You need to service all auto transmissions - even the sealed ones. This report is the how, why and when - for ordinary and severe applications, as well as whether you need to fit an aftermarket cooler or not.

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This report answers the following question, from a dude named Barry:

I really enjoy your videos as they give us non experts factual information that's difficult to get from other sources that are often less reliable.
I'd like to get some facts from you about what are harmful temperatures of the oil in automatic transmissions. I drive a 2.8l Prado with auto transmission & occasionally tow a camper trailer weighing 1.6 tonnes. Connected to the OBD2 port in the Prado is a ScanGauge2 that allows me to see the readings from many of the sensors on the car. I've been particularly interested in the readout of the transmission temperature.

"Some time ago you published a video that recommended using auto setting to tow & not to use the transmission as a "manual". Normally when towing I'd set the transmission into the 1:1 ratio 4th gear & tow in that mode. After watching your video, on the next camping trip to western Qld I put the Prado into Drive to see what, if any, difference it made. It did drive a little differently & shifted gears a lot more as it moved between the 5th & 6th overdrive gears to 4th gear or lower depending on the terrain.
The transmission temperature in this auto mode was regularly in the range of high 90 to over 100 deg C with the ambient temperature in the mid to high 20's.

"Next I went back to my default towing mode with the transmission set to 4th as the highest gear ratio used. There were less gear changes (no surprises there) but the transmission temperatures were definitely lower, mostly in the 80's & occasionally into the low 90's. Ambient temperatures were the same as before. The downside was that the fuel consumption was slightly higher. At a service after that trip I got my mechanic to change the transmission oil as on inspection it appeared much darker than normal. I'm not sure whether this was a result of the way it was driven on this trip or not.

"My understanding is that the optimum temperature for longevity of an auto transmission is somewhere in the 80 deg range & anything higher was likely to cause long term damage if the oil wasn't changed regularly (the Prado service schedule is for an oil change each 40,000 kms or 2 yrs). Can you give me some guidance on whether this is correct or not & what regularly running at the types of temperatures seen in auto mode are damaging. There must be many Prados (and other makes) out there that are used for towing & driven in a set & forget fashion that could be just waiting for an expensive appointment with a service centre.

"I'd appreciate your comments on this subject."

Many thanks

Barry