so why do transmission temps go up with overdrive on ?

OD on lowers engine speed, but raises trans temps ....hmmmmm

just never knew the reason for the increased temp, not used to an automatic with overdrive and never really paid actual attention to trans temps before this 4 runner.

and yes when climbing hills etc, my temp does drop with OD off

*this is somewhat specific to this trans*

OD on = torque converter only locks in 4th gear (OD)
OD off = locked out of 4th gear and torque converter locks in 3rd.

If you are on the gas enough to unlock the torque converter in 4th/OD, then you are better off turning OD off and getting the trans to lock in 3rd gear. Watch the tach carefully and you can see this happening.

If moving your food a bit on the gas makes the RPMs go up and down (instead of it staying directly locked to road speed), then the torque converter is unlocked and it will heat up the fluid.

-Charlie

PS. It really only matters for long hills, etc. You don't have to be too anal about it. It is best to keep the trans temp below 200, but a few seconds/minutes above that is normal.

PPS. More modern transmissions will lock the torque converter in more (sometimes all) gears to get around this issue and increase efficiency.

To me these trucks trans' unlock the OD way too easily causing slippage and hence the heat rise you see almost imediately when either climbing or during stop/go traffic.

I went bypassed (from the oem trans cooler) and used only an external trans cooler with a 2300 cfm fan for several years and experienced temps up to 237 while climbing say the west texas mountains or lots of stop/go in 100+ degree weather. Now I learned like y'all said to turn OD off on climbs and it makes a huge difference, stays under 200. OD off however does Not help with stop/go driving, at least no in austin.

So recently i noticed my original oem radiator developed a hairline crack and I decided it was time to replace it and go back to OEM raditor cooling + the external cooler/fan.

Its run in series first going thru the external cooler then radiator. Now on small to moderate hills I don't turn OD off and the temps only rise to 180 then drop within seconds of leveling off by 20-25 degrees. In town stop/go temps stay under 185 then drop 20 degrees once at highway speeds leveled off.

So if you want to really keep those trans temps in check go this route. Luck.

Most people with trans heating up to much are by passing the radiator cooler. Not only does it keep temps in check, pretty much around the same as the radiator temps, it also helps properly warm up the trans.

I still dont know why so many bypass the trans cooler when a replaced radiator will suffice. Almost all manufactures run trans coolers through the radiator for a reason. If they felt air cooled would be better they would go that route.

If I am driving around Austin I just stay out of the 4Runner lol. Too much stop and go IMO. I just go in the Corolla with a CVT because there's no shifting and it's way easier on a tranny. Austin = sucks.


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So you have a second electric fan on top of the normal fan? Is that switched power or has a temp sensor or What?

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Yes it's switched. Before I had to use it anytime ambient temps were 85+ (and even then stop/go & bigger hills would yield low 200s for a minute), now its off all the time. I may use it during low speed wheeling situations. My fan is only 10" (80watt) yet manages to help the a/c condensor out some when idling longer periods too.

But yeah like you said, I don't know what I was thinking bypassing the oem radiator trans cooler (over reacted to the pink milkshake potential), glad it didn't cause me any tranny issues. I have done trans fluid drain/fills & have run a magnefine for over 100k. Luck all.