jwahaus

Just trying to figure out what my auto-transmission is doing. I know that basically it is a 3-speed transmission + overdrive. What is weird is that there seems to be 6 or so drive ratio's (and no I'm not counting the Lo4 gears).

The ratio's I'm guessing relate to:
1st gear
2nd gear
3rd gear
3rd gear + overdrive
3rd gear + overdrive + transmission lockup

It seems also that sometimes it will use:
2nd gear + overdrive

I say this because if I'm cruising along 50MPH or so and encounter a steep hill I'll notice the transmission down shift. If I then turn off overdrive the RPM's jump up even more indicating that it was in: (2nd + Overdrive???)

Something I can't quite explain is when under heavy acceleration but not quite full throttle, say 2/3's throttle I'll notice the RPM's increasing at a steady rate until about 3000 RPM's at which point they stay steady for a second or so and then continue on to higher RPM's. At first I thought my engine was stalling but if I also watch the speedometer the speed continues to increase at a rapid pace while the RPM's hang out at 3000. It seems there is some sort of “clutch” at work here. Anyone know what is going on? Is this transmission lockup I'm noticing?

I sure do miss my 5-speed sometimes when driving this truck (except for my daily fight with stop-n-go which is why I got the auto-tranny in the first place). With the auto you're never quite sure what gear it's in.

FYI: 2002 Tacoma 3.4

Robinhood150

I used to think the same thing when I first got it, but I realized its the torque converter locking and unlocking.

jx94148

That first "Downshift" you feel when starting a hill at speed is the Torque converter unlocking which allows the motor to rev a little higher while still in the top gear.

I feel the same about automatics. Its always trying to second guess you and it can't see how long or steep any hill is. I can. I'll pick the gears based on what my eyes and ears tell me, thank you very much.

My99

Yeah that used to drive me nuts. When the torque converter locks up, is that what makes a tranny get hot quickest? I know very little about auto tranny's. What I'm asking is, when pulling a trailer or otherwise causing the truck to kick in the torque converter, would you be better off just turning off the OD and running in 3rd gear without the torque converter locking up? Or can anyone follow what i'm trying to say at all?

jwahaus

I'm not sure turning off OD will prevent the torque converter from locking. Like I mentioned above, it seems to be locking in second gear under moderately fast acceleration once I hit 3000 RPM's.

BTW when I'm towing I turn off the OD, I don't like to let the engine bog under a heavy load. It would be nice if we also had control over the torque converter. It seems logical (thinking it is clutch like) that it would be generating heat from friction but I really don't know.

TechWrench

Actually, as far as the torque converter is concerned, when it locks up, heat generation almost completely stops. Most of the heat in an automatic trans is generated in the torque converter, but only when it is not in lock-up. This is because the converter is using the fluid as a 'coupling' device and the 'friction' induced in the fluid causes it to heat up. Once the converter locks up, this process stops, and very little heat is generated.

My99

Okay, I guess i'm unlcear on when the convert is locked and when it is not locked. For example, in OD there seems to be 2 different RPM's for the same speed depending on how hard you mash the gas (of course if you mash hard enough it will kick down into lower gears). So is the converter locked when there's the least tension on a given gear and the RPM's are lower, or is it locked when you start going up a hill with your cruise on and it sorta "kicks" down (but stays in the same gear)?

Fink

All I know is when I need to pass somebody on the highway, I turn OD off and VROOM! There I go!! I also leave OD off when merging onto the highway so that I have more time in 4th gear (or whatever gear that is).

FWIW - My local dealer said when towing, its a good idea to turn OD off to prevent it from shifting into a higher gear with all the extra weight the trailer adds.

Just my $.02

Fink

TechWrench

When the converter locks up, the RPMs will drop because any slippage in the converter stops. If you are at a steady speed, and fixed throttle, the converter should lock-up. But if you hit the throttle too hard, or the load increases to much (hill) the converter will drop out of lock-up and the RPMs will increase slightly.

My99

Ahh, and thus it is better to drop the gear into 3rd (out of OD) if towing a trailer and you find yourself constantly kicking down (unlocking the conver in OD) and heating things up.

Thanks!

Robinhood150

Just to illustrate what's happening here and why the fluid heats up...

The fluid is constantly being sheared by the moving parts in the TC and tranny. When this happens energy is being transfered to the fluid itself. Believe it or not, when you stir your iced tea with a spoon, the temperature rises an infinitesimal amount.

One of my professors helped design a nuclear power plant and they tested out some gigantic water pumps. The pumps alone transfered so much energy to the water, that the water started to boil.

TechWrench

When you manually shift from OD to 3rd, all you do is keep the trans from using the OD gear. The converter can still lock-up if speed, load and temp signals are proper. The reason for not using OD when towing or climbing a hill is the torque range of the engine. If the engine is at too low an RPM, it will not be able to pull under load as well as at a higher RPM.

AgRunner06

It will also keep the temp down because it will unlock much less frequently than if it was in OD.

When I turn off OD, D will lock up almost immediately and stay locked up when accelerating unless I have my foot all the way in it (which never happens).

The higher rpms in OD is just the torque converter unlocking.