When testing voltage to the fuel pump using a multimeter-- with the key on the 'on' position and checking for voltage.....why is there voltage and then it goes away after a couple of seconds? shouldn't the voltage stay on? shouldn't the pump keep running? what is the logic behind this?

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Ben
1987 190d 2.5Turbo

Safety system. Depending on the vehicle, if there is no ignition or oil pressure activity for three seconds + or - the fuel circuit is shut off. Prevents you pumping fuel at 3 bar all over a hot engine in the event of a crash.

Generally speaking you get just a little voltage to "prime" the fuel system and make up any pressure that has bled down, then you only get voltage again once the computer has detected cranking and/or engine start.
This keeps the fuel pump from runing unnecessarily, saving battey power, and wear on the pump (excessive wear, since it's just deadheaded against the regulator). By only operating the pump when the engine is running, there is also a safety measure to help gaurd against fire in the event something has gone terribly wrong.

MV

Good to know. Thanks.

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Ben
1987 190d 2.5Turbo