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I have noticed that on many cars using hydraulic power steering, if you fully turn the steering wheel to the left or to the right, you hear a sound coming from somewhere in the hydraulic power steering system. On electric power steering systems, however, you don't hear such a sound.

What is making this sound?

Is it harmful to fully turn the steering wheel to the end?

marked as duplicate by Pᴀᴜʟsᴛᴇʀ2, Community Dec 26 '15 at 10:51

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Hydraulic power steering systems use pressure to assist in turning the wheels. The more assist is needed the more pressure is provided. When the wheels reach the end of travel the maximum pressure is applied. When this occurs a special pressure relieve valve activates to make sure that the pressure does not exceed safe limits. This is the sound you hear. This in no way damages the steering system. The pump is capable of producing much more pressure and the pressure relief valve makes sure it does not damage itself.

With electric steering systems the system is aware where in the travel it is. When the lock condition is reached the system stops providing assist.

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