Rough downshift...

[sdeeter19555]

What would cause a harsh downshift under acceleration? I was starting to see it mainly in a 5th to 4th downshift, now I'm starting to notice it in the 4th to 3rd. It's a pretty hard bump...not normal.

 

Any thoughts to start looking at...no codes, no other symptoms. Truck has 125k on the clock, trans has been maintained well (multiple fluid/filter changes).

 

Steve

2012 2500hd

 

 

[Formulabruce]

Transmission pressure problem. Boost valve in the Pump, or could be shift solenoids ( replaceable from underside). But since you have it in more than one shift sequence, I am thinking you will need a trans rebuild, seals replaced. I would highly recommend a BIGGER transmission cooler at that time. You may have trans codes that do not show up on a regular scan tool. A pressure check while driving ( transmission shop) will tell the story. Good Luck!

[sdeeter19555]

Really, a rebuild with 125k of a mainly daily driver? How are these guys getting 200k of hard use? I already have one of the biggest coolers installed in tandem with the OE cooler. Rebuilding it wasn't what I wanted to hear...

I guess I'll take it in and see what they say...the problem is it isn't consistent. Today was perfectly normal...

Steve
2012 2500hd

[Formulabruce]

They will put a pressure gauge on the transmission , and it has a long hose that will go inside the drivers window to be monitored at shift points.

[sdeeter19555]

I've more or less concluded this is something to do with a solenoid. It is inconsistent, one time it is smooth, next time harsh, next time less harsh, next time smooth...this whole ride home (300 miles), it happened once...every other time was smooth. Trans shifts perfectly fine up through the gears, even at WOT. Only when it drops a gear under load does it sometimes give me this almost delayed engagement.

Is any reprogramming needed to swap transmission solenoids, or can they just be swapped out? I would just have them swapped if the latter.



Steve
2012 2500hd 6.0l

[Animal1]

What did you eventually do to fix this harsh downshift under acceleration?  Mine does it 5 to 4th gear.   It will manually downshift smoothly.  Only happens in drive.   Thanks!

[Supreme Pizza]

Probably not, but you could look at the U-joints, cross member rubber mounts, motor mounts, etc. 

[Animal1]

That’s definitely not any of those suggestions.  Remember, manual shifting up and down is smooth whether accelerating or decelerating.  It’s not any mount because that would act up at other times too.  

[Supreme Pizza]

1 hour ago, Animal1 said:

That’s definitely not any of those suggestions.  Remember, manual shifting up and down is smooth whether accelerating or decelerating.  It’s not any mount because that would act up at other times too.  

 

    I agree. As I said, Its probably not. Never the less, its takes 5 minutes to check them, and if he runs out of other options, it would be worth having a look. Who knows, maybe the load limiting function is not working properly AND one of the mounts has a little slop. Its not likely - I agree. It is possible though.

 

     "brainstorming"

 

     Lets all throw out ideas, and he can check them one by one in order of likelyhood.

 

     I would suggest checking the most likely causes first, and working your way down to the unlikely possible causes.