New Owner - Coolant Loss = Potential Trans Issue?

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Jul 24, 2020
Hey all, new owner here! So I bought a 2007 GX470 from Arizona last week and road tripped back to the midwest. Closer to the end of the trip, I checked everything at a gas station and realized the coolant overflow tank was low. I added some and made sure I watched the temp gauge like a hawk until we got home. I ordered the parts including a new radiator but I was reading in older threads about something with the transmission being cooked due to a coolant loss? Can someone explain this is lay mans terms or link to the correct thread? I tried searching around but am either searching for the wrong thing or found threads that were too detailed or out of context. Obviously not going to drive until I replace the parts but had no clue this was a somewhat common issue... The morning after we got home, I checked the radiator fluid itself and added coolant to see how low it actually was. I added about half a quart to the radiator itself. Again I was watching the temp sensor like crazy and the transmission was still operating correctly, should I still be concerned?

Edit: I work on cars alot but up until now, I've always had small sports cars so this is definitely new territory for me, apologize in advance if anything sounds stupid or obvious.

I do not know of any specific issue with low radiator fluid and the GX transmission. In general though automatic transmissions are normally cooled via the radiator and a small external cooler. If the radiator loses it's coolant the transmission can overheat causing transmission damage. In my Mustang days I ran an external cooler that bypassed the radiator to keep trans temps down (This was a transmission with a high stall converter). Not sure if the GX guys do that. Maybe the experts will chime in soon.
Hows the maintenance history on it? I ask because it could be as simple as the coolant not being refilled correctly or it could be a very old coolant that simply ran low over the years. Not as quickly as water, but coolant does evaporate over the years.

If it were my rig, I'd top off the reservoir today, drive it around per usual. then tomorrow morning i would pop the hood to see if the reservoir is still "full"/at the recommended line.
I could be way off base here, but the main transmission + radiator failure issue I recall is the strawberry milkshake problem with the ‘96-‘02 4runners with an AT.

I don’t know of any particular radiator induced transmission problems for the GX470 that can’t happen to any car where the transmission is cooled by the radiator.
Whoops, just realized I didnt explain the reason why I'm replacing the radiator. So when I noticed the low overflow tank, I checked everything else and seen that the upper radiator hose was leaking the coolant and could see the coolant in that area as evidence but stupidly, when I was messing with the upper radiator hose, I accidentally snapped the overflow hose at the radiators plastic bit. There is no more off that plastic bit being exposed for the overflow line to fit back on. Unless find something that would work at Ace Hardware, honestly I'd prefer the safety of just getting a new radiator for peace of mind. The coolant wasn't mixed with oil (to my understanding) but thats why I was so confused when I seen other posts talking about how the radiator cools the transmission as well.

Also I understand the temp gauge is not accurate AT ALL, but I was just including that to at-least say that it didn't actually "overheat" I guess.
I think you're worried over nothing. If your engine didn't overheat due to coolant issues, neither did your trans. There's a potential for coolant and ATF to mix in a failed radiator, but that seems to be more a GM problem than anyone else.

If you're springing for a new radiator, buy one in a Toyota box for $150 instead of a Lexus box for $450. Also, be sure to open the block drains.
Just replaced my radiator with a Denso from Rock Auto. Same as Toyota/Lexus but way less $. It’s probably a 2 banana job, real easy. Only special tool that makes it easier is a long reach hose clamp plier to get the clamps off the radiator hoses.

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