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I turn the key to the on position and it constantly buzzes, an obviously malfunctioning fuel pump.
Any pump runs (buzzes?) as long as it gets voltage from it's fuel control circuitry. The fact that your '76 control circuit is erroneously sending voltage to run the pump continuously with the engine off doesn't mean the pump is malfunctioning at all. It's just doing what it's "told".
I can't speak to how your '75 164 works. I can only describe what your '76 should do, based on my experience, Bosch manuals, and wiring diagrams. It's true that some later K-jet cars do buzz the pumps briefly, due to their type of fuel relay. But your car does not use that type of relay.
However, after reviewing the wiring (it's been quite some time), I can see where there could be a very brief voltage pulse to the pump when turning the Key On. In your case, I'd guess that you may have a bad Main FI relay, or open safety contacts on the airflow meter. Either one would allow constant voltage to the pump with the Key On.
My intent was to save you from needless pump expense — and to let you know about a flawed Safety circuit that your mechanic is content to overlook. It seems you've already decided the pump needs replacing, and aren't concerned about the Safety issue, so I'll ring off here.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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