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My son's 1984 760T with M46/OD recently started dropping out of overdrive in hot weather. The problem is worn overdrive actuatin piston seals. The original seals had a blue Teflon sealing ring, and the seals give trouble with age. The upgraded new pistons use a nice fat rubber o-ring which gives better service. I ordered two new pistons (part number 6814454), and two new o-rings (part number 6814455-9), and the gasket (part number 380618-9) between the OD and the transmission. Once I had the parts I jacked the rear of car up, ,started the engine, and shifted through the gears and into OD. Once into OD I stuck the clutch in and hit the brakes. This procedure unloads the pressure on the transmission output shaft splines so the OD will slide of of the transmission. You can also shift into reverse and very firmly let the clutch out and achieve the same thing.
The picture below shows where I like to put my jack stands.
The next step is to remove the driveshaft and transmission crossmember. The transmission will drop slightly, and this helps lots to get at the eight 7/16" nuts which hold the OD to the transmission. I use a fine tooth 1/4" drive ratchet and 7/16" socket to remove the eight 7/16" nuts. This will nicely let you get the even the top two nuts. The ratchet and socket might not work on a 240, but they do the job fine on a 700 series. Be sure to clean all of the dirt you can from the joint between the OD and transmission. It is easier to clean it off before it gets on to the inside parts.
Sometimes a little gently tapping on the output flange helps get the OD off of the transmission. If you didn't unload the OD from the transmission output shaft, you are going to be wishing you had.
The new and old pistons can be seen below. They fit tighter than the old pistons, even before the seals go into the bore.
The OD with new pistons and gaskets is shown below.
Clean the back of the transmission off and remove all traces of the old gasket. It will look like this when you are ready to slide the overdrive back on.
It might take several tries to get the pump collar (for lack of a better description) around the cam on the output shaft. Slide the OD back on and start re-assembly.
Once done, re-fill with type F ATF or synthetic motor oil. This overdrive works perfect again.
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john
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