2001 2500HD Allison Tranny stuck in Gear

[kellgy]

I have searched the forum but did not find my specific problem discussed. The other day I got in the truck, the tranny clunked when put into gear, check engine light came on, and it was stuck in 1st gear. I drove it a few feet, put it in park, then back in drive and now it is stuck in a higher gear reaching 2000 rpms @ 30-35 mph. When I turn it off I can repeat being stuck in first and third gear consistently. It appears to be a communication problem rather than a mechanical problem IMHO.

Another thing that happened: the display did not register it was in drive or any other gear except reverse and the indicator showed reverse after a short delay when put into reverse. After leaving it sit for a couple of days, the gear display lights worked but the transmission problem persists.

I understand the reliability of the Allison and hope it does not need replacement. I have a scanner and am going to check codes tonight or in the morning.

I do not work much on engines much but am willing to trouble shoot and fix it myself or at least try to avoid being ripped off at the repair shop.

Any advice would be appreciated.

[kellgy]

I scanned the truck and it said 2 codes. I inadvertently deleted them thinking they were from another car I have. Anyway, when I cleared them, drive the truck and the tanny worked fine. During the test drive the radio reset to 1:00 repeatedly probably due to my cassette adapter used for my iPod. Also, the security light came and stayed on during the drive. Now I strongly suspect it's electrical.
Restarting the truck the security light did. not come on. I guess no radio until its fixed. Has anyone else had unusual activity like this?

[DmaxMaverick]

What seems like "stuck in gear", as you describe it is actually "Limp Mode". It limits the gear range, and engine speed to 2000 RPM's. Add to that, your display behavior, and the usual suspect is the NSBU (Neutral-Start-Back Up). Failure is usually caused by snow/ice packing up into it, but being a 2001, it could just be normal wear due to age. It could also be a dirty or loose connector. The NSBU is located on the side of the tranny, where the shift cable attaches. The early models were black, and the later models are tan. The later models are supposed to prevent failure due to the snow/ice issue. If you have to replace it, it isn't a complicated or too involved job than can be done in your driveway. Just follow the specific procedure.

If you are using a simple code reader, it will only show P0700, in relation to the transmission. P0700 is the DTC that commands the MIL/SES lamp to ON. You need a more featured scanner to read the actual TCM codes. Meaning, if you got 2 codes with a simple reader, that means you still have one unexplained code. It is possibly a U code, associated with instrumentation. You'll have to confirm that.

The good news is, it doesn't sound like a problem with the transmission itself.

[kellgy]

Stuck in gear limp mode happened traveling across the country when my injectors gave out last summer. This new condition was not the limp mode I experienced last summer since I was not limited to 2000 rpms and retained normal power but was limited to one gear that was clunky when engaged.

Next step: crawling under to find the NSBU and learn about changing it myself. Thanks! I sure hope that is all it is. I will be driving with the scanner in to monitor things if that will not harm anything. I imagine it is a simpler one like you mentioned.

[More Power]

The NSBU is the usual cause of losing the PRNDL display, but the other electrical sounding problems you mentioned could also be the cause. If this were me, I'd check battery condition, battery cable condition, and the primary grounds under the hood before jumping to the NSBU.

What color is the body of the NSBU your truck has now? If it's black, it's the original unit, which was replaced with a tan colored upgraded unit a few years after the 2001s became available. Jim

[chessy56]

I've had this several times with my '01 2500HD. Some posts on this switch have suggested that the connectors may be difficult to remove, particularly if this is the first time the switch is being replaced (connector shells tend to crack). My problem wasn't so much the switch, as it was the repair connector installed by my local Chevy dealership. If you find you need to replace the switch harness, I would suggest soldering the wires together and using shrink tubing to insulate them. Pay attention to where the wires go (which terminal & wire). Then, use electrical tape to bundle the splices together. I also went a step further and used RTV silicone to seal the seam between the 2 switch halfs and used dialectric grease to seal the seam around the collar that fits over the selector shaft of the trans and the connector shells. With the cheap crimp connectors the dealership used on the replacement harness, I was constantly changing NSBU switches and chasing limp mode problems. I haven't changed one for the last couple of years now and have in excess of 100K miles on it. I don't like crimp connectors- I solder them together now.

[jbplock]

.. I don't like crimp connectors- I solder them together now.

Solder sleeves (https://www.google.com/webhp#q=solder+sleeve) also work also for splicing wires
http://www.aeroelectric.com/articles/pigtail/Solder_Sleeve.jpg
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&docid=EXOopN28vp3b9M&tbnid=jeGxRfj0LPfppM:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.aeroelectric.com%2Farticles%2 Fpigtail%2Fpigtail.html&ei=DsuuUpC0LNGMkAero4CABw&bvm=bv.57967247,d.eW0&psig=AFQjCNGT1L6-MLiqiZRtWS1seNCsktJXiw&ust=1387273357513013