With mountain driving you have braking issues that you can easily deal with by utilizing engine braking like everyone has chimed in with. But, you also can have huge issues with overheating your auto transmission.
Having a way to monitor your trans temps is the first hurdle. If you have a 96-98, the ECU doesn't provide auto trans temp info. If you have a 99-02, you can either use a Scan Gauge or a Bluetooth OBDII reader sending information to a phone app like Torque Pro. For 95-98 owners, you can install a sensor into one of the transmission lines and run it to an aftermarket gauge.
Once you are monitoring your trans temp, you will notice how quickly your temp will rise when climbing grades. Turning overdrive off can help a lot if your'e at a speed between 3rd and Overdrive. This locks up the torque converter and will drop your trans temp 20+ degrees. If you're climbing up a steep slow grade, knowing where your trans temp is will let you know when it might be prudent to stop and give your transmission a chance to cool down. At 175 degrees, your trans fluid could last you 100,000 miles. As the temp increases, the life of the fluid decreases. At 230 degrees, which is a temp commonly hit by people who drive in the mountains, your trans fluid will only last 30,000 miles. The idiot light Toyota provides for the trans temp on the dash supposedly goes off at around 300 degrees. At that point, it's too late. You're trans fluid is completely toast. The light is basically warning you to stop so you don't do further damage and get yourself towed somewhere.
With an external trans cooler ran in-series with the stock trans cooler in the radiator, I've still seen high temps on particularly steep ascents. One in particular was a 15-20 degree climb up to someone's house in the Santa Cruz mountains. The road was long and consistently steep which caused the temp to rapidly climb. I was almost going to pull over and give my trans a rest when we finally reached the house. I hit 240 degrees. A trans cooler ran in-series will drop your coolant temps around 20 degrees. So, without the cooler, I could have hit close to 260. At 260, your trans fluid will only last you 10,000 miles.
Monitor your trans temps. It might be the difference between a nice long life for your original transmission or an expensive and labor intensive replacement.
Here's a video showing how turning off overdrive can drop your trans temp:
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If you would like to see how I installed my external trans cooler, here's a video that includes replacing the radiator and thermostat as well:
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