Tempermental Allison

[1stdiesel]

Bought 2002 HD 2500 Duramax, there are no codes listed for trans, what happens occasionally is when it shifts to 2nd or 3rd when driving normal, there is a very short rev-up of engine rpm when making the shift. You can hear and feel a definate blip in the throttle when it shifts. It works perfect most of the time but all of a sudden it will do it for no reason when making the shift . It appears the shift calibration somehow gets off. Whats wrong? Something not quite right, it is slightly overfull on trans fluid, has 109,000 miles on truck, truck is perfect, never has pulled, guy who had it never worked it, vin#1GCHK23182F147678, 5 speed auto, allison

 

[1stdiesel]

1stDiesel here again. One more thing in trying to pinpoint the issue. Like I say, when it shifts into 3rd, it shifts firm and snug but at the completion of the shift there is a very short rev-up and instantly rpm settles back down. I refer to it like you could blip the throttle while making the shift. It only does it once in awhile but when it does it you know it. Anyone experience this? any imput Mike? Thanks!:woott:

 

[IOWA LLY]

I would start with changing the spin on filter unless you know it has been changed recently, drain a little fluid out to get it down to the proper level, then get someone with a proper scan tool to re-set the taps and allow it to re-learn and see if that clears it up.

 

[IOWA LLY]

1stDiesel here again. One more thing in trying to pinpoint the issue. Like I say, when it shifts into 3rd, it shifts firm and snug but at the completion of the shift there is a very short rev-up and instantly rpm settles back down. I refer to it like you could blip the throttle while making the shift. It only does it once in awhile but when it does it you know it. Anyone experience this? any imput Mike? Thanks!:woott:

I didn't read this before I posted. Does this truck have any kind of programmer or performance module on it?

The "blip" you describe sounds like an improperly written tune.

 

[1stdiesel]

No, this truck has never had a programmer on it according to the previous owner of 7 years when he purchased it at 15,000 miles. I haven't had it scanned yet but what you say makes sense, maybe a glitch in programming.

 

[THEFERMANATOR]

Sure sounds like it's a tuned ECM that somebody got the torque limiting tables too high on.

 

[jheyob]

Sure sounds like it's a tuned ECM that somebody got the torque limiting tables too high on.

X2, but the blip would happen consistently if that were so.

 

[THEFERMANATOR]

X2, but the blip would happen consistently if that were so.

Not always. I run an 02 OS in my BURB and was running an 03 TCM and would get the randomn blip out of it every once in awhile, never could get it figured out until I changed my tuning in the TLIQ tables.

 

[1stdiesel]

Thanks guys for the input :thumb: On another diesel forum I had guy say his truck started out doing this, then more often, then started throwing a code, and finally rebuilt trans because of a clutch pack slipping. I'm pretty sure my trans is fine mechanically, my blip is to crisp to be coming from a slipping clutch pack for sure when I'm driving it normal with very little power on it, this is when you can feel it the worst.

 

[03demax]

How long has it been since the fluid and all of the filters were changed. I had a funny shift when I had my 03. I changed the fluid and the spin on. still had the problem. dropped the pan again and changed the internal...problem solved!! IDK if this your problem.

 

[1stdiesel]

Previous owner said he had it flushed 2 yrs ago, I didn't get particulars on it and I feel he is honest. I would think they would have dropped pan and changed internal filter but then I have read that Allison doesn't recomend it till overhaul. They say just change the external spin-on. Think I should get it flushed again if there's junk letting loose and clogging filter? These type of problems are hard to pin down.

 

[Mike L.]

Try draining some fluid out of it and get the level correct. You don't want it over the half way on the hot grid.

 

[Colt]

I just went past some thread on plugged cooling lines on the Allisons after clutch packs glazed off and that sure has some value if you don't know the trans. care before you got it. Just for a thing to know if you hadn't heard that. I had to check this for a guy before he went into his looking for similar issues to yours I guess so. Good luck...

 

[1stdiesel]

Thanks Mike, 03dmax, and Colt. It's been so cold that I hv'nt dropped a little oil out of trans, it's a strong 1'' above max hot line when trans is barely warm and if that don't do it I'll drop filters and oil, hopefully I will not need to get it reflashed. Thanks again !:thumb: