"just like I've done Dozens of times." OP

Spraying expensive hard to diagnose electrical sensors in an engine bay with water is risky. Lots of people do it, and swear by it, but there have been countless breakdowns due to the practice. Water does not conduct electricity, but when it dissolves an ionic substance like a metal oxide, it does. If there is already a small amount of corrosion, that salt dissolves readily in water and will short out an electrical current. Even without pre existing corrosion, adding water will cause corrosion. Do you think all of the sealed electrical connections are still water tight after 24 years?
I don't have specific info for that model, but wiring harnesses can collect water and corrode. There could be a crimp done at the factory that has finally split due to corrosion, or a sensor got wet, and fried. I would be hunting around with an ohm meter. Check all the wires and connections associated with that system. I wish I could be of more help. Good luck.

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Chris
84 280sl
82 300d euro