Q: have 5v at fuel pump but 12v at and though the pump relay were did the volts go? on 1998 Mercedes-Benz ML320

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no fuel at engine, good ground at pump conect. bad voltage at fuel conect.
(1) Answer
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With the connection disconnected at the pump what voltage do you have? Not sure with Mercedes some cars use a two stage pump and pulse voltage to the fuel pump so it doesn't work so hard when maximum fuel pressure is not needed, and then gives full voltage when under vehicle is under full load.
Look on Youtube I saw a good video on "testing voltage drop".
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