I'm acquiring an '86 190E 2.3 from a friend, with a very puzzling problem. As he describes it, the car is completely dead unless you manually jump the starter solenoid, at which point it'll start and everything comes to life.

Any advice where to start to a W201 newb?

__________________
'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".

Ground wire? Can be rotten inside.

__________________
MB 300CE Turbo '88
MB 300E '86

Possible. I believe before jumping the solenoid, everything is dead in the car - like the battery is disconnected.

__________________
'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".

ground cable chassis to engine rotten?

2-0 ground issue. Good advice, guys!

__________________
'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".