07 Denali shifting wrong in tow haul

[zunman]

Hi all. I have a 2007 denali xl with around 90k on it. It has 26" rims also. I have had no issues with the tranny until today. There was a early s-10 blazer with a v8 swap behind me wanting to play. We were waiting to get on a highway from a stop. I punched it to the floor and started ripping up the highway. After letting off and driving a normal speed, the tranny started shifting up and down alot and seemed like it didnt know what to do. Once placed out of tow haul, it shifted nice as usual. I brought it back out a while ago with no hard pulls in tow haul and its doing the same thing. It just keeps shifting up and down repeatedly. Any thoughts? Maybe just a bad servo? I just had a service done 3 months ago too. Fluid level is fine too. The tranny temp went up to 160 too. Hopefully not a tranny rebuild right before x-mas!

 

[GM Customer Service]

zunman,

I am sorry to learn that you are experiencing an issue with your vehicle regarding the transmission. I cannot guarantee anything but I would like to look into this concern for you. Please private message if I can be of any assistance.

Regards,

Jennifer T.
GM Customer Care

 

[livingez_123]

Do you have the PCM adjusted for the different tire and wheel package? The trans is probably hunting for the right gear because of the difference in tire height.

 

[NathanJax]

The trans is probably hunting for the right gear because of the difference in tire height.

x2

Was thinking the same thing

 

[zunman]

So today I called my tranny guy. He said due to it only acting up in tow haul, it is probably computer related. So i called the shop who tuned and dyno'd it, he said the computer does somewhat learn the wheel size and may be trying to compensate in some way. My thought is why is it acting up several months and a 1000 miles later though? He said to put back on the stock wheels and disconnect the battery while doing so. After doing that it seems to now be fine with about 15 miles.
So Im hoping when I go to put the 26" wheels on in the spring, Ill just have him reprogram to the size of the 26 and no issues! Has anyone else had these symptoms due to larger wheels with the 6l80e tranny?

 

[zunman]

Tyranny is still shifting bad. Guess I'll take it to the dealer.

 

[zunman]

It was a bad brake switch for the record.

 

[Gatorlac]

are you using tow/haul without pulling anything?

 

[NathanJax]

It was a bad brake switch for the record.

that's weird

 

[redemn93]

are you using tow/haul without pulling anything?

good question. i was thinking the same. probably not the best thing to do.