elnido

My 95 T100 has my first auto transmission in a long time. I find myself switching the O/D off and on quite a bit, like shifting a five speed. Mostly to "optimize" the perceived performance (ie not shifting in and out on hills, slowing down, etc.) So far no problems, any thoughts? Is this transmisson abuse?

Jeffires

It's been used regularly for the same things on my '91 since it rolled off the lot and still no problems *knock on wood*

Mohamed

As long as you are not using it as a manual tranny to slow down the vehicle or upshifting at like 80 mph...it's fine, it is as if the tranny is down shifting to get to another gear when you hit the gas. Personally, I just leave it on, unless I'm towing, and mash the gas to downshift to a lower gear.

elnido

I used to just leave it on, then after reading about t/c lock up, I noticed I spend a lot of time driving on hills (colorado) in O/D with what feels like the t/c not locked. IS IT BAD TO USE IT TO DOWNSHIFT AT NORMAL SPEEDS?

Honger

If you are driving in frequent enough hills I'd just leave it off so as to avoid t/c locking issues. I wouldn't use it for performance issues because as Mohamed mentioned, you can just use your gas petal to control this. I'm not sure there is a problem with switching it off/on a lot... Jeffires is a perfect example of this. However, you could always end up wearing out the switch or something.

Additionally, if you drive in a lot of hills, I'd consider a transmission cooler. These are known to extend the life and reduce wear on AT transmissions.

Regards,
Joel

WATRD

All it does is lock out the overdrive gear. Turning off overdrive is just like downshifting in a manual and not using 5th gear.

Use it all you want. It's not going to hurt anything. In fact, if you attempt to switch O/D off when you are going too fast to downshift into the lower gear, it won't let you. It beeps, the O/D Off light blinks and it won't switch gears until you slow down a bit, then it locks out O/D and the light comes on solid.

Ben03

I've been using my O/D button to downshift my truck since day 1. Nothing has happened so far nor do I expect anything negative to happen. As one put it on this site, " . . . think about how much a transmission shifts on its own anyway, us telling it to downshift or upshift is no different than it doing on its own . . . ". In addtion to that, as the computer eventually "learns" our driving style, the truck downshifts on its own in certain instances. My truck, BTW is an 04 Thundra, V8, 4X4, LSD, TRD, Tow . . .

Honger

Ha! Learn something new everyday! I think I may try this out on the way home. Thanks WATRD!

WATRD

I discovered it by accident on a long road trip. You have to be going fast enough that downshifting would cause it to come close to the redline.

Mohamed

True on your statement, but downshifting by hitting the O/D switch going down a hill or even fast enoguh generates a lot more heat and is bad for the tranny torque converter, it's not quite the same as a manual tranny, where you rev match the gear when downshifting...

I'll have to get back as to more detail on this a bit later as I am coincidentally taking automatic tranmissions right now. Geez, a torque converter is quite a complicated thing, we just got into this subject last night. Another interesting thing with the torque converter is the link is a fluid link, not mechanical link such as a clutch and flywheel. Because of such, and the centrifugal force applied, you only get 90% effectiveness and the last 10% is lost as heat, so you can see that downshift on a high note would be quite bad for a torque converter, you'd be surprised at how powerful these things can be, and thus the reason for lower gearing in an automatic versus manual tranny vehicle, but the heat generated is quite a lot.

ricqik

I gotta ask, how fast was it? On my tundra, no matter how fast I go it'll shift back into 3rd if I turn the od off. Even at 105mph. That's because the 00-04 tundra's 3rd gear goes all the way to the 105mph speed limiter and still has I think about 400rpm left before hitting it's 5400rpm redline rev limiter.

WATRD

Let's just say that I was on an open stretch between Moses Lake and Vantage Washington, the conditions were good and I needed to make up for lost time.

I saw a car parked on the side of the road way up ahead and needed to shed some speed, so I clicked the O/D off. It beeped and the orange light blinked at me until I coasted down a ways, then it shifted and damn near pitched me through the windshield.

My Tacoma may not be representative with 35" tires and 4.56 gears either, but I don't think I can hit the speed limiter in 3rd, never tried.

Mohamed

I have to refresh my statement here, I can't believe this, but my teacher DOES downshift his automatic tranny going down a hill. He says he's been doing this for quite a long time and said if he didn't, his brakes would cook all the time coming down a hill from where he lives at, but what's amazing is he states he's had no issue with his tranny and his van is over 200K miles , in fact states he has had no problem with his old Astro van's tranny doing this.

Honestly, you make the call, but personally, I still wouldn't downshift my auto to slow down my truck, like I said, I would be afraid of the torque converter heat generated, even with the lock up inside the coverter.