Transmission issue or wiring??

[Sickofthisshet]

Let’s start with 2002 Chevy 2500HD 6.6
I am chasing an issue I am sick of. I came to a red light and went to go and my truck would not move in drive. It did go in reverse so I was able to back up into a parking spot luckily. Get it home, disconnect battery and it still only had reverse. I replaced the NSBU switch and totally messed up, it would only start in neutral. So bought a new one and wiring harness and no luck. Changed transmission speed sensor. Took the steering column apart tonight to see the tow haul wire cut in half? It doesn’t even look broken, it looks cut. I spliced those back together and tow haul light popped up showing me it works. Put the truck in reverse and it locked in and moved. Tried drive and it would not move. Went back to park, tried reverse again and on the dash it was showing I was in neutral instead of reverse. Went down one to neutral and the R on the column was blinking. Back to park, went down to drive and drive started blinking. I noticed the transmission shifter interlock solenoid looked loose so I pushed on it and D stopped blinking, and it also went into reverse fine, then I went to drive again, back to reverse and it wouldn’t work then.
So if I pull the shifter down 1 from park it goes into neutral, pull it down 2 it goes into reverse and blinks, pull it down 3 it dies into drive but no engagement and does not drive, pull it down 4 it’s in 3, pull it down 5 it jumps to 1st, pull it down 6 it goes to 2 blinking.
Do I have something backwards on the NSBU?? I feel like after I spliced the tow wires together, connected the battery, it reset something for just along enough to go into reverse and threw itself into limp mode maybe? Even disconnected the battery does not do anything or allow it to go into gear so me messing with those wires under the steering column made something happen…

 

[2004LB7]

Welcome

When you replaced the switch, you made sure to have the selector in neutral first and it didn't move when loosing the nut? And you didn't remove the retainer clip on the switch until you had it in first?

 

[Sickofthisshet]

Welcome

When you replaced the switch, you made sure to have the selector in neutral first and it didn't move when loosing the nut? And you didn't remove the retainer clip on the switch until you had it in first?

Thankyou!

And well the second one my husband replaced, I’ve asked him 19 times if he’s positive he lined it up correctly and he assumes because it starts in park/doors unlock when you shift and put back into park, that is has to be correct. Once it didn’t work I disconnected the battery and it’s been sitting dead for months. Jumped off and still would not engage in any gears so I don’t think I reset anything yesterday to make it finally go in reverse again/then limp mode. It didn’t do that until splicing back together the tow haul wires and moving stuff around under the column.

 

[Bdsankey]

If the NSBU is reading incorrectly it's one of 3 things:
1) Bad NSBU
2) Broken/damaged wiring
3) Improperly adjusted/installed NSBU

As far as parts, I cannot stress enough how important OEM GM or Allison parts are for these types of things. OEM quality sensors (not even ACDelco) are always the way to go when available. They're costly but there is a reason they tend to carry that cost.

Based on this thread and the information presented I'm guessing it's #3. To properly change the NSBU you need to:
1) Chock the tires/engage parking brake
2) Put the transmission in neutral
3) Remove shift selector cable & arm
4) Remove the old NSBU
5) Clean the area of any debris
6) Install the new NSBU
7) Reinstall the shift selector cable & arm
8) Verify proper readings, if not reading correctly adjust until it is (this is why the part is slotted)

 

[Sickofthisshet]

If the NSBU is reading incorrectly it's one of 3 things:
1) Bad NSBU
2) Broken/damaged wiring
3) Improperly adjusted/installed NSBU

As far as parts, I cannot stress enough how important OEM GM or Allison parts are for these types of things. OEM quality sensors (not even ACDelco) are always the way to go when available. They're costly but there is a reason they tend to carry that cost.

