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Low Voltage At Fuel Pump

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  • Low Voltage At Fuel Pump

    I'm restoring a Blackfin with a 454 Marine Power inboard. I have done all the routine maintenance for winterization and when I tried to start her up she would not start. I have traced the culprit to be the fuel pump (no gas to carb). It is an electric fuel pump that pumps great with 12 volts applied with a jumper from the battery. It is wired through an oil pressure switch that contains three wires.With the key in the ON position I have the following voltages at the oil pressure switch:Red/ yellow: 12.6 voltsYellow/ red: 0Red: 0With the key in the start position I have the following voltages:Red/ yellow: 10.5+ VoltsYellow/ Red: 10.6+ VoltsRed (at fuel pump): 3-4 VoltsMy gut tells me that the pressure switch is most likely the culprit, but I have limited experience with inboards. According to the Marine Power manual it says to first check for 12 volts at the fuel pump (which I do not have, I only have 4 volts at best with the key in the start position) and then check the pressure switch next. The level of oil on the dipstick is also within normal level.Where do I go from here? Will a faulty oil pressure switch cause these problems or do I have other problems?


  • #2
    Re: Low Voltage At Fuel Pump

    It's either rusted and corroded terminals or a bad switch.
    Don S.

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    Please, no PM's (Private Messages) regarding boat/engine problems.
    That is what the forums are for.
    Only forum/moderator issues will be answered in PM's.

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    • #3
      Re: Low Voltage At Fuel Pump

      The oil pressure switch contacts come into play only after the engine builds oil pressure. During startup, the pump must receive power from a bypass relay or other bypass circuit. When oil pressure is sufficient to close the switch, the pump receives power through the oil pressure switch. If the engine dies in the event of an accident the fuel pump is shut off automatically to prevent it from pumping fuel onto the possible resulting fire. 10.5 and 10.6 volts at the oil pressure switch sounds low. You are losing over 2 volts across the switch. Does your instrument panel have an oil pressure light or analog gauge? If its a gauge is it electric or fed by a small oil line?

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      • #4
        Re: Low Voltage At Fuel Pump

        My oil pressure gauge is electric.I can't really account for the voltage drop I was getting unless I have worn the batteries down from all the testing that I've done today. I have traced each wire individually that is involved with the fuel pump and pressure switch, no splices and all connections are corrosion free.

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        • #5
          Re: Low Voltage At Fuel Pump

          The oil pressure gauge has nothing to do with the system at all. The oil pressure switch is seperate from the gauge system and is only an open or closed switch.I'm not sure how Marine Power does it, but Mercruiser comes off the starter solenoid during the cranking process and completely bypasses the oil pressure switch. Then when the oil pressure builds the pressure closes the switch and operates the pump from the ignition (purple) wire.I've also noted the Marine Powers wiring is always lacking. Had one that depended on the gas in the carb to get it started long enough so the oil pressure would build and run the pump......had to redo that for the customer because he could never get it running.
          Don S.

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          Please, no PM's (Private Messages) regarding boat/engine problems.
          That is what the forums are for.
          Only forum/moderator issues will be answered in PM's.

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          • #6
            Re: Low Voltage At Fuel Pump

            I think that Marine Power must be fairly similar to the diagram.The yellow/ red wire comes from the starter solenoid and the starter relay and goes to the pressure switch.The red/ yellow wire comes from the relay between the coil and starter relay.The red wire comes from the pressure switch. There are only three wires going to the pressure switch, one of which is the red wire that goes to the fuel pump.Thanks for the input, but this has me stumped. I am only getting 4 volts to the fuel pump with the key in the start position which won't make the fuel pump work, with 12 volts on the pump it gushes fuel.

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            • #7
              Re: Low Voltage At Fuel Pump

              He has an electric oil pressure gauge and he indicated that the fuel pump is indeed wired through that sender. A simple on/off switch will not make the oil pressure gauge work. That's why I asked if he had an idiot light or analog gauge. Oil pressure operates a pressure transducer in the oil pressure sender to provide the variable voltage to the gauge in relation to oil pressure. A simple on/off switch would either have the gauge pegged or zero'ed.

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              • #8
                Re: Low Voltage At Fuel Pump

                Here's a possible solution you could think about.Run a wire from the fuel pump (red side) to a push on switch on the dash. Run a positive to the other side of that switch. The switch should be one that is only 'ON' while it's held down (called 'momentary') when you want to start the engine, just hold the switch in for 10 seconds (to pump fuel up to the carb), release the switch and crank the engine. This removes the need for relays and bypasses without defeating the safety issues of the oil pressure switch.0.02Chris..........
                The world takes on a whole new perspective when viewed from 100' below...



