Catalyzes the NAD(+)-dependent oxidation of L-carnitine to 3-dehydrocarnitine. carnitine + NAD(+) = 3-dehydrocarnitine + H(+) + NADH Amine and polyamine metabolism; carnitine metabolism. Homodimer. In the N-terminal section; belongs to the 3-hydroxyacyl-CoA dehydrogenase family. L-carnitine dehydrogenase subfamily.