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Old 10-31-2011, 12:30 PM   #6 (permalink)
Hubert Farnsworth
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Join Date: Feb 2011
Location: USA
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Planet Express Ship - '94 Geo Prizm LSI
90 day: 36.57 mpg (US)
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Quote:
Originally Posted by Ryland View Post
This should have been covered in drivers ed, but seeing as how they don't teach people how to drive any more I'm not surprised that it was not.
The over drive button controls if the torque converter gets to lock up, stop and go traffic can be rough if it's constantly locking and unlocking, same with rolling hills at 45 to 50mph but for a lot of normal driving you want it to lock up so you have a more direct drive in the transmission.
Using it can also help your transmission to last longer.
The Overdrive lock switch has nothing to do with the torque converter locking up, but would prevent access from the over drive gear ranges, which on a four speed automatic would mean that the car would not shift beyond third if it was engaged prior to the vehicle shifting up, it may also alter the programming to hold onto lower gears until higher revs than in overdrive. In a 5 speed automatic with a single overdrive gear it locks out 5th, if there are two overdrive gears (4th and 5th) it would then again act like the 4 speed and not shift into the higher gear ranges.

In hilly or mountainous terrain where the car would start to hunt for gears because your desired speed was too high for the lower gear, but too low for the next higher gear according to the computer which is trying to maintain power output and fuel economy will generally default towards trying to hold onto speed. The excess shifting builds up heat and wears out the fluid and could burn up the clutch packs and brake bands within the automatic. In normal driving it is recommended to always drive in overdrive.

In the case of the 2012 focus that was mentioned its really a manual transmission with electronic servos actuating the gear shifts between two sets of input shafts and clutches in order to take advantage of the greater mechanical efficiency of a manual, but the simplicity to the user of the automatic's point and drive abilities.

The torque converter lock up is dependent upon engine speed, wheel speed, engine load, which is why you might mistakenly believe that by blocking the car from overdrive would affect this, as once you were in the speed range to lock up the torque converter it would unlock prior to shifing to the next lower gear.
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