John Wayne's Son -- Cops Called 'Cause He Lacks Class!

John Wayne's son had a visit by the law Friday night ... after a fight over a horse Mercedes.

Law enforcement sources tell TMZ ... Patrick Wayne's wife, Misha Anderson called the cops at around 9 PM, after they got into a heated argument over the car. The way cops tell it ...75-year-old Patrick bought his 45ish-year-old wife a Mercedes C class which pissed her off because she wanted an E class.

Our sources say the couple said they were in the process of breaking up ... which is the weirdest part, because why would you buy the wife you're divorcing any car ... even a C class?

Nonetheless ... cops say they calmed them down and left.