An MB dealer diagnosed this problem. They said both fans operate normally.

On the way home however, the temperature gauge was pointing about 100 degrees C (I was driving slowly in residential area with air conditioner on). For curiosity, I stopped my car and opened the hood. Only one of the auxiliary fans was rotating. The other was completely still.

Is this normal?

At some stage, only one fan rotates? When the temperature becomes higher, then both fans rotate?

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.