Here’s what happened to Vin Diesel, the subject of so many “The action hero for the New Millennium” cover stories back when “Pitch Black,” “The Fast and the Furious” and “xXx” were hot. Vin never learned the Eastwood Rule.

Back in the day, nobody made more action films than Clint. And not all of them were masterpieces. But Eastwood, legend has it, would take his script and mark out big chunks of his dialogue. It wasn’t just that he could act without words, or that words were never his strong suit. He knew bad dialogue is a serious buzz kill for a guy carving out a piece of movie cool for himself.

Diesel, all muscles and ex-bouncer beefiness, wants to talk. He let all the hype get to his big, bald head and he figured he was a better actor than he is. He talked himself right out of A-pictures and straight into B-movies like “Babylon A.D.”

Just how bad things have gotten is evident in “Babylon,” a gritty sci-fi road picture in the “Children of Men” mold, with Diesel playing a mercenary hired to escort a waif from a convent in Mongolia to a crowded, advertising-riddled New York of the future. Diesel talks. A lot. He tries to do the cynical mercenary-style narration.

“Save the planet? What for? And from what? Ourselves?”

He’s good at hard-boiled. Just in small doses.

“Karl! I thought you were in Sudan, killin’ babies!”

Diesel isn’t the whole problem with this “road picture” that gives up the road far too soon, this futuristic dystopia that doesn’t have enough of “Beyond Thunderdome’s” wit, enough of “Blade Runner’s” style or grit. No wonder director Mathieu Kassovitz (“Gothika”) has all but disowned it.

Michelle Yeoh is Rebeka, a kung-fu nun who helps Toorop, Diesel’s merc, take the moody Aurora (Melanie Thierry) across borders in a post-nuclear/post global-warming world of refugees, guns and Russian tanks rolling hither and yon. More should have been made of this arduous journey, but the film shows signs of serious cutting that the director had nothing to do with.

Toorop was hired by an apparently dubbed Gerard Depardieu, playing Gorsky, a mob boss. A religious cult led by the cynical Charlotte Rampling wants the girl. So do others, stuntmen/hunters who bounce around in that popular “parkour” style of leaping tall buildings in many, many bounds.

The movie has a nice look – gray, rusted metal exposed to Siberian and Bering Strait snow. What it lacks is coherence and a compelling reason for us to take this often-abrupt journey with Aurora, Toorop and Sister Rebeka.