Mystery gear between 3rd & 4th ??

[SoCal LS-L]

I just remembered something that kept happening on the freeway back from NV..... when I had my cruise control on..... sometimes when first starting up a hill in "D" going about 70, the transmission would kick up a notch and the engine would go from about 2100 RPM to about 2600 until the climb ended, then it would kick back to about 2100. However when I took it off cruise and selected "3", it would be at about 3000RPM at 70mph...... quite higher than the mystery "gear".

Could this possibly be the torque converter unlocking?? It always locks up normally about 15 minutes into freeway driving.... so does it unlock purposely for hill climbs or high load situations? I would just like to know if my transmission is on the way out

[crazyhorse]

In a word, yes. The TC will unlock before the trans downshifts. You'll pick up a couple hundred RPM when it does.

[bheinen74]

the 2600 in D and the 2100 in D is the torque converter lockup
when it locks up, its OverDrive, which i why its turning less RPM.
Driving along, it will be in lockup or OverDrive, until it starts climbing a hill, which can kick it back down to D, thus the RPM increase while climbing a hill. Then it will go back into OD, or lockup of TC

OD and lockup is same. it's essentially a 5th gear over drive reduction
we have 4 gears, but when its TC locked that is our OD. driving in manual mode prevents lockup

[Crazy_pilot]

I'm guessing you don't have this issue in your location, but on particularly cold days here the trans cooler does it's job too well, and when I'm on the highway the airflow will chill the fluid to the point that the TC unlocks because the ATF is no longer hot enough. It stays that way for a couple minutes, then re-locks. Pretty annoying, actually.