The 126 odometer is gear driven. Likely a gear has slipped into the wrong position.

This link may be helpful. How to repair your odometer for pocket change

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.