Mondeo turbo disconnected

[Stowelly]

Hi all.

I recently brought a 56 plate ford mondeo 2l tdci, it used to be a taxi only doing short journey's around town, it has only done 155k , it doesnt tick over very well so my brother who is a mechanic gave it the once over for me and he found that the turbo had been disconnected from the actuator, so he reconnected it and we took it for a spin a lot of black soot was chucked out but after a while it cleared up, it seemed to have made a difference when it idles, we think it doesn't idle very well as it was always sitting with the engine running, what we don't understand is why was a perfectly working turbo disconnected? There is no engine management lights on and it hasn't gone into limp mode, so at a loss as to why.