Someone Talk to Me About Pinning the Transmission T-Stat

So, I have seen several posts about pinning the transmission thermostat to improve transmission operating temperature and prevent the transmission fluid from burning up. My question is why would pinning the thermostat open help in this regard? Isn't the thermostat opening on its own once the transmission is up to temp?

I'm sure there will be some responses stating "its 15 seconds and it can't hurt" but isn't it just as important to get the trans up to temp as quickly as possible as it is to keep it within the specified temp range?

Photo from another thread for reference:

 

Maybe if you knew you were going to tow heavy for the day?

I’ve only done it to check the transmission fluid levels.

 

I’m guessing so the temp is constantly flowing and able to cool more effectively instead of just being kept in basically an oven.

 

That doesn't make any sense though right? Why would the thermostat not already be open if the temp was high enough for it to need to be open in the first place?

 

It doesn’t to me except for extreme cold climates.

 

I haven't done it, so I wont speak from experience, just conjecture.

I believe most transmission thermal events are pretty transient (going up a hill for a minute or 2). Pinning the thermostat starts the system off (before the hill) at 10 or 20 degrees cooler... So you have more thermal mass to absorb that transient before it overheats...

I assume it doesn't do much for a condition that is just constant for 30 minutes...
Just conjecture.

 

Now that DOES make sense. Do we know what temperature the T-Stat is supposed to open at?

 

Its hot
I will try to find it, but I don't have much data with me at the second
(I want to say its about 190F, but still looking)

 

Probably buried in the Service Manual somewhere:
https://drive.google.com/file/d/1cx5nnlnCzlhI45D1vbBMj1ni983SKdKf/view?usp=sharing

I believe I saw a separate detailed AT description document floating around here somewhere?

I also believe there is one for the MT as well that I would appreciate getting a hold of.

 

Because it only opens around 190deg, same as the engine thermostat. So no matter how big a trans cooler is, temp will never drop below 190 or so. Which is great setup for cold climates and epa bs. Not for hot climates. Mine's been pinned for 2 yrs.

 

Is 190* within the effective temperature range of the trans fluid? Do we know what the effective range of the Toyota trans fluid is? I guess I'm using the term effective range and thermal breakdown point interchangeably here.

 

160-285

 

@gearcruncher

 

So we're talking almost a 100* buffer between when the thermostat is supposed to be open and the thermal breakdown point of the oil?

 

I believe @gearcruncher usually posts the historical time/temperature data for fluid damage.

Interestingly, the ECU does track fluid degradation on some models/configurations, but I don't think I ever checked techstream for it myself.

https://www.tacomaworld.com/threads/atf-thermal-degradation-estimate-from-ecu.535949/

 

Wouldn't let me quote, that's from gearcruncher

 

Very interesting. I'll try to take a look at techstream today and see what it says.

Great info. So basically, at normal operating temperatures with the thermostat governing fluid flow, we are talking a 65k MAX lifespan for the "lifetime" ATF fluid?

 

I don’t think this should be the takeaway. There are many many automatic Toyota transmissions still in service with well over 200,000 miles still shifting as designed with the original ATF

 

This is an old thread, any new material on it? I have a 2022 trd offroad. How do I know if it has the tow package? It has a hitch. any thoughts?