Overdrive Off = Transmission Temp Drop

mtbtim

mtbtim

Elite Member
Hey Dudes,

I learned the trick of turning off Overdrive on this forum to reduce transmission temps while climbing grades. It seems this is still not a well-known fact to many 3rd Gen owners so I figured it was time to make another video. I didn't have my expert video producer/editor @infamounsRNR around but i put together passable product. The lighting while driving washed out some of the trans temp information I was trying to show on my phone but there was enough footage that showed the trans temps when overdrive was off and on that the video worked to document the difference.

Here's the video and Enjoy the Show!

<iframe width="560" height="315" src="https://www.youtube.com/embed/BEZfDOkGVno" frameborder="0" allowfullscreen></iframe>
 
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It's a shame that the TCC doesn't lock up earlier than 40-42mph with overdrive disabled. My 04 grand Cherokee will lock up TCC at 30mph with OD turned off. Which is real nice.


Sent from my iPhone using Tapatalk
 
Thanks for info Dudes. I was wondering what the lowest speed the torque converter would lock up at.
 
I've really not understood what a torque converter even does... as it what benefit it provides. My last vehicle was a manual, so not sure why autos don't just shift through the gears like normal? Is it supposed to be more "smooth" for the driver or something?
 
gamefreakgc said:
I've really not understood what a torque converter even does... as it what benefit it provides. My last vehicle was a manual, so not sure why autos don't just shift through the gears like normal? Is it supposed to be more "smooth" for the driver or something?
Click to expand...

It's the same principle as the clutch in a manual transmission, it's just a different way of solving the same problem: Allowing the engine to turn independently of the transmission. Except it does it without driver interaction, as needed in a manual. At least that's how I understand it, lol.
 
gamefreakgc said:
I've really not understood what a torque converter even does... as it what benefit it provides. My last vehicle was a manual, so not sure why autos don't just shift through the gears like normal? Is it supposed to be more "smooth" for the driver or something?
Click to expand...

In a manual the clutch sits where the torque converter is on an auto, and when you push the pedal down it physically disconnects the engine from the transmission. The torque converter is a fluid coupling, allowing a permanent connection between the engine and transmission, because it allows the engine to idle while the transmission is stopped. It does this through some nifty fluid dynamics.

Warning, more info than you may want below.

A torque converter has two modes, below and above stall speed. Below stall speed the TC is basically a REALLY tall gear that allows some slip because of the fluid nature of the connection. When you step on the gas the TC very quickly spins up to stall speed, at which point the fluid dynamics inside change and it essentially up-shifts to a new gear. This effectively simulates how a clutch works. Some power is lost through the TC, making it less efficient than a manual. Which is why in top gear it has a lockup feature, which simply means it engages a shaft that creates a physical connection through the TC, eliminating the fluid coupling as if the TC didn't even exist. In a 4 speed auto you will feel a 5th "shift", where the rpms drop a little bit, when the TC locks up. If you disable overdrive, i.e. 4th gear, the TC will lockup in 3rd at a lower speed, which is good for towing because the transmission gets really hot when it's not in lockup and pulling extra weight. You make more power out of lockup because the rpms are a bit higher at any given speed, but it's less stressful on your transmission to stay in lockup as much as possible. Some guys install a switch so they can manually force the TC to lockup whenever they want. Drag racers do it, and some that tow a lot do it.
 
Very interesting... I didn't know that.

My commute is almost exclusively 30 - 40 mph. I wonder if I could increase fuel economy by disabling overdrive?

Hmm...
 
I got OD off all the time except on freeways. Tim, you has the driver side roof handle. I installed it and find it very useful when you want to readjust the seating position.
 
WallyWest said:
In a manual the clutch sits where the torque converter is on an auto, and when you push the pedal down it physically disconnects the engine from the transmission. The torque converter is a fluid coupling, allowing a permanent connection between the engine and transmission, because it allows the engine to idle while the transmission is stopped. It does this through some nifty fluid dynamics.

Warning, more info than you may want below.

