Can someone explain transmission "half shifts" ?

[03v8raleigh]

This might be a weird question but hear me out..
So say I'm cruising at 70mph with cruise control on. Engine is at 2000 rpm in 5th gear. I get to an uphill section of highway and the car needs to downshift to 4th to maintain speed with the cruise on.. but right before it shifts to 4th, the rpm's instead jump to ~2300, and engine noise increases, but stays in 5th. If the hill is steep enough, it will then abruptly drop to 4th (the steeper the hill, the quicker it drops to 4th. And if the hill is just right, it will stay in this "half gear," or gear 4.5, as I like to think about it, for the entirety of the hill and never have to actually drop down to 4th. I can replicate this same shift pattern without cruise as well, by applying just the right amount of pedal pressure.

What is going on? I assume this is normal, as my family's '04 toyota sienna does the same thing, and has for all of its 250,000 miles. But I have no idea how this can be possible since there are only 5 gears in the transmission, and there should be no way for the rpm's to change without actually shifting gears..

So, anyone know how this magic is happening? Am I the only one that notices this?

(My vehicle, for reference: 2003 runner, v8 with 150,000)

 

[TechWrench]

This might be a weird question but hear me out..
So say I'm cruising at 70mph with cruise control on. Engine is at 2000 rpm in 5th gear. I get to an uphill section of highway and the car needs to downshift to 4th to maintain speed with the cruise on.. but right before it shifts to 4th, the rpm's instead jump to ~2300, and engine noise increases, but stays in 5th. If the hill is steep enough, it will then abruptly drop to 4th (the steeper the hill, the quicker it drops to 4th. And if the hill is just right, it will stay in this "half gear," or gear 4.5, as I like to think about it, for the entirety of the hill and never have to actually drop down to 4th. I can replicate this same shift pattern without cruise as well, by applying just the right amount of pedal pressure.

What is going on? I assume this is normal, as my family's '04 toyota sienna does the same thing, and has for all of its 250,000 miles. But I have no idea how this can be possible since there are only 5 gears in the transmission, and there should be no way for the rpm's to change without actually shifting gears..

So, anyone know how this magic is happening? Am I the only one that notices this?

(My vehicle, for reference: 2003 runner, v8 with 150,000)

What you are experiencing is the lock-up torque converter unlocking under load. This is a normal condition. When the torque converter clutch releases (unlocks), the direct connection between the engine and transmission reverts to a fluid only coupling, which will result in a 250-300 engine rpm increase to maintain current speed.

 

[03v8raleigh]

What you are experiencing is the lock-up torque converter unlocking under load. This is a normal condition. When the torque converter clutch releases (unlocks), the direct connection between the engine and transmission reverts to a fluid only coupling, which will result in a 250-300 engine rpm increase to maintain current speed.

Interesting, thanks! So in the interest of causing as little wear and tear as possible when doing highway driving, (I always turn off cruise control on uphills so I can avoid unnecessary transmission shifts when cruising) should I try to keep the truck from running in the +300rpm mode as well? As in, would it be healthier for the transmission to stay in the direct mechanical connection?

Trying to make this truck last as long as possible and most of its future miles will be highway miles. Want to avoid having to replace the tranny at 200,000 miles like our van.

 

[Ron4RNevada]

Whether the torque converter is locked up or not will have no effect on life of torque converter.

 

[TechWrench]

Interesting, thanks! So in the interest of causing as little wear and tear as possible when doing highway driving, (I always turn off cruise control on uphills so I can avoid unnecessary transmission shifts when cruising) should I try to keep the truck from running in the +300rpm mode as well? As in, would it be healthier for the transmission to stay in the direct mechanical connection?

Trying to make this truck last as long as possible and most of its future miles will be highway miles. Want to avoid having to replace the tranny at 200,000 miles like our van.

Unless the Lock/unlock cycle starts happening multiple times within a min. or so, it should not cause any immediate damage to the torque convertor or transmission. But, the longer the torque convertor remains in the unlocked condition, the higher the trans temps will tend to go. It is the torque convertor that generates the majority of heat in a transmission system, when it is unlocked. A long, prolonged pull uphill, under load, will cause the trans temp to rise significantly. Once the hill climb is over, and load stress on the trans returns to normal, the temps will return to normal (assuming your cooling system is functioning as it should. Since you have the V8, you should have a aux trans cooler in front of the A/C condenser, which should help with regulating trans temps. If you intend to tow with any regularity, a transmission fluid change earlier than the normal factory recommended interval would be a good idea.