400E oil pressure sender
 

So I replaced my oil pressure sender and the gauge still goes straight to the top and stays there as soon as I start the car:confused:

This is normal. At any rpm over about 12-1500 rpm the oil pressure gauge should peg. It will also peg at idle with a cold engine. When fully warmed up the oil pressure will probably show 1.5 - 2 bar at idle on your M119.

Rgds,
Chris W.
'95 E300D, 353K

If you're saying it stays there permanently there's something wrong, probably a bad lower engine wiring harness.

If it doesn't begin to get lower at idle (probably settling around 1.5-2bar at idle when warmed up.....and stays solid pegged at all times then it might be grounded somewhere. Also, when starting it should be more of a "rise to the top" movement than a quick flick to pegged.....quick flick = grounding issue, steady rise = normal

Jumps right to the top on start up and stays there, must be a ground problem!

Which is to say your lower engine wiring harness needs to be replaced or rebuilt.