Allison transmission

Beaux Duke · December 23, 2002

Saturday morning I got in my '01 2500HD and started it up. Put it in reverse and nothing happened. No noise, no bump, no click... nothing. Shifted into Park and back to Reverse again... still nothing. Went from Reverse to Drive... BAM! Transmission engaged with a very loud noise and shook the entire truck violently.

Shifted to Park and back to Reverse... still nothing. Dropped it in Drive again... BAM!! Engaged violently again. Noticed the "SES" message in the information center had illuminated so I put it in park, shut it down and called GM Roadside Assistance.

Tow truck guy arrived and asked me if it would start. I said, "Yeah, but it won't shift."

He got in, started it, put it in Reverse and backed it onto the bed of the wrecker with no problem. Go figure.

Anybody heard of this problem before?

Woody1 · December 23, 2002

I just laughed after reading that. Not at your problem but the wrecker driver being able to get in it and go! I cant help ya, but thanks for the laugh anyway!Let us know what you find out what caused it, cause i got one just like yours, only without the wrecker man and no tranny probs.

Shaners · December 23, 2002

I think I remember some earlier Ally trannies having some problems like that. They seemed to be happening more often if the truck was parked on an incline a little bit. I don't remember what the cure was though.

99silveradoz71 · December 24, 2002

Ah yes, the phantom symptoms...hate when that happens, hopefully the dealer can figure it out.

BROKER · December 24, 2002

OUR 02 HAS A BRAINFART LIKE THAT ONCE IN A WHILE.Then everything is fine for another 20thou! I've left it alone so far.

Beaux Duke · December 25, 2002

Talked to the Service Rep right before they closed on Christmas Eve. Looks like I get a new transmission.

Not exactly thrilled about that.

The Service Rep said I get a new trans because GM won't let it's service people work on the Allison. Only Allison certified mechanics can work on them. If no one is certified, then out it comes and a new one goes in.

I asked what sort of problem they found and was told they didn't have the code readily available and to call back after Christmas. I'll update this thread after Christmas.

Oh yeah, it was kind of funny at that Woody1. You could have knocked me over with a feather when it went into reverse and acted like nothing had happened. I know the tow truck guy thought I was crazy.

CMNTMXR81 · December 26, 2002

That's done on other transmissions (namely the 6 speed manuals) due to their complicated design and certified technician availability.

It's also cheaper 9 times out of 10 to simply replace it anyway and I concur with GM's thinking. I'd rather have a brand spanking new trans than one that Bubba had popped the case on and took a crapshoot to figure out what was wrong, attempt to fix it, put it all back together, then reassemble your truck only to come back two months later when Bubba's first fix didn't work and eventually led to greater problems as well as you having to bring it in AGAIN to be service for the same problem.

Think about it, it saves GM money, it saves you time and hassle (by only having to service it once) and you get a brand new trans out of it. What could be better than that? And don't say "It shouldn't have broken in the first place!" Coulda, woulda, shoulda I'd reply!

Beaux Duke · January 2, 2003

I was hoping it was something simple like a bad modulator valve or such.

I agree. I don't like the idea of Bubba tinkering around in there. But I also don't get all warm and fuzzy with the thought of Bubba replacing it either. I'm sure I'll be crawling around under there after I get it back making an inspection of Bubba's work.

kylant · January 5, 2003

beaux,

when this happened did the PRND321 display flash or go black? if so, it could be a NSBU switch, a common failure on earlier model trannies. they have a new updated switch.

Or, did you have your truck sitting for a few days before this event? If so, on some trucks there have been issues with the torque converter draining all the oil. then the tranny senses loss of oil pressure during intial start up. during this time if you put the tranny into gear you get your symptoms, then the tranny goes into limp mode.

go check out thediselpage.com and look under 2500HD & 3500 truck/drivetrain thread. Do a search for NSBU and one for transmission engage. You will find so much information there, you could spend all day reading it.

I highly doubt your tranny is bad. Could be a simple new switch (NSBU) or a false code.

Don't let the stealer replace your tranny until these are addressed.

Good Luck

Beaux Duke · January 6, 2003

Sorry kylant, I don't remember what the shift display looked like. At any rate, the deed's been done.

Got home Friday and the Dealer had left a message on my answering machine. The trans had come in earlier than expected and they had installed it. Unfortunately they are now having problems with the TCU (Transmission Control Unit) and had to order a new one. It should arrive Monday (1/06/03) and hopefully I'll be able to pick up the truck Monday evening or Tuesday.

Now I'm wondering if it was the TCU all along.

Professor · January 6, 2003

Most likely not the transmission or the TCU. There have been several problems with the NBSU as kylant stated. It has been discussed frequently and in detail on thedieselpage.com. It is a shame if the dealer had you down for this long and it turns out to be the switch because they did not know what they were doing.

Beaux Duke · January 7, 2003

Got home Monday and had a message from the Dealer on my answering machine. The trans & TCU were installed and working OK but the power steering pump went out while they were moving the truck around the shop area. They're having one overnighted and it should be ready today (1/07/03).

Do you suppose the whine they were hearing was coming from the P/S pump and not the trans???

odoh · January 7, 2003

I had an funky/ruff shifting issue. One dealer said it was normal. Another dealer quickly offered to replace it. I quickly nixed his proposal. 10Days, an TCU and valvebody later, its normal for the first time in its life. I too am reluctant to have dealer working on it because to many of them, the D/A is still relatively new. GM repair bill was 2900 including courtesy vehicle. ~ odoh

Beaux Duke · January 8, 2003

Finally got the truck back and everything seems fine.

Backed out of the driveway this morning, shifted to Drive and no hard shift like the previous transmission.

Steering seems to require a little more effort but I sort of like it this way.

The Service Rep was not letting go of any info as to what was wrong with the trans. They said they'd get in touch with me if and when Allison let them know what was wrong with it.

A guy at work said he had problems with his Duramax/Allison HD at around 20k miles too. His dealership apparently had a service tech qualified to work on the Allison. He had the valve body replaced in his but he didn't have the same symptoms as mine.

If I get any info back from Allison/Dealer I'll update this thread.

All is right in the world, again.

:thumb: