same thing happened to me a couple of weeks back. it was a stripped gear. manual tells you not to reset the trip odomoeter when vehicle in motion - it happens.

you should pull the gauge and diy the stripped gear or send it to a chop. a diy job will not cost much.

a shop will charge you about $80 to $120 to fix the gauge and abou $60 for r & r.

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1986 500sl