Refilling the trans

The shop manual says to refill the MOD trans using the fill plug and it will just flow into the overdrive unit. Since I’ve completely drained the trans and OD, and it’s still on the bench, is there a better way? In other words, should I fill/partially fill the OD unit as well to prime it, or does the oil just flow that easily between the 2 units?

In principle it is like in this drawing: when you fill trans, oil flows over the “wall” also to the OD. But not back, so if you drain trans, OD will not drain. No need to prime OD, it is full of oil if trans is full.

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Thanks. That’s what I thought, but I drained everything… trans & OD. The trans had been sitting since at least the 1970s, so I got as much out as possible, opened all the plugs, cleaned the filters, etc. That’s why I needed to know if it would be better to add some to the OD, too.

Is there any reason that I can’t remove one of the switches on top of the trans and fill it from there? I know the hole is smaller, but it seems like an easier task than trying to pour a liquid through a hole in the side of the trans!

Yes, you can fill it from top. No need to remove switches, this bolt (arrow) is good for filling. Remember to open the original filling hole on side so you know the correct level of oil. And keep transmission level.

Thanks! Mine doesn’t seem to have those top fills. No trouble to remove the switch though.