word choice - "... that had stopped half an hour ago" VS " ... that had stopped half an hour before/earlier"

This is a sentence I read:

The engines that had stopped half an hour ago were in action again.

In my opinion, ago in the above sentence is used incorrectly. It should be replaced by before or earlier:

The engines that had stopped half an hour before were in action again.
The engines that had stopped half an hour earlier were in action again.

I think ago can only used in the past tense rather than past perfect tense.
Here is a correct sentence, making the correct use of ago as follows:

The engines that stopped half an hour ago are in action again.

Is this thinking correct?