Time clause in Reported Speech

My book (FCE Use of English) says that the tenses do not change in Reported Speech when the reported sentence contains a time clause. And then we've got an example:

[1] "The car broke down when I was driving to work," he said.

[2] He said that the car had broken down when he was driving to work.

yet in another example, it goes at follows:

[3] "I was locking the car when a traffic warden turned up," she said.

[4] She said that when she was locking the car a traffic warden turned up.

Can you explain why in the first example the tense changed in the first part of the sentence despite the time clause ("when") in the sentence? Would the rule be that we don't change tenses only after the time clause?