Is it really possible to overfill one's transmission with fluid?

Dear Car Talk

Dear Car Talk | Mar 01, 1995

Dear Tom and Ray:

Does it harm an automatic transmission if the fluid is well above the "full" mark on the dipstick?

TOM: Apparently it does. You may notice a little warning that says something like "Caution: Do Not Overfill. Overfilling May Lead to You Making Your Mechanic's Boat Payment This Month."

RAY: But I have to admit, I've never personally seen a case where overfilling has caused any damage to an automatic transmission. I've just seen the extra fluid leak out.

TOM: I suppose what can happen is that the oil can foam and "froth," kind of like a root beer float. And if it's foamy, the fluid won't be able to adequately lubricate the transmission, and it gets damaged.

RAY: But this only a theory. We've never actually seen any evidence of this occurring. So in the interests of science, I'm conducting a little experiment as we speak. On my way home last night, I stopped and put a quart of IBC Root Beer in my brother's transmission. And I guess we'll see what happens.

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