Quote:
Originally Posted by Hubert Farnsworth
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.
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Yup, you've got it exactly. The overdrive button should only be used when you're climbing hills (usually heavily loaded), and the transmission keeps shifting up and down trying to maintain your speed. You can also use it on long downhills to keep the transmission shifted down and give you more engine braking.