Is it safe to pin open Transmission thermostat?

I would have to agree that pinning it open probably won’t hurt anything during the hot seasons. But would personally take the pin out in the dead of winter to help get fluid temps up quicker. I’ve notice very slight slipping when it’s 32* or below and the fluid temps are still low

 

Not my experience. When I run with the thermostat pinned, it is significantly slower climbing temp as would be expected - but seemed to stabilize around 175-185 which is about 20 degrees cooler than its operating temp with the t-stat doing it’s job.

 

This is consistent with what I read in this thread, which I still haven’t finished but it is pretty rich in user data on the subject. It’s an older thread though so I didn’t know if any consensus has changed since then, hence this post.

https://www.tundras.com/threads/install-complete-on-additional-transmission-cooler.12385/

 

Breather in transmission works both way. It is for pressure equalization.

 

I never saw 185. It drops to at least engine coolant temperature + a few degrees, which is about 210-230 F in my case. Perhaps, because I have 3 skid plates. In my Volvo installing a skid plate raises ATF temp by 15-20 F - that is with extra cooler and no thermostat.

Air cooler will cut the excess temperature a lot. It will not affect cold temperatures that much, because valve body internally has a spring loaded valve that diverts pressure away from the cooler to avoid its rupture. Cold ATF means a lot of linear pressure developed by the oil pump, most of it gets purged by the safety valve. Not so much cold ATF gets to the cooler to begin with.

 

Just a note, you do not collect moisture in your transmission, so there is nothing to "burn off". The only reason engine oil collects moisture is due to the combustion process. Same reason the diffs and t-case do not collect moisture.

 

I pinned it open after work. I only drove around for 20 minutes max, and after a harder throttle pull and parking it the hottest I saw was 175ish at the pan. 80 degree ambient. I’ll keep an eye on it and report back. Also for those interested, I’m testing the IATs post intercooler of the SC after doing the throttle body coolant bypass and initial testing shows a roughly 10 degree lower delta for that mod compared to before as well, which I was definitely skeptical of its effectiveness prior.

https://www.tundras.com/threads/general-supercharger-thread.85513/page-289#post-3209054

 

I've kept mine pinned since installing my transmission cooler. I have also replaced with all synthetic fluid. Took a trip from Las Vegas to the bay area and back and didnt see temps past 175 to 180 on the freeway.

This included driving up the mountains in San Bernardino Mountains and Tehachapi Pass on the way back. Truck and transmission performed flawlessly.

 

So at least for Toyota higher mileage I know that their transmission take a little bit of time to heat up and shift properly. But before that the transmission is not happy. Like iirc the transmission is locked in the lower gears (1-2-3).

And like anything mechanical they like their normal operating temperature which for transmission is higher temperature. And because of this your are putting more stress to your transmission while coming up to the normal temperatures. So if keeping the pin in causes it to heat up slower, my thoughts would be not to do that.

I say this as my opinion only and nothing scientific to back this up this claim. Just my humble opinion.

 

I just installed an external transmission cooler on my 2020 Tundra. Does fluid pump through the external cooler at all times, regardless of whether the pin on the thermostat is pulled or not? Sorry if this is a stupid question. Trying to understand how it all works.

 

The thermostat opens and closes as heat builds up and is released through the cooler. Be sure to pin it open anytime you service the transmission.

 

I was just curious if the pin being in would send oil to the cooler at all times opposed to the pin being out and the oil only being sent to the cooler when the thermostat reaches a certain temperature.

I notice that the cooler is super hot whether I drove a couple hours or only 10 minutes without towing. Trans temperature stays between 190-200 not towing anything and weather is over 100 degrees. Before, the trans would get up to 220, no towing and weather in the 80s.

 

Yes, pinned in it will always circulate. Mine unpinned runs about 190-205 pulling a trailer around town in s4 on the highway at 70mph in 100 temps. Without a trailer in D about the same.

 

Good to know, thank you sir!

 

I know this post is over a year old and I also don't contribute very much but I do have a lot of information on different toyotas and their transmissions. Currently we have a 2021 highlander with 105k miles pinned the thermostat day I bought it new. 2020 tundra that didn't come with a auxiliary cooler so I bought all the factory parts installed myself and pinned the stat open at 5k miles truck now has over 50k. Wife drives a 2019 4runner that did not come with a auxiliary cooler so I added a 30k btu b and m cooler In line with the factory radiator cooler. Finally we have a 2019 tacoma trans has been pinned since we bought it new and it has just over 60k miles on it now. All vehicles are driven 65 miles each way back and forth to work all highway winter trans temp doesn't exceed 140 degrees and summer temps rarely get above 170. I service all transmissions with idemitsu or Asian ws fluid every 15k miles. People tell me it's a waste of money to do the drain fill at that interval but I have 0 transmission issues. The temperature is measured with scan gauges in all vehicles and I just put back the same amount of fluid that comes out.

