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We're on an eight-week trip towing our 5800-lb travel trailer. We typically tow with the OD off because that's the conventional wisdom as I've seen it... turning the OD off eliminates constant locking and unlocking of the torque converter, which generates excess heat... an automatic transmission's worst enemy. The manual states that we should tow with overdrive off to maintain engine braking, but when I need engine braking it's simple matter of pressing the switch to gear down from fifth to fourth, or gearing down to third (or even second) manually.

As an experiment we've spent the past few days of our trip towing with the OD on and I've noticed that there isn't constant locking and unlocking of the torque converter. That is, in fifth gear I can't recall ever seeing it unlock without a subsequent downshift. We've done some varied driving (Black Hills of South Dakota vs the I-90) and where conditions and laws permit we typically tow at 104 km/h (65 mph, the limit of the trailer tires).

When we're cruising in 5th (~2000 rpm) and start an ascent, the torque converter unlocks (~2200 rpm) and then it drops into 4th (~3000 rpm) It does often drop into 3rd (~4000 rpm) and has gone into 2nd (~5000 rpm) on occasion.

It's the reverse on a descent... from 4th, upshifts into 5th, then locks the converter a few seconds later.

A friend with an '01 mentioned that the torque converters on early Sequoias were weak, but he didn't know if that extended to the '05 (which got variable valve timing and another cog in the tranny).

Is there any real reason I should be towing with OD off?


Cheers,

John.
 
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