air conditioning fan works only intermittenly on 2003 Cadillac DeVille DTS

Why does air conditioning fan work intermittenly? IT comes on when right door slams or when going across a rough railroad track

Asked by for the 2003 Cadillac DeVille DTS
1 answer
The blower motor brushes are worn and the slamming door causes them to make contact, at least temporarily. Just been through this exact scenario.