The smear-in-rain diagnosis

Free auto & mechanics tip: The smear-in-rain diagnosis (step 1 of 12 from "Wipers, Washer Fluid & Cabin Filter").

If the wipe leaves a rainbow film or a greasy arc, first wash the outside of the windshield with glass cleaner and a clean towel, then wipe the old blade edges with a damp paper towel. Road film, tree sap, and rain-repellent wax are the usual smear. If it clears, you needed glass, not parts. If the rubber is split, missing chunks, or the frame is rusted into a banana, you need blades.

A squeal on a dry windshield is often a dirty rubber or a blade left in summer sun. A chatter (hop) is often a bent arm or the blade sitting too steep — not a reason to buy the most expensive 'beam' on the shelf yet. One wiper working and one dead is a motor or linkage problem; this course will not fix that. Stop if the arm does not spring back against the glass.