The upper wiring harness allows communication to the fuel injectors, Mass Air Sensor, coolant temperature, intake air temperature camshaft position .. etc. Depending on the values communicated to the Engine Control Module, your engine will adapt to various conditions.

I would have someone read the engine codes. Fault codes could be causing your poor performance. I.E. When the ignition control unit recognizes a specific type of failure, it sends to switch off all affected fuel injectors. Reading the fault codes could also help verify all connections on the new wiring harness are functioning properly.

I have a 93 400e also and I never get a "check engine light" even with various fault codes.

I'm sure there are others here on the forum that can further elaborate.

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Russell McMahon
Tulsa, OK
2001 CLK 320
1992 400se
1993 400e
2000 320ML