 

Just drove 65 miles from harrington de to wilmington de for work outside temperature is 53 degrees.

 

I love an old thread...

update on my pinned '20

Houston is around 80-85 now. Africa humidity will soon be here.

Transmission tops out at 177.

it heats up to 140 in about 10 minutes or so.

normally, it is 80-90 degrees above ambient. Winter is similar, although ambient delta is a tad higher.
Yes, Houston has winters, they are simply not like y'all's.

Hwy cruising in TC lockup, it's around 165-170 degrees.


I've pondered why would I want the trans to be 195-200 degrees if I could make the trans be 175 degrees?

I'm still pondering.


and before ya'll pontificate about how the Toyota engineers designed it that way...do you mean the same Toyota engineers that removed my factory cooler? those engineers?

 

And they removed it to save money not because the transmission doesn't need it. The pictures I sent are from my 2021 Highlander 3.5l 106k miles transmission pinned since day 1 no problems and I get 2 mpg more than epa highway rating in cold weather and 4 mpg more when outside temperatures are above 70 degrees.

 

I pinned mine a few months back just for shits and grins. I didn't think it would do anything, I figured once the trans warms up the thermostat is wide open so the temperature would reach the same equilibrium temperature. And I was totally wrong. My transmission used to hang out at 220 on the interstate doing 85, now it's at 190 tops. I'm blown away there was a 30deg temperature drop. The thermostat must not fully open is all I can figure and pinning it maximizes flow. Mines staying pinned for life now.

 

I have the same experience - about a 20-25 degree drop from 200 to 175-180.

 

One for rational thought and common sense.

 

rational? hmmm

unpinned - it runs ~205-210 degrees

pinned - it runs ~175 degrees

it would seem quite rational to me.

and it does provide additional cooling. clearly.

I was of the same belief - until I (and others) demonstrated it.


confirmation bias is a thing.

 

one more.

Re: warm up cycle.

the transmission appears to warm up via the puck. therefore; in strong correlation to the engine.

and who, on God's green earth, worries about the warm up cycle of a Tundra transmission.

for the record. I notice very little difference in the warm up cycle.

 

I would suggest to try it yourself. It only takes a few minutes to pin it open, and you can pick up a cheap bluetooth obd dongle to see the temp differences. Does a 30 degree drop in temperature keep a transmission alive longer? I can't answer that, but it certainly makes me feel like it will.

 

agreed.

a few of us have done just that.

confirming that it does, indeed, lower the trans temp by ~20 degrees.

and a cooler trans is a happy trans.


let's use me as a fine example*:

I drive 90% hwy
speed limit
TC locked up in 6th gear.
cruise control

why would I want the trans to hover at 210 degrees (It does/did in Houston), when I can easily make it hover at 175 degrees?




*please disregard that I am pushing 600hp, as I rarely use that amount.
But when I do, my transmission takes the hit from 175 degrees, not 210 degrees. thus, able to absorb a bit more friction/heat/slippage.

 

I pinned mine when I first got our travel trailer every time we used it. I added an additional cooler after the first tow.I see a 20 degree temp difference pinned. Just towed 2000 miles un pinned temp stayed at 203 208 at 65 highway in s5. Torque converter was usually at 205 210 but would come down to trans temps within a couple degrees,temps for this trip were 48 degrees to 88 degrees.I have cat plates on so that's why I left it un pinned this time. Towing a 6800 to 7000 lb load.

 

I have a factory cooler. Mine is unpinned. Towing through hills at 65 in s5 it runs right around 203-205. If I downshift and it unlocks the converter to pull a hill it’ll jump to 225 on the converter and then when it locks up it drops right back to 203-205. This is how Toyota designed it and this is how I’m gonna leave it.

 

@Silver17 did you end up getting updated numbers, pinned, while towing?

My aftermarket 15-row Setrab cooler was yielding the exact same results as you, which I guess is comforting in a way. Sad I missed this thread last year.

Anyways, I'm chasing these mythical sub-200*, and conveniently, part of my Stage 1.5 install is to add a 20-row cooler since it all needs to be relocated for the intercooler, anyways. I won't have any mountain hauls before then, but it'll be interesting to see if the issue is really just the sizing of the coolers (factory and aftermarket 15-row) as referenced earlier, or if it is just the design of the tstat.

The new cooler should be ~28% more effective, which should (in theory) put me down at tstat operating temp instead of perpetually open.

 

I do a lot of heavy towing. I do drain and fills every other year.

I don't consider 225* detrimental to the oil when it hits that temp.

I'm not bothering with pinning it open.

 

You should let Toyota know that they have been spending million of dollars on a thermostat that is not needed.

It's there for a reason. These transmissions are extremely reliable and there is absolutely no reason not to running as Toyota designed it.