I am certainly no guru, but here's my two cents:

I think there is a reference potentiometer inside the ETA that controls the idle. Maybe you could have disturbed it in the process of rewiring. If the ref pot goes bad, I read that they are not sold anymore. Is there any way to pull the codes?

If my ETA went bad, I would consider looking for the dealer who would sell a new one at the lowest cost. By the time you wait for someone like 4mercedes to rebuild it, and then take the chance that the rebuild is DOA (it happens), it's probably not worth the down time.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles