PRND321 underline issue

[ieetgluu]

A couple months back I noticed that my CTS was reporting the gear as Park while I was driving around. I then noticed there was no underline under Drive.
Park, reverse and neutral will always have the underline. Once I shift to drive the line will stay under N for a second then disappear all together. Truck will still drive, seemingly normal, but I've noticed if I shift out of drive to neutral then back to drive sometimes it does goofy stuff like seems to stay in neutral. This has only happened once or twice, but I don't typically shift like that.
CTS will report Park, reverse and neutral as does the underline, but then drive will be reported as Park.
Occasionally it has "cleared up" while driving. CTS will show correct gear and D will be underlined again. Although lately it doesn't clear up anymore lol

Symptoms seemed similar to those reported with NSBU switch. I replaced it and was of course amazed that the issue persisted. I don't usually care to shoot the parts cannon at something, but it seemed like a safe bet given the symptoms.

Trans built by Evan several years ago

Any ideas out there? Thanks guys for even reading. I've tried searching, never come up finding this exact issue.

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[ieetgluu]

Forgot to add, it has popped the p0700. I cleared it the first time, eventually it popped again and I've just left it. I've not yet been able to pull the actual code from it. I know, it would probably help here. My autocal will pull trans codes, right? It's been years since I've messed with that thing.

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[Ron Nielson]

It kind of sounds like TCM, but that's a wild guess. I suggest you call Evan. He's a very smart guy and likely the shortest path to the solution of your problem

 

[TheBac]

Until you know the actual code, you have no idea where to start. Do things in the proper diagnostic order. Get the code read first.

 

[2004LB7]

I'm going to guess, NSBU switch but codes from the TCM would be nice

 

[Bdsankey]

Without codes we're flying blind. I had a customer with a similar issue.

He changed to a brand new GM NSBU, no fix.
Changed his pigtail, no fix.

Then he called me to diag the truck and we had no communication with the after he put my new built trans in. Replaced the TCM with a known good used TCM and boom everything worked as it should.

Long story short, grab the codes and see what it tells you. It could be simple.

 

[ieetgluu]

Thanks guys. I do have another tcm that Evan had sent me years back. The one in it now bucks in the 3-4 shift under moderate to heavy throttle. The other tcm(I think he said it was from an 03) shifts beautiful but I lose elevated idle and o/d lockout which makes my tap shifter semi useless.

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[darkness]

Thanks guys. I do have another tcm that Evan had sent me years back. The one in it now bucks in the 3-4 shift under moderate to heavy throttle. The other tcm(I think he said it was from an 03) shifts beautiful but I lose elevated idle and o/d lockout which makes my tap shifter semi useless.

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It’s probably an 01/02 tcm but there are updated os’s that allow you to keep odlo with a separate switch and then use the cruise for elevated idle. Sounds like you have an lly?

 

[ieetgluu]

Yes sir, 04.5

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[ieetgluu]

P0708
Transmission Range Sensor “A” Circuit High


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[2004LB7]

P0708 Transmission Range Sensor A Circuit High

Might want to give an NSBU switch a try as it's a relatively easy and cheap fix if that's the issue

 

[1FastBrick]

P0708 Transmission Range Sensor A Circuit High

Might want to give an NSBU switch a try as it's a relatively easy and cheap fix if that's the issue

He tried that already.

"Symptoms seemed similar to those reported with NSBU switch. I replaced it and was of course amazed that the issue persisted. I don't usually care to shoot the parts cannon at something, but it seemed like a safe bet given the symptoms.

Trans built by Evan several years ago"

 

[2004LB7]

He tried that already.

"Symptoms seemed similar to those reported with NSBU switch. I replaced it and was of course amazed that the issue persisted. I don't usually care to shoot the parts cannon at something, but it seemed like a safe bet given the symptoms.

Trans built by Evan several years ago"

I missed that. Bad wire? Maybe installed it with the indexing wrong? Bad TCM?