                1972 Bertram 'Bahia Mar' 20
                2006 Mercruiser 4.3MPI (0W617679) w/Alpha 1 Gen II (0W829301)
                **
                Please don't send service questions in private messages. That's what the forum is for. Messages of thanks, always accepted.
                **
                Member of the month - April 2013. http://www.iboats.com/blog/achris-member-of-the-month/

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                • #9
                  Re: Low Voltage At Fuel Pump

                  Upinsmoke, there are 2 seperate units, a gauge sender, and a oil pressure switch, the sender is probably up near the distributor, or there is a tee by the switch.Jeff, does the red wire hook to the pressure switch on the same terminal as the yellow red wire? It should, otherwise there is no way to get power to the pump while cranking.Regarding your 3 volts, check the red wire with an ohm meter, from the OP switch to the fuel pump. It should be almost 0, if there is high resistance, there could be a break in the wire, you need to find the cause of the problem and fix it correctly, not just start adding switches and wires to bypass everyting just to make it work.
                  Don S.

                  sigpic

                  Please, no PM's (Private Messages) regarding boat/engine problems.
                  That is what the forums are for.
                  Only forum/moderator issues will be answered in PM's.

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                  • #10
                    Re: Low Voltage At Fuel Pump

                    Don, the red wire does not hook to the same terminal as the yellow/ red wire, although it seems like it should. I was wondering if the switch somehow was supposed to direct this voltage from the yellow/ red wire to the red wire until the oil pressure switch opened allowing the red/ yellow wire to take over while using the oil pressure switch.

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                    • #11
                      Re: Low Voltage At Fuel Pump

                      hello marine power did use some single pole double throw oil pressure switches. they will use one contact normally closed for cranking and the other normally open for running. the normally open side will close with oil pressure. if the diaphram inside leaks oil will contaminate the contacts and you get low voltage. but it goes back to what DonS said, you have a faulty switch or corroded terminals at the switch/switch wiring. as a quick test jump the yellow/red to red and retest the pump with the key in start position. then jump the red/yellow and retest with the key on. if both tests pass the switch is bad. if they dont keep moving aft in the harness until you find the break. you can also do a voltage drop from yelow/red and red/yellow to red and see if the switch contacts are bad. its not complicated just basic electronics. the test method for me is the easiest and quickest unless I am on a time clock.
                      no tech questions by PM, they wont get answered.
                      you have to be trusted by the people that you lie to .

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                      • #12
                        Re: Low Voltage At Fuel Pump

                        Agreed, but combination senders are in use -- hence my question since he may have been reading the voltage drop caused by the gauge transducer. He's losing over 2 Volts between the battery and the sender (problem #1) and another 6 - 7 Volts through the sender (problem #2). Make sure 12V is available at the start relay input. If not continue back to the starter. In start mode check the start relay output. If less than 12V is present you have a bad start relay. If good, repeat that test at the input to the switch. If more voltage drop is shown the connections are bad between the relay and switch. If ok to this point, remove the sender. Apply 6 PSI or more of air pressure and measure resistance. If low resistance or no resistance is measured, the sender is bad.

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                        • #13
                          Re: Low Voltage At Fuel Pump

                          sorry Upinsmoke, if, when pressure is applied, the resistance is low or zero, then the (oil pressure) switch (item #5 in Don S picture) is GOOD! It's designed to allow current to flow to the pump when the oil pressure is up.
                          The world takes on a whole new perspective when viewed from 100' below...



                          1972 Bertram 'Bahia Mar' 20
                          2006 Mercruiser 4.3MPI (0W617679) w/Alpha 1 Gen II (0W829301)
                          **
                          Please don't send service questions in private messages. That's what the forum is for. Messages of thanks, always accepted.
                          **
                          Member of the month - April 2013. http://www.iboats.com/blog/achris-member-of-the-month/

                          Comment



                          • #14
                            Re: Low Voltage At Fuel Pump

                            Agreed -- zero is on, its the low resistance that causes the voltage drop.

                            Comment



                            • #15
                              Re: Low Voltage At Fuel Pump

                              High resistance causes voltage drop, very low or zero resistance cause no voltage drop.
                              Don S.

                              sigpic

                              Please, no PM's (Private Messages) regarding boat/engine problems.
                              That is what the forums are for.
                              Only forum/moderator issues will be answered in PM's.

                              Comment

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