Hi,
English for this topic can be rather idiomatic. Here are some general comments.
Which is more correct?
A broken car or a brokendown car.
'Broken' commonly refers to something that is now in more than one piece.
eg a broken window eg a broken plate
'Break down' is used for vehicles primarily.
eg My car broke down yesterday eg my car has broken down.eg I can see a broken down car eg I had a breakdown yesterday on the highway.
It can also 'A broken washing machine or a brokendown washing machine.
Very often, for machinery we say that 'it is not working'.
eg my watch is not working, but not my watch has broken down' and not 'my watch has broken.
Clive