 2 plugs on it. Make sure you get the right one as there are two models of NSBU. One with 2 plugs, one with only 1.


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## Western1 (Dec 4, 2008)

Like Phil said! Most common problem is the NSBU switch. Go to Allison dealer as he stated. No aftermarket. Inspect old one to see if there is moisture or corrosion.


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## Rtlofishing (Mar 23, 2018)

I have a new switch in the shop that I ordered to put on it but wasn’t sure once I was actually able to source the codes. Would that have anything to do with the noise before it coded out and went limp before the reset. I was going to check my connections this weekend at the TCM to make sure all the pins were seated. I was leaning towards a new switch also. Prior owner said he changed it and it didn’t help at all.


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## Philbilly2 (Aug 25, 2007)

If you did not say that the indicator was not in the correct spot... maybe.

That indicator not being correct is a dead give away to the NSBU. 

Start there and see what happens. It is a 15 minute job to change.


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## Mark Oomkes (Dec 10, 2000)

Philbilly2 said:


> If you did not say that the indicator was not in the correct spot... maybe.
> 
> That indicator not being correct is a dead give away to the NSBU.
> 
> Start there and see what happens. It is a 15 minute job to change.


If it was a Furd you'd have to remove the cab...


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## Randall Ave (Oct 29, 2014)

Mark Oomkes said:


> If it was a Furd you'd have to remove the cab...


Only to change the oil.


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## Rtlofishing (Mar 23, 2018)

Also going to check my transfer case been reading a lot about the 7034 code being related esp when the cases get wear on the inside. That is where the mismatch in speed is coming from tripping the code. Lots to check.


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## Philbilly2 (Aug 25, 2007)

Rtlofishing said:


> Also going to check my transfer case been reading a lot about the 7034 code being related esp when the cases get wear on the inside. That is where the mismatch in speed is coming from tripping the code. Lots to check.


It is very possible that you have pimp rub, but you need to get that indicator to match first.


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## OneBadDodge06 (Sep 22, 2004)

Worked on a 4L60E a while ago that had a P0700 code and it ended up being the TCM. Don't know if this applies to an Allison


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## Rtlofishing (Mar 23, 2018)

Indicator working fine post code clear and driving out of overdrive. Transfer case has no fluid in it and hole in case about the size of a English pea. Noise coming from case and with the heavy pump rub I'm thinking it's causing the chain and shaft to do weird stuff off acceleration which would be un torqued causing the last sensor to see something it didn't like with speed causing the ratio fault. Pulling the case and making that repair seeing how it's gotta be done anyways.


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## Rtlofishing (Mar 23, 2018)

And trans was 2 qts low on fluid to boot.


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## Philbilly2 (Aug 25, 2007)

OneBadDodge06 said:


> Worked on a 4L60E a while ago that had a P0700 code and it ended up being the TCM. Don't know if this applies to an Allison


PO700 is just a "general" trans code. It will appear with most all transmission issues.


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## Philbilly2 (Aug 25, 2007)

Rtlofishing said:


> And trans was 2 qts low on fluid to boot.


Did you use the correct method to check level?

(Truck running w/ shifter in N)


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## Rtlofishing (Mar 23, 2018)

Yes sir.


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