Origin of the South African English word "skadonk"?
Solution 1:
According to Oxford Dictionaries Online, the spelling is skedonk:
[South African informal] An old, battered car.
'Back then my hippy, varsity friends and I all piled into a few skedonks and headed for the mountains.'
ODO says the word is of unknown origin, but your suggestion that it's onomatopoeic seems reasonable.