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How common is a bad shift cable?

GM Guy

Manual Trans. 2WD Enthusiast
Messages
4,838
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837
Location
NW Kansas and SC Idaho
Hey all,

A buddy of ours ran onto a LB7 duramax, IIRC its a 01 or 02 CCLB DRW D/A K3500, and he wants to flip it for a grand, no touchie (in his words, he would just turn it into a moneypit)

He took it in on trade from an old rancher, it has had a bale bed on it for only 30K. The rancher put the bed on his 06, and bought a new one for highway duty, so it sounds like he was just rolling into newer equipment while the cattle prices are high.

Apparently it needs door hinges on the drivers door (IDK how bad he let it go, havent seen it yet) it had a set of injectors 2-3 years ago (still runs great, no hazing, no blowby) but the transmission is "sluggish" and apparently needs a new cable, as it pulls hard when you are trying to shift.

I am assuming like most folks I talk to with a duramax, the did not know that the spin-on filter needs changed often, so I am hoping that a new cable and a fluid change will take care of this one.

So basically, I am wanting input from you guys as to how common the cables go bad, and how hard they are to change.

Thanks!
 
The cable isn't to hard to change, never really heard of them going bad. On the early trucks you also need to make sure the NSBU switch isn't packed up with crap behind it. The early trucks did not use a NSBU that sealed up to the trans very well, and they get packed up and bind up the switch. Changing them out can be tricky sometimes as it looks straightforward, but it isn't completely. The hardest part is getting the 2 plugs out of the NSBU as GM used a hardening contact grease in them when they put them in at the factory. And only use the grey ALLISON NSBU if you replace it. I know the aftermarket switch is cheap, but it gives ALOT of trouble.
 
well, packed crap is a distinct possibility for this truck. :)

What are the symptoms of the NSBU switch failing, and what exactly does it do?

As far as parts, luckily 30 miles away, we have a certified Allison shop, so I bet I can get the parts cheap there. filters included.

Thanks!
 
The NSBU is a nuetral safety backup switch. It is mounted onto the shift shaft right behind the shift arm where the cable hooks up. Allison learned by 2006, and moved it inside the trans by the valve body, but 05- it was external. It tells the tcm what gear you have selected, turns on the reverse lights, and is the nuetral safety switch. GM cheaped out early on and used there cheaper switch like the 95+ trucks used, but it had problems with the allison as its not a precision switch, and didnt use a seal behind it which allowed crap to get down into the switch binding up the linkage, or worse, switch failure which gives you the shift range inhibited or the flashing gear display on 01-02 trucks.
 
Ok Gents, the truck is in the yard. :)

The shift cable is completely locked up, wont move at all, so its sitting in neutral. I pulled the codes, and they are:

P0841
P0700
P0841P
P0700P
The last two might just be a random duplicate due to using a cheapie 30 dollar code reader.

So, what are the thoughts? New shift cable and NSBU? I read somewhere about an internal harness and solenoids. Is this something i can tackle myself, take to GM, or take to Allison?

Thanks!
 
Ok Gents, the truck is in the yard. :)

The shift cable is completely locked up, wont move at all, so its sitting in neutral. I pulled the codes, and they are:

P0841
P0700
P0841P
P0700P
The last two might just be a random duplicate due to using a cheapie 30 dollar code reader.

So, what are the thoughts? New shift cable and NSBU? I read somewhere about an internal harness and solenoids. Is this something i can tackle myself, take to GM, or take to Allison?

Thanks!
Replace the shift cable, but the P0841 is a BAD code to see and is most likely NOT your NSBU. The P0700 just means the TCM has requested teh MIL on, but P0841 is for the C solenoid stuck off. This code is going to require some knowledgeable diagnostics, and most likely some work inside the pan of the trans. It could be a wiring problem, bad C solenoid, bad PSM, sticking C shift valve(if it's sticking, you have some SERIOUS ISSUES to address), valve body problem like a warped valve body, or bad TCM. A HIGHLY doubt a NSBU will do ANYTHING for this except leave your wallet lighter from buying a NSBU.
 
Thanks for the info Ferm, it is much appreciated!

Is the shift cable something I can tackle myself?

Secondly, will an Allison shop be plenty able to diagnose the C solenoid, even though its in a GM vehicle, or do you think GM would be the best place to take it?

Third, what is the best case scenario? could it be a ground or voltage problem (batteries are mis-matched one is older, and due to it being stuck in neutral, we are constantly hooking it up and unhooking it) Would poor fluid/ low fluid and clogged spin-on affect much in this scenario?

Is there anything on the external wiring that could have been knocked loose by mud?

Finally, how sealed is the NSBU? I was wondering if there would be a reason to replace it after living in a muddy dirty environment like it has for the past 30K miles. If they are sealed tight and wont be damaged by hard crusty mud, then I will leave it be.
 
Fwiw, our 01 8.1l has never had an nsbu failure and its been a feed truck for 300k. Trans was replaced around 120k as well a the tcase, maybe they updated it then but I doubt it.
 
Fwiw, our 01 8.1l has never had an nsbu failure and its been a feed truck for 300k. Trans was replaced around 120k as well a the tcase, maybe they updated it then but I doubt it.
It was most likely changed out. GM started using the ALLISON switch around late 02 because of all the problems there black switch gave.

