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!