A torque converter has two modes, below and above stall speed. Below stall speed the TC is basically a REALLY tall gear that allows some slip because of the fluid nature of the connection. When you step on the gas the TC very quickly spins up to stall speed, at which point the fluid dynamics inside change and it essentially up-shifts to a new gear. This effectively simulates how a clutch works. Some power is lost through the TC, making it less efficient than a manual. Which is why in top gear it has a lockup feature, which simply means it engages a shaft that creates a physical connection through the TC, eliminating the fluid coupling as if the TC didn't even exist. In a 4 speed auto you will feel a 5th "shift", where the rpms drop a little bit, when the TC locks up. If you disable overdrive, i.e. 4th gear, the TC will lockup in 3rd at a lower speed, which is good for towing because the transmission gets really hot when it's not in lockup and pulling extra weight. You make more power out of lockup because the rpms are a bit higher at any given speed, but it's less stressful on your transmission to stay in lockup as much as possible. Some guys install a switch so they can manually force the TC to lockup whenever they want. Drag racers do it, and some that tow a lot do it.
Click to expand...

That's actually very helpful. Because my 4Runner puts out a whole lot more power than stock (I've added roughly 100 more HP) it will sometimes peg itself at 3k RPM's and just sit there at about 1/3 throttle, when I want to use more of the powerband. I knew this was because of the torque converter but didn't really know WHY it was doing it. I think I'll try driving with O/D off for a bit and see how that changes things.
 
nissanh said:
I got OD off all the time except on freeways. Tim, you has the driver side roof handle. I installed it and find it very useful when you want to readjust the seating position.
Click to expand...

It was a gift from Sean from his many visits to our local Pick and Pulls.
 
tvpierce said:
Very interesting... I didn't know that.

My commute is almost exclusively 30 - 40 mph. I wonder if I could increase fuel economy by disabling overdrive?

Hmm...
Click to expand...

In theory yes, if you're not hitting fourth gear anyway the only effect disabling overdrive has is giving you lockup, which should increase mpg.

gamefreakgc said:
Because my 4Runner puts out a whole lot more power than stock (I've added roughly 100 more HP) it will sometimes peg itself at 3k RPM's and just sit there at about 1/3 throttle, when I want to use more of the powerband.
Click to expand...

Not sure I know what you're getting at.

Do you mean you give it some gas and feel like it should downshift for more power but it doesn't? If so, have you tried using the ECT PWR button? I leave that on all the time, it raises the shift points so you hold each gear longer when accelerating, and it will downshift quicker too in the situation I think you're talking about.

You'd think that would impact your gas mileage, but it doesn't seem to. I get 17 - 18 mpg average on my commute, and it's not highway. Plenty of stop and go.
 
WallyWest said:
Not sure I know what you're getting at.

Do you mean you give it some gas and feel like it should downshift for more power but it doesn't? If so, have you tried using the ECT PWR button? I leave that on all the time, it raises the shift points so you hold each gear longer when accelerating, and it will downshift quicker too in the situation I think you're talking about.

You'd think that would impact your gas mileage, but it doesn't seem to. I get 17 - 18 mpg average on my commute, and it's not highway. Plenty of stop and go.
Click to expand...

Yes, I also leave the ECT Power on full-time. I even have a shift-kit installed where it modifies the valve body to shift more aggressively and hold gears longer. The issue I think is that it has much more power because of the supercharger and fuel mods, it'll be spinning nicely and scooting along but since the throttle isn't open that much the torque converter won't lock up and sort of saps the power. I think I might just tighten the transmission kick-down cable a bit more to downshift easier, I did that before I had the valve body shift kit installed.
 
WallyWest said:
In a manual the clutch sits where the torque converter is on an auto, and when you push the pedal down it physically disconnects the engine from the transmission. The torque converter is a fluid coupling, allowing a permanent connection between the engine and transmission, because it allows the engine to idle while the transmission is stopped. It does this through some nifty fluid dynamics.

Warning, more info than you may want below.

