What things can I check, since I replace all things?
on 2000 Jeep Cherokee

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My son drove in to town and the car was fine, he filled the up the jeep and after that it made a loud noise , would drive over 30mph. It was jumping and running really ruff. I took it to a mechanic, he said it needed a new alternator, battery, and serpintine belt. Replaced all, Still only registering 9volts, running really ruff. Light on says check guages, sounds bad, no over heating or anything.
(2) Answers
Make sure he didn't put diesel fuel in it. Have somebody else (ask around for references) diagnose the electrical system!
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