E420 just cut off in mid drive and refused to start. The symptoms pointed to a faulty K40 relay that is known to give up. The following film describes some of the steps for looking up if the relay is fixable or not.
pn: A2105400072
1996 - 1997 E320
1997 E420
1998 - 2000 SLK230
K40 pinouts --
A.1 Air pump switched ground input
A.2 Power output relay K40k9
MOTOR
A.3 Power output relay K40k9
MOTOR
A.4 Air pump voltage supply
B.1,2 Circuit 30 main power input
B.3 Power output to fuel
pump
B.4 Same as pins B.1,2
B.5 Main ground
C.1-3 Pins not used
C.4,5 ABS switched power output
D.1 Horn contact signal input
D.2 Main power input Ignition on
D.3 Main power input Ignition on
E.1 Fanfare horns power supply
E.2 Pin not used
E.3 Fuel pump activation signal input
E.4 Power output to engine
MOTOR
E.6 Same as pin A.2