I have a Mercury Capri 1991 convertible. The car was owned by someone
else briefly before I got it, but who ever had it, probably put motor
oil in the place where transmission oil should go. My probelms started
with the transmission when I tried to change the transmission fluid,
then the transmission started acting radically. The car wouldn't go in
reverse, it losted power while driving on the freeway.
I caulked this to transmission damge from the change of the types of
fluids. Before I changed the transmission fluid, it was viscous, and
black , like motor oil.

I add more fluid, mirculously I was able to get the car to go in
reverse without having do anything to it, but the transmission wouldn't
engage well at low speeds, but started driving ok at high speeds like
on the highway.
I was stingy putting the new fluid in cause I didn't want to overfill
the transmission.
Recently, To make a long short short. I had to change one of the belts
that go to the alternator, simply cause I drove the car and saw the
battery come on, luckily I arrived home to time as radiator fluid
started venting from the hood of the car. The next day, I discovered
the belt ( alternator ,waterpump, engine) was missing. I seeked to
replace the belt. With an original belt as a template, ( but somewhat
damaged) I replace the belt with a new one, however it was extremely
difficult to install the belt as I guess it was a very tight fit, plus
there was no room for adjustment on the alternator. The Alternator was
set at its most minumin adjustment. In order to get the belt on the
roller, I ended up accidentally rotating one of the rollers located
near the engine, oil pan. Turning that roller made al the differance in
installing that belt, it helped give me enough torque to place the belt
on the roller. Now that belt seems a bit tight ( but the car doesn't
make that squealing noise that a car makes when a belt is slipping on a
roller, that car used to be real annoying to drive that way).

But now something as changed,with way the tranmission works. The
tranmission didn't seem to work 100% on that car, giving poor
performance at low speeds ( city driving), but good performance at high
speeds as the automatic transmission wouldn't engage very well. Now the
car won't run well at high speeds now. I added some transmission fluid
as maybe the car was driving like there wasnt enough fluid in it, the
car showed some improvement, but still drives on the freeway poorly as
it refuses most to engage at high speeds making it so that it runs at
high rpms, so that I have to slow down. The engine sounds like its
working harder. Why did this happened? Could it be caused somehow by by
my repairs, the new tigher alternator belt, or did I somehow upset the
timing by having to turn that roller to get that belt on, thusly
affecting the transmission performance , or could the car have suffer
sever engine damage when it overheated ( because of the missing belt),
it didnt seem like it. The car used to drive well on the highway. I
could travel out the city 50+ or so ( with a questionable transmission)
miles, now I dare now travel outside the city. Can anyone help?