I didn't consider that the relay could be the source of the DC voltage on pin 10 without washing out the crank signal and causing everything to quit, but I guess it could be doing that very thing.

If you jumped pins 7 and 8, start the engine, and measure voltage between pins 10 and 11, it could tell a lot. A steady 5 volts DC would indicate a wiring problem, while an AC voltage that varies with engine speed would be a pretty good indicator that the relay was at fault.

Good catch.

EDIT: 5 volts DC could also indicate another faulty component on the RPM signal line.

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1989 300 SEL that mostly works, but needs TLC