Trouble Shifting in Tow/Haul mode

[Paul Brambilla]

So I'm having this strange issue on my 07 Yukon Denali that only seems to happen when in Tow/Haul mode.

First to Second to Third all seems to work as expected. However, once I try to get up to speed it just won't shift into 4th. My RPM's will get up to 5000 or so, at which point if I back off the gas pedal it seems to do this strange thing where it attempts to shift then immediately goes back into 3rd.

What makes this strange is if I put the truck into manual mode, with our without Tow/Haul on, I can absolutely shift into 4th or 5th just fine. This leads me to think that my transmission is probably fine, but maybe it's some sort of computer/sensor problem?

The truck has 137K miles and I just purchased it a few months ago. It came with a 90 day powertrain warranty that indicates it covers any problems with the transmission related to not shifting properly. However, I suspect they will try to convince me it is something not covered by that warranty.


Any suggestions?

Thanks

 

[Paul Brambilla]

Just a few more details as I took it out on the highway again. When in drive + Tow/Haul: It shifts great from 1 -> 2 -> 3. At about 3000 RPM's it shifts immediately into 2nd which spikes the RPM's up to over 5000. At this point I turn off tow/haul and it returns to normal.

Repeating that same experiment, but in M mode + Tow/Haul with the highest gear selected at 4, things work as expected. It Shifts from 1 -> 2 -> 3 -> 4.

I'm stumped...

 

[Paul Brambilla]

Well, the shop just let me know they've "fixed it" by simply replacing the brake sensor. They are claiming that when moving from 3->4 there was something telling the computer that the break was being applied which explains why it attempted to downshift.

I'm skeptical to say the least. If this were true why does it work just fine in M mode or normal drive?