The diode is probably not your problem - the coil inside the relay is (the electromagnet that makes the relay contact move).

A bad relay coil will do exactly what redlaser said above the way it's wired.

Since the coil isn't moving the relay contact anymore, it seems that would be the culprit.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.