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Old 05-30-2013, 06:01 PM
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Default Interesting information - Max Trans Temp

Since I got my new Tacoma, I have been towing an Airstream from Arkansas to Arizona last week. I have a Scan Gauge installed so I can monitor engine and transmission temps. On one hot, hill climb I saw a trans temp of 235 degrees. I was wondering what the temp was that turns on the A/T Temp Hi light on the panel so I asked Toyota.

Here is the answer:

Dear Terry Tubb,

Thank you for your follow-up email.

We apologize that the Owner's Manual does not include the temperature in which the automatic transmission oil light will illuminate in your 2013 Tacoma.

If you are towing with your vehicle, the automatic transmission oil temperature light may illuminate at 302 degrees. If the light comes on, it indicates that the auto transmission fluid temperature is too high. If the light flashes, it indicates a malfunction in the auto transmission system.

For further evaluation, information and assistance, we recommend you contact your local Toyota dealership.

Regards,

Kym Strong

Now we know - BTW I do have the Tow Package that includes a transmission cooler.
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Old 05-30-2013, 06:34 PM
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I got my old Tacoma to light up the A/T Temp light when I was climbing up Hayden Pass. I was in 4Hi and climbing up steep, loose shale on the west side. Lots of slipping... I parked for 10 minutes, then put it in 4LO and was fine the rest of the way.

I always wondered how hot that was....now I know.
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Old 05-30-2013, 06:42 PM
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Quote:
Originally Posted by Jacket View Post
I got my old Tacoma to light up the A/T Temp light when I was climbing up Hayden Pass. I was in 4Hi and climbing up steep, loose shale on the west side. Lots of slipping... I parked for 10 minutes, then put it in 4LO and was fine the rest of the way.

I always wondered how hot that was....now I know.
When watching the scan gauge, it is very obvious that dropping to a lower gear and higher RPMs help cool the transmission. Guess your experience proves it!
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"95" FZJ-80 - "White Top" F&R ARB Lockers, Slee F&R Bumper & Sliders, Warn Winch, 35s, Safari Snorkel, Low Range Gears, P/T Kit

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Last edited by ttubb; 05-31-2013 at 06:38 PM.
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Old 05-31-2013, 07:48 AM
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Terry that is a beautiful set up. I'm jealous.
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Old 05-31-2013, 10:38 AM
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IMO, 302F is way beyond the safe point for ATF. Even Ford Super Duties (4R100) are not supposed to go past ~220, but frequently run 190-210. To me, that's hot.

My FZJ-80 never saw any temps above 180, even when towing in Omaha at 106F ambient temps. It normally ran at 140F.

The temp at which the 'AT Hot' light comes on in Toyotas (including FJ62s) is super high and to me, far beyond reasonable (if it's the sump fluid temp). If my ATF got anywhere near that temp, I'd immediately do a fluid change and add a big cooler. YMMV.
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Old 05-31-2013, 11:33 AM
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Quote:
Originally Posted by SteveH View Post
IMO, 302F is way beyond the safe point for ATF. Even Ford Super Duties (4R100) are not supposed to go past ~220, but frequently run 190-210. To me, that's hot.

My FZJ-80 never saw any temps above 180, even when towing in Omaha at 106F ambient temps. It normally ran at 140F.

The temp at which the 'AT Hot' light comes on in Toyotas (including FJ62s) is super high and to me, far beyond reasonable (if it's the sump fluid temp). If my ATF got anywhere near that temp, I'd immediately do a fluid change and add a big cooler. YMMV.
302 is critical temp! ATF looses it's lubricating properties at 290+ and at 300+ the expansion of steel and aluminum parts is extremely dissimilar. In other words, if your light comes on you need to pull over and let it idle for a bit and cool it back down! Autos in modern cars have a tougher time than the older ones due to higher operating temps of the modern engine. Older vehicles rarely had external tranny coolers and now almost everything has them.
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Old 05-31-2013, 12:19 PM
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Some auto tranny fluids will start to flash in the low-to-mid 300F range, typically most by 350F. Synthetics usually are safe above 400F. ATF datasheets spec viscosity at 100C. I guess the warning light really is a warning light, so if it comes on you are taking a ride in the danger zone and like Steve says, flush and fill would be wise because you might be cooking the lube.
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Old 05-31-2013, 04:40 PM
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http://www.txchange.com/heatchrt.htm

A heat vs. trans. life chart, FWIW.
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Old 05-31-2013, 06:42 PM
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That chart is probably overly dramatic (and they conveniently sell tranny coolers), but the fundamentals seems true that you don't want the ATF going beyond about 225F.

The thermal expansion coefficient of materials is constant, most aluminums being about 12 to 13 ppm (e.g. microinch) per F and irons are typically about 6 to 7. My main worry would be ruining the fluid more than the marginally worse metal expansion. Joining dissimilar metals is a bad idea but unfortunately common on our junk.
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Last edited by DaveInDenver; 05-31-2013 at 07:06 PM.
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