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I suspect that originally there may have been some sort of governer or something that assisted in shifting it, but if there was, it was long gone by the time I got the truck. There was a cable-actuated lever on the side of the overdrive unit, as well as an electric solenoid. The cable was activated by a nice chrome T-handle that was mounted to the bottom of the dash and said "OVERDRIVE" on it in large text. Pushing the handle in put the transmission into a freewheeling mode where the engine would go down to idle if I lifted my foot off the accelerator when I was driving down the road. There was a pull switch on the dash that activated the solenoid. The solenoid would engage the overdrive gear. I'd have to let off the gas and turn on the solenoid simultaneously, and once the engine RPM dropped the correct amount, the overdrive gear would engage with a CLUNK. I used it all the time because it got me better gas mileage, but it was a PITA. There were two things that bugged me about it. One was that if I didn't turn on the solenoid at exactly the right time, the overdrive gear wouldn't engage, and I'd blow out the solenoid fuse. The other annoyance was that if I forgot to turn off the overdrive, and came to a stop, I couldn't disengage the overdrive until I was moving again, and it would make 1st gear more like second, so it was a bit difficult to get going again. I definitely prefer the 700R4 transmission I have in my truck now, even if it isn't the most robust transmission ever made. It only cost me $65 at the local pick-n-pull wrecking yard and many hours of my own time (I like working on my truck anyway), so I'll enjoy it while it lasts.
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