Final drive fluid test?

[Dave C.]

Doyt,
I own two cars with A/T and have made a bit of a study of the "ATF in final
drive" subject.  Both my cars have about 150k miles on them and still have
their original transmissions ... both are non-turbo.

Some people say there is no physical way for the fluid swap between AT/diff
to happen.  Based on what I know of their construction I suspect this is
true, although I have not dissasembled these units myself.  They are two
separate units... there is no seal with ATF on one side and diff fluid on
the other.  The actual problem is fluid loss. Maybe thru evaporation?  Low
fluid is what kills the differential.  Incorrect fluid level will also kill
an AT.  

The most common leak in the AT would be around the 3 inch round cover on
the passengers side of the transmission held in with a big circlip.  This
is the 2nd gear brake band piston cover.  It is sealed with an O-ring and
the heat of the exhaust hardens the o-ring over time. The piston assembly
can be removed and the O-ring replaced. It's best to replace the piston
seals at the same time since they are rubber and harden also. If the piston
seals have hardened they can affect the shifting, pressure is lost past the
seals.  This can be done with the trans in the car on the type 44's.  I've
had this leak in both of my cars and did the seal replacement myself.

If I needed to replace an AT or diff I would look seriously at a used one
or else ship it out to the rebuilder in Florida recommended by Dave Head
... assuming he's still in business.  I'd sooner pay the shipping cost than
take a chance on anyone locally ... this is after asking around and not
finding anyone who would recommend a rebuilder in Central OH.

If anyone knows of an actual incident where ATF leaked into the final drive
(or the reverse) I'd like to know about it.
----------------
Dave C.