1983 Mercedes Benz 280sel

Electrical problem
1983 Mercedes Benz 280sel 6 cyl Two Wheel Drive Automatic 500000 miles

My car is a 1983 Mercedes Benz saloon Model 126 (280 SE Automatic). I suspect it is the fuel pump relay or the fuel pump itself. Usually there is a soft buzzing noise as I turn the ignition key and the engine fires instantly. Lately, sometimes there is no buzz and the engine wont't start unless an assistant puts some pressure down on the relay. How do I check whether the relay or the pump itself is faulty. I have checked the connections for the relay and they are OK
April 5, 2009.

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