I'm having a problem with my 1981 240D. When I drive on the highway and turn the steering wheel a bit it starts to shake badly. What could be the problem?

Does this happen with turning either direction ,, right or left ?
Does it do it more at some speeds than others ?
If yes, what are those speeds ?
Have you checked your wheel bearings lately? When did you check them last ( number of miles ago)?

It happens with both right and left.
Seems to start around 50mph
No, haven't checked wheel bearings lately.

I would start with the tires on this one. Get them checked for balance and wear pattern and go from there.

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Diesel-guy

My suggestion is to jack up first one front side and then the other, and check for looseness in the suspension - such as ball joints and steering components.

You may have worn tie rod ends, loose ball joints, etc.

Just grab the raised wheel and try to forcefully shake it side to side and top to bottom. There should not be much play.

Ken300D

Set your wheel bearings while your tires are off being checked for balance..and you may want to check the front shock absorbers also...they won't cause this... but will allow it to feel much worse.. or can get worn out if this goes on long.....