A stranger that helped us, turned out to be an auto mechanic.
Explanation:
The difference between that and which clauses are that one is used for essential information and the other is used to attach non-essential information to a sentence. The ones aimed at essential information are called defining, while the other ones are called non-essential. That is for essential data, while which is for non-essential data.
In our example, the information "helped us" is essential to the sentence so it is an essential sentence and we need a defining clause.