A torque converter has two modes, below and above stall speed. Below stall speed the TC is basically a REALLY tall gear that allows some slip because of the fluid nature of the connection. When you step on the gas the TC very quickly spins up to stall speed, at which point the fluid dynamics inside change and it essentially up-shifts to a new gear. This effectively simulates how a clutch works. Some power is lost through the TC, making it less efficient than a manual. Which is why in top gear it has a lockup feature, which simply means it engages a shaft that creates a physical connection through the TC, eliminating the fluid coupling as if the TC didn't even exist. In a 4 speed auto you will feel a 5th "shift", where the rpms drop a little bit, when the TC locks up. If you disable overdrive, i.e. 4th gear, the TC will lockup in 3rd at a lower speed, which is good for towing because the transmission gets really hot when it's not in lockup and pulling extra weight. You make more power out of lockup because the rpms are a bit higher at any given speed, but it's less stressful on your transmission to stay in lockup as much as possible. Some guys install a switch so they can manually force the TC to lockup whenever they want. Drag racers do it, and some that tow a lot do it.
Click to expand...

When you really think about it, the engineering behind a torque converter is freaking amazing. "Nifty" as you put it.

Is there a certain RPM or speed range that 3rd maxes out on, or depends on the road you're on? (Straight vs up/down hill)

I've always wondered that as when I have O/D off, I notice anything after 40MPH the RPM's start really climbing, feels un-natural.

At the same token, I've been on downhill mountain passes with OD off and I was in the 60s/70s MPH range (Lower RPM).
 
gamefreakgc said:
I've really not understood what a torque converter even does... as it what benefit it provides. My last vehicle was a manual, so not sure why autos don't just shift through the gears like normal? Is it supposed to be more "smooth" for the driver or something?
Click to expand...

It's amazing that it came from GM out of all people at that.

https://en.wikipedia.org/wiki/Hydramatic

https://en.wikipedia.org/wiki/Torque_converter

The easiest way to think about it is that the engine has to always be moving when it's turned on, or you'll have big issues. There has to be a way to disconnect the transmission gear from the crankshaft of the engine and allow the crankshaft to always be doing that rotation until which time the vehicle is powered off. Otherwise, imagine trying to apply brakes to a moving crankshaft, you'd mess up everything trying to rotate the crankshaft as they continue to move.

The clutch does it in a manual. The TC does it in an automatic through fluid coupling.
 
Last edited:
1996 SR5 will not go into overdrive!

mtbtim said:
Hey Dudes,

I learned the trick of turning off Overdrive on this forum to reduce transmission temps while climbing grades. It seems this is still not a well-known fact to many 3rd Gen owners so I figured it was time to make another video. I didn't have my expert video producer/editor @infamounsRNR around but i put together passable product. The lighting while driving washed out some of the trans temp information I was trying to show on my phone but there was enough footage that showed the trans temps when overdrive was off and on that the video worked to document the difference.

Here's the video and Enjoy the Show!

<iframe width="560" height="315" src="https://www.youtube.com/embed/BEZfDOkGVno" frameborder="0" allowfullscreen></iframe>
Click to expand...

Truck runs in 3rd and wont go into overdrive. I changed shift solenoids, filter and fluid with no help. It shifted much better but still will not go into o-drive. On three occasions after that it surprised me by actually trying but jumped up for two to three seconds and right back out. This happened weeks apart. Truck has 256,000 on it and runs like a dream otherwise. Very strong. I drive it a lot between VA and Texas and I don’t like running over 3 grand the whole way! Please help!

Thanks for these videos and your knowledge! They have helped a lot, plus we drive the same model!
 
jezzbuzz said:
Truck runs in 3rd and wont go into overdrive. I changed shift solenoids, filter and fluid with no help. It shifted much better but still will not go into o-drive. On three occasions after that it surprised me by actually trying but jumped up for two to three seconds and right back out. This happened weeks apart. Truck has 256,000 on it and runs like a dream otherwise. Very strong. I drive it a lot between VA and Texas and I don’t like running over 3 grand the whole way! Please help!

Thanks for these videos and your knowledge! They have helped a lot, plus we drive the same model!
Click to expand...

I'm not a transmission expert. Swapping out the shift solenoids like you did is what I would have tried to fix the issue. Maybe somebody will chime in with some insight on what could be causing it to fail to shift into overdrive.
 
Thanks for getting back!
I have read that a temp sensor of some kind will cause the lack of o-drive problem. Any info about this would help for sure. What sensor, location, and will it help?
 
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