Based on this thread and the information presented I'm guessing it's #3. To properly change the NSBU you need to:
1) Chock the tires/engage parking brake
2) Put the transmission in neutral
3) Remove shift selector cable & arm
4) Remove the old NSBU
5) Clean the area of any debris
6) Install the new NSBU
7) Reinstall the shift selector cable & arm
8) Verify proper readings, if not reading correctly adjust until it is (this is why the part is slotted)

Thankyou, I’m pretty sure I got a cheap ACdelco. Just wanted to see if it even fixed the problem but I can always buy another and install it myself so i know for sure. The first one that was installed like I said would only crank and start in neutral, I’m not sure if he broke it uninstalling because I had to buy another one. I keep making comments “are you sure…” because I was 100% certain that was the problem (a year ago, it’s been a YEAR!)
So can this one be taken off and reinstalled or is it no good?

Also let me ask this, when I put it in reverse it goes to neutral. When I put it in neutral it goes to reverse (blinking). So I need to pull the shifter down just once right? (technically would be R, but it skips R and over to N)

 

[Sickofthisshet]

If the NSBU is reading incorrectly it's one of 3 things:
1) Bad NSBU
2) Broken/damaged wiring
3) Improperly adjusted/installed NSBU

As far as parts, I cannot stress enough how important OEM GM or Allison parts are for these types of things. OEM quality sensors (not even ACDelco) are always the way to go when available. They're costly but there is a reason they tend to carry that cost.

Based on this thread and the information presented I'm guessing it's #3. To properly change the NSBU you need to:
1) Chock the tires/engage parking brake
2) Put the transmission in neutral
3) Remove shift selector cable & arm
4) Remove the old NSBU
5) Clean the area of any debris
6) Install the new NSBU
7) Reinstall the shift selector cable & arm
8) Verify proper readings, if not reading correctly adjust until it is (this is why the part is slotted)

Are YT videos allowed here? I posted what it is doing. The lights on the dash aren’t visible in person. It wasn’t until I was using my phone as a flashlight I realized my camera was picking it up ? So I havnt been able to even know that it wasn’t shifting correctly until now. Just figured since when put in park the doors unlocked that everything must be right. So considering that - would that be a bad switch?

Source: https://youtu.be/fw9bFMVCnx4?si=9scMfOslBfLMmR2e

 

[Sickofthisshet]

Hey, so scratch my post….i called to get the part number of what was purchased and he bought one for a 4 speed.
I’ll go lay in bed and cry now.

 

[Bdsankey]

Dual Plug Style: 29540479
Single Plug Style: 29541852

These are the genuine GM numbers. You want only Genuine GM or Allison for this component. If you can't source one locally let me know and I can get one headed your way. Hopefully you have the single plug style as they're far cheaper.

 

[TheBac]

Hey, so scratch my post….i called to get the part number of what was purchased and he bought one for a 4 speed.
I’ll go lay in bed and cry now.

Well, at least you found the problem. Frankly, none of us would've guessed that.

 

[Sickofthisshet]

Dual Plug Style: 29540479
Single Plug Style: 29541852

These are the genuine GM numbers. You want only Genuine GM or Allison for this component. If you can't source one locally let me know and I can get one headed your way. Hopefully you have the single plug style as they're far cheaper.

No I have the 2 plug - 11 prong ?

That may be a piece of the issue but I also checked around at other sensors and connectors and discovered the harness connector at the bottom of the pan is caked in mud and transmission fluid I guess. It also was not even plugged in correctly, it was backwards. I left my house and drove about 3 miles before the red light where it wouldn’t go and only would engage in reverse. So I cleaned it real good and I’m going to see if I can confused the TCM just enough to not recognize the NSBU is bad and see if it will go in drive. I’m currently waiting for everything to dry out to connect back. But with the amount of fluid in it I am assuming it’s a failed O-ring… however since it was backwards I’m wondering if that could have pushed tranny fluid through it? If I see fluid again after plugging in correctly I’ll move to replacing the whole harness I guess.

