4matic rear differential has hydraulic lines going to it?
 

OK,

I can only guess that this limited slip differential has hydraulic lines going to it to lock it up. I've completely disassembled my wife's old 4matic, and when I got to the differential, I saw this hydraulic line going to it.

What is it for? Does it detect slippage and engage the front wheels, or does it add pressure to the clutch plates and further engage the limited slip of the wheels?

Those would be for the ASD clutches. ASD includes the warning triangle in the speedo.

Sixto
87 300D

When the 4-M detects wheel slip it locks the t-case. If slip continues it will then additionally lock the rear diff.

so that hydraulic line is to lock the differnetial. Any idea how many psi it takes? Since I have pulled the differential and will be offering it for sale I would like to explain why it has hydraulic lines going to it.

Is it still limited slip without the hydraulics, or are the hydraulics needed for it to function?