Thanks for the info Ferm, it is much appreciated!

Is the shift cable something I can tackle myself?

Secondly, will an Allison shop be plenty able to diagnose the C solenoid, even though its in a GM vehicle, or do you think GM would be the best place to take it?

Third, what is the best case scenario? could it be a ground or voltage problem (batteries are mis-matched one is older, and due to it being stuck in neutral, we are constantly hooking it up and unhooking it) Would poor fluid/ low fluid and clogged spin-on affect much in this scenario?

Is there anything on the external wiring that could have been knocked loose by mud?

Finally, how sealed is the NSBU? I was wondering if there would be a reason to replace it after living in a muddy dirty environment like it has for the past 30K miles. If they are sealed tight and wont be damaged by hard crusty mud, then I will leave it be.

The shift cable shouldn't be that tough of a job to do, just have to get access to under the carpet where the cable runs is all. The adjuster is at a midway point in the cable whereas others is at the end, but it's not hard to adjust. The issue could be a short in the wiring or who knows. Like I said, that code is going to require some diagnostics with a DVOM 1st to see if the circuit is shorted out or open, and also to check the PSM to see if it is working or not. You can try clearing the code, and seeing of it comes back right away or not. The ALLISON TCM does NOT like low voltage, and can do all kinds of wierd stuff from low voltage during cranking. As to the NSBU, is it black or grey? If it's black it is a GM design switch, and not sealed well at all. If it is grey, it is an ALLISON design switch, and is VERY well sealed.
 
Thanks for the info.

I think right now the plan is get it all cleaned up, change cable, change oil and filter, probably change batteries, and cross fingers and hope for the best. :)

Also, I thought I read somewhere that the TCM on the shroud should have the little cover removed for better airflow? This one still has a cover on it.
 
No, leave the cover on it to prevent it from getting arced. Jump starting is the #1 killer of tcm's when the cover is removed. Any wire with voltage hits the housing and kiss the tcm goodbye. Thegen iv tcm used on 6 speeds is meant to he left uncovered for cooling.
 
OK, finally had a warm day to do some washing.

I do have a black NSBU.

I have not unhooked the cable, but I can wiggle the arm a little, so it seems the cable is definitely the issue. I might split it at the adjuster and use pliers to drive around the yard.... :) The records at the dealer show a new cable in 2013, so I assume they only did the lower cable and the one on the floor seized.

So what is the deal with the adjuster, what is the proper procedure for dialing it in correctly?
Is that basically the hardest part of the whole install? looks pretty easy underneath, but I bet from the adjuster to the column I will be having fun...

Ferm, if you were in my shoes, how would you proceed with the P0841?
 
Break out the ohm meter, get a wiring diagram, and start checking the wiring first, next the psm, then solenoids, if all checks good swap in my spare tcm to see if its an erroneous code.
 
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Would Allison provide a wiring diagram, or should I just buy a GM service manual set off ebay? I will have to get some assistance from a buddy or my old man, as I am pretty electrically challenged, so that is why I am stalling... :)

Also, got to tinker with it a little today.

got the adjuster apart under the cab floor and was able to easily shift the trans.

But, any time we would pull it in drive, the gear selector would flash and it would not even try to engage. I then thought it might have something to do with not applying the brake first, so we shut it down, put it in neutral, refired, and then held the brake as I pulled it into gear.

Also, fluid is at the bottom of the cold crosshatch, and the drivers side is uphill slightly, so it might be a little low on fluid. Do these have a level sensor that needs to be made "happy" before it is OK to engage?
 
As to the cable, adjusting it isn't hard, no different than any other. Put the shifter in park, make sure the trans is in park, then lock the adjuster in place.
 
No level sensor in them. If it is col, then the bottom of thecold range is fine. The flashing gear display is normally a bad nsbu, so when you were washing it, you might have got water in it(which was the problem with the black switches). I'll try and copy the pinouts and stuff off of alldata later tonight for you.
 
I see I left off at the end of the 2nd to the last paragraph. putting it in gear while holding the brake had no affect.

So a bad NSBU will prevent it from dropping into gear then? I will get a new cable from GM, and a few spin-ons and a NSBU from Allison and see If I can get it going.

Also, I hear talk about an easy to lose filter magnet. I assume cheaper from Allison?

Finally, thanks for the generous offer on the diagrams Ferm! Hopefully I can put them to use, or find someone who can! :)
 
OK, change of plans.

One google search on how to get the big plug out of the right side got to a page about oil leaks, and the guy only getting a leak after messing with the plug, so I figured with my luck, I better leave it alone.

So, diving right in to the NSBU where she sits.

Got the old one off fine, and scraped and dug behind it till clean. I will not be trying to blast it perfectly clean with water, just used air, dont want to shoot water past the shaft seal, or blast dirt against it.

So, now my question is:

What does a guy do with that black plastic cover that comes with the new NSBU? does it go on the NSBU before the nut is installed, or is it just a guard for shipping?

I understand the metal clip is for alignment purposes and should be removed upon completion of installation.

I plan on anti-seizing the bolts ever so lightly, and putting a very thin film on the shifter shaft itself to combat rust.

Is there anything I am overlooking?

Thanks!
 
OK, looked around and figured out the black shield.

Dielectric grease or Electrical contact grease in the plugs, or since there is still some of the sticky shit in there, leave it alone?
 
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