Here's an exploded view of...
Here's an exploded view of a '49-50 Ford overdrive unit. A planetary gear set (bottom of diagram) provides the overdrive ratio; 6919 is the governor, 6916 is the solenoid.
Overdrive Errors
Q. I recently read an article about older overdrive transmissions in another magazine. The author claimed that anytime the overdrive was engaged, regardless of speed, the car was freewheeling-there was no engine braking on deceleration. A reader responded explaining that when the overdrive was engaged and the vehicle achieved approximately 27 mph that the overdrive was activated and there was compression braking. The author stuck to his explanation. Who's right?
Recently I bought a '50 Ford with a 283 Chevy and the original overdrive three-speed transmission. I've got to put the engine back together (it was apart for a rebuild when I got the car) and I'm debating if I should keep the transmission once I get it running.
Jerry Petty
Via the Internet
A. I've got a '50 Mercury three-speed overdrive behind a Flathead in a Model A and it's great. Here's how it works. There's a control on the dash, when the handle is pulled out the overdrive is locked out and the transmission operates like a normal three-speed.
When the handle is pushed in the overdrive is engaged, but not activated-that doesn't happen until the car reaches approximately 27 mph. At that point a switch in the transmission mounted governor closes, which activates a solenoid that locks the planetary gear set in the transmission that provides overdrive-but that lockup doesn't occur until the driver lifts off the throttle momentarily. Under normal circumstances overdrive will work in second and third gear. Using overdrive in second in traffic works well-drop below 27 or so mph and the transmission drops into "2nd direct" when traffic speeds up, lift off the gas and it will shift into overdrive. When you want to accelerate, flooring the throttle will activate the kick-down switch that causes the transmission to shift out of overdrive, let up and it shifts back in.
As far as freewheeling is concerned, with the overdrive handle pushed in the car will free-wheel below the governor cut-in speed, above that there will be compression braking. This also means that if you park the car with the overdrive engaged, it will roll. However, if you shift into reverse, a mechanical linkage inside the transmission locks out the overdrive. Hang onto that overdrive, you'll love it.