On Mon, 21 Aug 1995 John Mardy wrote: > My TR250 hated to back up. It felt like the parking brake was on. > The manual says that "internal damage" can result from trying to back > up while the OD is engaged. John, Yes, here's what happens. When going forward, the planetary gear set in the overdrive has the option of "overdriving" the output shaft of the OD unit. This will occur when you engage the solenoid and the sun gear ceases to turn. Then the engine drives the planetary gears, and the annulus then turns faster than the planetary gears driving it. At this point you're overdriving. Now, as a safety backup mechanism, to guarantee that the car will always be in 1:1 (direct) or overdrive when going forward, the annulus is connected to the sun gear via a sprag (one way) clutch behind the planetary gear set. This basically acts like a ratchet. When overdrive is engaged, the output shaft spins faster than the input shaft, and the clutch does nothing. When OD is disengaged, the driving force from the input shaft 'locks' the sprag clutch, and the output shaft is forced to turn AT LEAST as fast as the input shaft. When reverse is selected, it's important that the overdrive does not try to engage. Doing so would make the the annulus try to overrun the sprag clutch, but this time in the reverse direction. No can do. When the planetary gear set and the sprag clutch fight, the sprag clutch wins. So the cone clutch which controls the planetary gear set is forced to spin against the OD case, which is of course stationary. The faster you go in reverse, the harder the hydraulic pressure forces the spinning cone clutch against the stationary OD case. As the cone clutch wears, OD engagement becomes less forceful. When the cone clutch is worn out, the OD will not engage. So, should you worry? No not really. If your OD works, be happy and think about other things. If it doesn't engage with quite the fortitude of days past, you'll know why, and next time TRF has a special on A-type cone clutches, you'll say to yourself, "Hey, there's something I need". Yes you can put it in yourself, and you can remove the OD from the car without taking the transmission out. It's a short weekend job. Greg
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