Sounds like the steering box is way too tight. There's a "high spot" for lack of a better word, in the center of the steering position to ensure that steering is more accurate when you're driving in a straight line.

Probably someone was tired of it wandering and tightened it up.

You need to use two tools simultaneously on the adjustment screw - one on the lock nut and one on the screw. Hold the screw in position and loosen the lock nut. Then, back out the screw a little and hold it in position while re-tightening the lock nut. Spin the wheel back and forth across center a couple times to see if you still feel the thump. If you do, repeat the process until you can't feel it bind when the wheel crosses center.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250