I had the same symptoms on my 420. It cost $44 and change for a set of new brushes. Works great now. Of course, I R&R'ed the starter myself. Requres a 10 mm allen socket, swivel head ratchet, wobbly adapter and three feet of extension.

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Gregga
'89 420 SEL "Moby Dick"
'30 Ford Coupe
'31 Ford Slant Windshield Town Sedan
'33 Ford Pickup
'34 Ford Pickup
'01 Honda Passport