2000 Yukon XL Not downshifting??????

[Oshkosh]

Hi,
Had a new issue pop up today after not driving the truck for a couple of days.
I started the truck let it warm up for a couple of minutes went for a ride and noticed it kind of doggie off the line once rolling.
I stomped on it at about 20mph and just kinda bogged as it picked up speed.
I then manually shifted down to 2nd and it sounded normal and took right off as I manually shifted up threw the gears.
It spins them up in reverse when stomped..
Now 48 hours ago when I parked it ,it was fine no issues what so ever.
It isnt throwing any codes, wasnt doing anything out of the ordinary before today.The fluid is clean and up to level...
Any Ideas? Would something go bad that quickly?

 

[ezdaar]

Never, I repeat NEVER manually down shift a auto trans unless SPECIFICALLY built to do so.

It spins up the sunshell to ridiculous speeds instantly! this is the num one reason they blow up.

Pour in some Seafoam for trans, internals might be sticking.. Run it for a week, then change fluid.

Tho its sounds like engine wants a tune up.

 

[Oshkosh]

Well it didnt over rev..and motor is running fine...

Never, I repeat NEVER manually down shift a auto trans unless SPECIFICALLY built to do so.

It spins up the sunshell to ridiculous speeds instantly! this is the num one reason they blow up.

Pour in some Seafoam for trans, internals might be sticking.. Run it for a week, then change fluid.

Tho its sounds like engine wants a tune up.

When I down shifted it didnt over rev, no more than a regular step on it downshift,That being said it is the first time Ive manually downshifted this truck. The engine is running fine/great, not a burp not a hicup why would you think it needs a tune up? Other than Gm's oil consumption issue it has been great.
The filter and 6 quarts of fluid where changed 2,000 miles ago so that should be plenty clean enough, I would think.As the old fluid wasnt discolored or burnt.
So it shifts and goes great manually and is an electronicly controlled transmission , maybe a sensor? pcm? I would have to lean to an electrical issue...

 

[chauncey0337]

When I down shifted it didnt over rev, no more than a regular step on it downshift,That being said it is the first time Ive manually downshifted this truck. The engine is running fine/great, not a burp not a hicup why would you think it needs a tune up? Other than Gm's oil consumption issue it has been great.
The filter and 6 quarts of fluid where changed 2,000 miles ago so that should be plenty clean enough, I would think.As the old fluid wasnt discolored or burnt.
So it shifts and goes great manually and is an electronicly controlled transmission , maybe a sensor? pcm? I would have to lean to an electrical issue...

Yeah, I don't see how a tune up would help you in this situation. This might be a long shot but check the engine bay and underneath the truck and make sure all wires and connections look good. There was a thread not too long ago where someones truck was running weird and found out it got invaded by some vermin and chewed up some wires. Might be worth a visual inspection.

 

[ezdaar]

that great that you dont manual down shift. Its not engine over rev your concerned about, its the internals of the transmission.
Specifically this, This is the sunshell. It cracks and explodes under manual downshifts.
As its rotating speed goes from (est) 9k rpm to over 18k rpm in a milisecond.




As far as the engine tuneup, I was basing that off of, the fact you said the engine feels like its bogging..
As far as trans fluid change, just 2k miles ago, thats even more reason something maybe sticking, as the fresh detergents "might" have free'd up some debris that have now lodged themselves into any number of moving parts that make the trans operate.

 

[Oshkosh]

When I googled the issue it mentioned sensors and connections, that being said worth a look for chew marks also...
Strange that I have driven the heck out of it the past two weeks(500 miles), park it for 48 hours and it acts up....
We did have a wind driven rain maybe something got wet...Not like I haven't driven in the rain or snow before...lol
The no codes has me baffled also...

 

[JochenWoern]

that great that you dont manual down shift. Its not engine over rev your concerned about, its the internals of the transmission.
Specifically this, This is the sunshell. It cracks and explodes under manual downshifts.
As its rotating speed goes from (est) 9k rpm to over 18k rpm in a milisecond.




As far as the engine tuneup, I was basing that off of, the fact you said the engine feels like its bogging..
As far as trans fluid change, just 2k miles ago, thats even more reason something maybe sticking, as the fresh detergents "might" have free'd up some debris that have now lodged themselves into any number of moving parts that make the trans operate.

Now maybe I am missing something, but help me with the "downshifting" part.

Are you saying that when I am driving downhill and I want the engine to help me brake (for example in snow conditions), I am not supposse to manually downshift from "D" to "3" or "2"?

Thanks.

 

[Oshkosh]

Thanks for the input...

that great that you dont manual down shift. Its not engine over rev your concerned about, its the internals of the transmission.
Specifically this, This is the sunshell. It cracks and explodes under manual downshifts.
As its rotating speed goes from (est) 9k rpm to over 18k rpm in a milisecond.




As far as the engine tuneup, I was basing that off of, the fact you said the engine feels like its bogging..
As far as trans fluid change, just 2k miles ago, thats even more reason something maybe sticking, as the fresh detergents "might" have free'd up some debris that have now lodged themselves into any number of moving parts that make the trans operate.

Well I dare say it is bogging because of the not down shifting transmission,starting out in a higher gear..The engine is fine.The trans isnt slipping or anything other than not automaticly downshifting.
If the valve body was gummed up or otherwise plugged it wouldnt shift either way would it?
Which leads me to think something electrical is not telling it to downshift.

 

[ezdaar]

Now maybe I am missing something, but help me with the "downshifting" part.

Are you saying that when I am driving downhill and I want the engine to help me brake (for example in snow conditions), I am not supposse to manually downshift from "D" to "3" or "2"?

Thanks.

All I am saying is that manual downshifting a Auto trans thats not built to handle the stresses is a leading cause for trans failures.

Now If your driving in snow and need the assistance of the trans to keep you from crashing, then by all means do what needs to be done.
1-2k for a new trans is far cheaper than a new 10-50k vehical or your life.

---------- Post added at 03:43 PM ---------- Previous post was at 03:39 PM ----------

Well I dare say it is bogging because of the not down shifting transmission,starting out in a higher gear..The engine is fine.The trans isnt slipping or anything other than not automaticly downshifting.
If the valve body was gummed up or otherwise plugged it wouldnt shift either way would it?
Which leads me to think something electrical is not telling it to downshift.

Everything you will want to know and more about your trans

Manuals,both hydro and electrical

Manuals,both hydro and electrical

 

[JochenWoern]

All I am saying is that manual downshifting a Auto trans thats not built to handle the stresses is a leading cause for trans failures.

Now If your driving in snow and need the assistance of the trans to keep you from crashing, then by all means do what needs to be done.
1-2k for a new trans is far cheaper than a new 10-50k vehical or your life.

OK, point well taken.

So let me re-phrase: "As long as it can be avoided, under no circumstances shall the transmission be downshifted manually while driving".

Does that sound better?