The severe service schedule for transmissions?

[lizpat]

A Hyundai TSB I found online seems to suggest that city driving and idling means you should follow the severe ATF change schedule of 30,000 versus 100,000 for normal driving. This contradicts what I have always thought about the issue. My thinking is that highway driving is more taxing on the transmission. Consider that to measure the ATF level, you were expected to do at least 20 minutes of highway driving to get the ATF temperature up to a normal level.

There is no urgency for me about this, as it is I already do a drain and refill at about 20,000 to 30,000 miles, but I am still curious about what severe service means for automatic transmissions.

 

[stranger706]

Hwy cruising is very easy on an AT providing you're not towing anything.

You're in top gear, with a constant load and constant temperature. You're not constantly changing gears and wearing out the bands and clutches. This is why city driving is so hard on ATs.

This is pretty true for the rest of the drivetrain as well. Hwy cruising = good.

 

[Donald]

If you look on the Amsoil website under their ATF, I believe you will see how they classify normal vs severe.

 

[95busa]

The torque converter locking and unlocking creates a bunch of heat. Highway cruise=locked torque converter=cool running trans.

 

[OrdnanceMarine]

If you have access to your transmission temp via something like a ScanGaugeII then you can see what heats up your transmission. Doing this with my 4Runner was eye-opening!

I'm guessing that Hyundai wants highway driving before checking your fluid level because this will provide for a more uniform temp that people are checking their fluid level. I know Toyota wants the transmission fluid level checked at something like 110F. Every AT vehicle I know of runs the ATF through the radiator core to help regulate the temp of the fluid for more consistent transmission performance as well as add a means of cooling the ATF. What I usually see is that then the torque converter locks up the transmission temp stabilizes at whatever the radiator core temp is towards the bottom corner of the radiator. Generally this ends up being in the 130-180F range, depending again on how hot that section of the radiator is.

To echo what was said above, when the torque converter is locked up the transmission is essentially generating very little heat and highway driving is usually comprised of exactly this. Driving that involves lots of shifts and an unlocked torque converter will add heat to the transmission but generally if you're not mashing the throttle over and over or moving a heavy load these periods of heating are relatively short and do not result in a hot transmission. The only time I've seen passenger cars with a hot transmission are on extended steep grades where the transmission does not lock up.