Final answer:
To convert the sentence into passive voice, the object comes first followed by the passive form of the verb and the performer of the action; thus, it becomes 'The old car was being repaired by John when I arrived.'
Step-by-step explanation:
When converting the sentence "John was repairing the old car when I arrived" into the passive voice, the focus shifts from the subject performing the action (John) to the action being performed on the object (the old car). The sentence in passive voice would be: "The old car was being repaired by John when I arrived." This maintains the same tense and meaning as the original sentence but emphasizes the object that is receiving the action.