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Discussion Starter · #1 ·
This happened all of the sudden. I can go in reverse all day long but no forward D321 will work. When selecting the range it'll think a few seconds then the box highlighting the selected range will flash. Happens in all forward gears. I thought enough online searching suggested the NSBU aka neutral safety switch so I replaced it. That didn't make a difference. I even spoke at length with an Allison service tech who said without any codes on the dash they're limited to where to start troubleshooting. I have no error or idiot indicators on dash suggesting a problem.
Have you experienced this?? Is the solution a backyard mechanic fix or factory fix? Thx!

Brad
02 2500HD Duramax
4x4 crew 162k miles
 

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I agree-- no codes-- just have to check all the wires, connectors -- might have a broken wire in the harness.

If you can get it scanned with a Tech II, that will help with the diagnoses.
 

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This happened all of the sudden. I can go in reverse all day long but no forward D321 will work. When selecting the range it'll think a few seconds then the box highlighting the selected range will flash. Happens in all forward gears. I thought enough online searching suggested the NSBU aka neutral safety switch so I replaced it. That didn't make a difference. I even spoke at length with an Allison service tech who said without any codes on the dash they're limited to where to start troubleshooting. I have no error or idiot indicators on dash suggesting a problem.
Have you experienced this?? Is the solution a backyard mechanic fix or factory fix? Thx!

Brad
02 2500HD Duramax
4x4 crew 162k miles
Have you driven through any high water lately?
 

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Discussion Starter · #4 ·
Nope, not driven through water. Just started one day. I relented and had it towed to the closest Allison service center. Thx!
 

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i was going to also suggest a NSBU but you changed it and it didnt fix it, let us know what happened to it when the tear into it at the Allison Service Center!
 

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Discussion Starter · #6 · (Edited)
UPDATE - So the NSBU was NOT the cause of this. I purchased the $38 Napa unit. Boy it was a pain to get the connectors off til I revisited this site to find that a hair dryer would help loosen the glue. After being towed to the Allison Svc Center, they found no codes while hooking their laptop to various parts of the truck. They dropped the pan and removed the "valve body", found some debris, dirty fluids, flushed the bodies, lines and such and replaced filter and fluids. The tranny worked fine after that. Word of advice..don't miss your tranny service interval like I did. That was $750 vs. about $40 if I were to replace the filter and fluids. I don't drive my truck much anymore but when I do its always towing something, Airstream, horse trailer, tractor...
 

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I just had something similar happen to my 01 but it turned out to be the range switch. That was only 400 but then i had them do a full flush and drop the pan etc. That's another 300 but at least it wasnt the tranny like I feared.
 
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