 

[ieetgluu]

Well, it's not looking like the tcm. I put my spare in today, which appears to be an '01, and that just made things worse. The only thing that would get the underline is Park. I never left the driveway to test for fear of being stranded. Shifts between PRND felt delayed and like it was confused.

So I put the original back in and it went back to a it was before, consistent underline of PRN, no D. Took it for a drive and decided to try 321, which I hadn't done previously. At this point things got weird. 3,2&1 all reported as P with no underline and it didn't feel like it was changing to those gears. Then, while in 1 the CTS started reporting 3. After that I lost the underline in all but Park, with reverse feeling rough as it shifted into and a loud metal on metal clunk into Park.

Yikes. I've been trying to convince myself to get something newer for quite some time now, this is pushing me closer. I want l5p and have been back and forth between older 6spd actual allison and newer, better looking l5p with the sort of allison 10 spd, but that's certainly a different discussion. Then there's the topic of going from non emissions to the world of issues and differences with dpf/def. Never mind the fact that's its all going for stupid money these days.

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[ieetgluu]

I'll also add that I inspected as much wiring underneath and in the engine bay as I could. Was expecting mouse damage but didn't find anything suspect. The NSBU harness does have a sharp bend in it where it joins another loomed bundle. At this point I'm suspecting a break or partial break in a wire at that point or the sensor in the valve body took a crap.

I had thought that the p0708 immediately popped when I turned the truck on after each tcm swap. I had disconnected batteries each time. But now I realize the tcm codes need to be cleared through a scanner and that battery disconnect won't do it.

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[Bdsankey]

I'll also add that I inspected as much wiring underneath and in the engine bay as I could. Was expecting mouse damage but didn't find anything suspect. The NSBU harness does have a sharp bend in it where it joins another loomed bundle. At this point I'm suspecting a break or partial break in a wire at that point or the sensor in the valve body took a crap.

I had thought that the p0708 immediately popped when I turned the truck on after each tcm swap. I had disconnected batteries each time. But now I realize the tcm codes need to be cleared through a scanner and that battery disconnect won't do it.

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On an 01-05 truck there is no sensor in the valvebody that designates what gear the shifter is in, only the NSBU does that.

What brand NSBU did you use? Is it a GM or parts store brand? Trace your NSBU wires from plug to plug and see if anything changes resistance when you wiggle the harness around. This will help you determine if there's a short or break somewhere. Visual inspection sadly doesn't always cut it.

 

[ieetgluu]

On an 01-05 truck there is no sensor in the valvebody that designates what gear the shifter is in, only the NSBU does that.

What brand NSBU did you use? Is it a GM or parts store brand? Trace your NSBU wires from plug to plug and see if anything changes resistance when you wiggle the harness around. This will help you determine if there's a short or break somewhere. Visual inspection sadly doesn't always cut it.

It's a GM part. Where is the other end of the plug located? Does it go straight to TCM? Does someone have a schematic or can point me in the direction? Hard to meter wiring without knowing what's what.

Thank you for the continued suggestions, guys

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[ieetgluu]

Side question, posted in the wrong area, did tapatalk or DF reset at some point? I've not been around much at all over the last few years, but I've far more than 20ish posts over 11 years. It's like history was deleted.

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[ieetgluu]

It's a GM part. Where is the other end of the plug located? Does it go straight to TCM? Does someone have a schematic or can point me in the direction? Hard to meter wiring without knowing what's what.

Thank you for the continued suggestions, guys

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Or perhaps it goes from NSBU to ECM and I was just plain looking in the wrong spot(even though they're right next to each other). I've found mouse wiring damage under the fuse box in the past, and of course the firewall material nesting in the valley when I did the head gaskets n studs.
Little fuckers.

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[2004LB7]

Side question, posted in the wrong area, did tapatalk or DF reset at some point? I've not been around much at all over the last few years, but I've far more than 20ish posts over 11 years. It's like history was deleted.

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Forum software upgrade a few years back. Some items are no longer supported. Other got a new name or location in the forum