I installed a temperature sensor on the top front mounting bolt of the alternator. The wire goes through a small grommet in the firewall to an LED display on the dash, just below the air conditioner knobs.

On a sunny 90 degree F day, the alternator housing hit 175 degrees F after 15 minutes of driving around.

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Interesting. I'm curious why you did this?

You are possibly the only point of data on record for that measurement.

I could not find any post about the alternator temperature under normal operation. The best I could find was that the engine block is somewhere in the 200 to 220 degree F range.

I will be charging an auxiliary LiFePO4 battery using a RedArc BCDC1240D, which pulls around 40A from the alternator (via the starter battery). Instead of getting a high-output alternator (Nations Starter and Alternator makes the 270XP, but not for the 2019 model year, which I have), I will be using Toyota's OEM alternator. The concern is that the alternator will overheat from the extra load from the RedArc charger. The temperature sensor will let me know if that is about to happen. Then I can turn off the charger using the "Charge LiFePO4" switch that I installed next to the temperature sensor LED display (replacing the OEM USB ports). That switch will operate a relay under the hood that sends or blocks power to the RedArc charger. When the alternator cools off a bit, I can push the switch again to resume charging.

Have you called them? It was my understanding that they do make one that will fit the 2019.

Thanks for the tip. Last I talked to them (a year or so ago?), they did not have one. But I just called, and they said they have both a 180A and 270A alternator for the 4.0 liter 2019 4Runner. They just have to update their website to show they are in stock and ready to go, but they can take orders over the phone.