 

[2004LB7]

ATF in the connector can cause weird issues with the transmission. If it's leaking, known issue, replace it

You can try cleaning it thoroughly to see of the issues go away (once you get a new proper NSBU switch) but it will eventually fill back up with fluid and act up again

 

[Sickofthisshet]

ATF in the connector can cause weird issues with the transmission. If it's leaking, known issue, replace it

You can try cleaning it thoroughly to see of the issues go away (once you get a new proper NSBU switch) but it will eventually fill back up with fluid and act up again

It filled up by the time I went back to plug it in. So will definitely get that taken care of. I tried fooling with it for 2 hours; finally unplugged the harness altogether just to see what happened, and it DROVE!!! I didn’t take it up the road out of fear but drove it around my pasture. It was sluggish and slow. Put it into 4lo and it pulled amazing and drove like it would normally. So I am going to get the right NSBU and change the O ring or should I just go ahead and do the whole harness?
Do you think this verifies either my NSBU went out at day or the fluid caused loss of connection?

 

[DAVe3283]

The 5-speed Allison transmissions has a hydraulic failsafe where 3rd and reverse work fully mechanically when there is no electrical power, which is why your truck would drive once you unplugged the connector.

If I were to guess, it would be the transmission connector filling up with fluid that stranded you the first time. A failing NSBU usually starts getting flaky where if you jiggle the shifter it will go back to working. Though I did have a cracked NSBU that filled up with water while I was driving through the snow, and it failed all at once like yours did, so either is possible, especially if it was raining at the time.

Just get a new internal harness and new genuine NSBU switch (sounds like that is on the way already) and it should work great again.

 

[Bdsankey]

The 5-speed Allison transmissions has a hydraulic failsafe where 3rd and reverse work fully mechanically when there is no electrical power, which is why your truck would drive once you unplugged the connector.

If I were to guess, it would be the transmission connector filling up with fluid that stranded you the first time. A failing NSBU usually starts getting flaky where if you jiggle the shifter it will go back to working. Though I did have a cracked NSBU that filled up with water while I was driving through the snow, and it failed all at once like yours did, so either is possible, especially if it was raining at the time.

Just get a new internal harness and new genuine NSBU switch (sounds like that is on the way already) and it should work great again.

I agree with this.

 

[Cougar281]

ATF in the connector can cause weird issues with the transmission. If it's leaking, known issue, replace it

You can try cleaning it thoroughly to see of the issues go away (once you get a new proper NSBU switch) but it will eventually fill back up with fluid and act up again

So I'm going to be the one to ask it.... Transmission fluid is non-conductive, and all of the electronics inside are submerged in it. In 6L and 6T transmissions, among others I'm sure, the entire TCM also gets submerged (or at least soaked) in it. So why does ATF in the main connector cause odd issues?

 

[2004LB7]

So I'm going to be the one to ask it.... Transmission fluid is non-conductive, and all of the electronics inside are submerged in it. In 6L and 6T transmissions, among others I'm sure, the entire TCM also gets submerged (or at least soaked) in it. So why does ATF in the main connector cause odd issues?

I'm not too sure either as your right about it being non conductive. But it's a well known issue. Maybe it gets between the mating pins and being non conductive it prevents a good connection???

 

[Cougar281]

I'm not too sure either as your right about it being non conductive. But it's a well known issue. Maybe it gets between the mating pins and being non conductive it prevents a good connection???

If that was the case, you'd expect dielectric grease in other electrical connectors would cause the same kind of issues with whatever's on said connector ?‍

 

[2004LB7]

If that was the case, you'd expect dielectric grease in other electrical connectors would cause the same kind of issues with whatever's on said connector ?‍

Yep. Head scratcher for sure

 

[Bdsankey]

Agreed, no clue "why" fluid in the connector causes issues but it always has.

 

[gmduramax]

If that was the case, you'd expect dielectric grease in other electrical connectors would cause the same kind of issues with whatever's on said connector ?‍

I thought you weren’t supposed to put dielectric grease inside the connection, just over the top to encapsulate it ??‍

Also that stuff you spray on battery terminals, do not spray it on first then make your connection, my dad learned that the hard way ?