I believe overdrive allows you to engage and disengage the highest gear. so when overdrive is on in a 4-speed AT, you have all 4 gears. when it is off, you have only the first 3.
the reason your jeep was shifting up and down rapidly with overdrive on is that the car is at an angle on the hill that your present speed and engine RPM kept the computer right on the line of when to up/down-shift. it couldn't decide which to do, so it did both, over and over. turning off overdrive disables that fourth gear, keeping the jeep in the 3rd gear and smoothly rolling up the mountain with less abuse to your tranny.
correct me if i'm wrong, y'all. i hate spreading mis-information.
Oh, and for spichman:
Don't think overdrive would do that with your tranny, especially if it is on level ground. That sounds like a different (and very nasty) tranny problem.
• 81 J20 8400#
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