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  1. #1
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    Default 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. #2
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Low Voltage At Fuel Pump

    It's either rusted and corroded terminals or a bad switch.
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  3. #3
    Supreme Mariner Silvertip's Avatar
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    Default 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?

  4. #4
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    Default 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.

  5. #5
    Honorary Moderator Emeritus Don S's Avatar
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    Default 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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  6. #6
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    Default 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.

  7. #7
    Supreme Mariner Silvertip's Avatar
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    Default 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.

  8. #8
    Moderator achris's Avatar
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    Default 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..........
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  9. #9
    Honorary Moderator Emeritus Don S's Avatar
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    Default 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.


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  10. #10
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    Default 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.

  11. #11
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    Default 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.
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  12. #12
    Supreme Mariner Silvertip's Avatar
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    Default 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.

  13. #13
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    Default 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...



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  14. #14
    Supreme Mariner Silvertip's Avatar
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    Default Re: Low Voltage At Fuel Pump

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

  15. #15
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Low Voltage At Fuel Pump

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


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  16. #16
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    Default Re: Low Voltage At Fuel Pump

    Here is a copy of my engine wiring diagram. Hope you can see it. http://i3.photobucket.com/albums/y76...ingdiagram.jpg Thanks,Jeff

  17. #17
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    Default Re: Low Voltage At Fuel Pump

    The simple version:the oil pressure unit has 3 screws and is plumbed into the oil gallery of the motor.key "on" no power supplied to fuel pump,"start position" power supplied to the fuel pump from starting circuit, "run position" when motor starts to run ,oil pressure trips the switch to supply power thru the maim ign. circuit.theres a "common lug" to fuel pump, the "normally closed" position to the starter circuit, "normally open" to the main ign. feed.
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  18. #18
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Low Voltage At Fuel Pump

    That's what I thought the Yellow red was doing. Yep, you need a new switch. To prove it, just put a jumper across the S and P terminals on the pressure switch, the fuel pump should run when you turn the ign. switch to start. Then, jumper between the I terminal and the P terminal on the switch. Turn the ignition switch to the run possition and the pump should run.
    Don S.


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  19. #19
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    Default Re: Low Voltage At Fuel Pump

    hello] that is a single pole double throw switch. it allows current from s to p terminals until oil pressure is high enough to move the switch. the current then flows from I to P. check your ign relay red wire input and the red/yellow out. it should read battery voltage when the key is on. if not chase that bad connection as well. but I already stated several tests to isolate the switch as bogus or not.
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  20. #20
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    Default Re: Low Voltage At Fuel Pump

    OK. I'll give it a try tomorrow. Ran out of daylight today after work. You guys know of a good source for the oil pressure switch if that turns out to be the problem? There are no Marine Power dealers in town so I rely heavily on ordering via catalog or the internet, although my local NAPA is able to get most marine parts overnight.I'll keep you guys posted and I welcome/ appreciate your suggestions.

  21. #21
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Low Voltage At Fuel Pump

    You can get a standard 2 wire oil pressure switch (normally open) from NAPA, get one in the 5 psi range and wire it like the merc.EDIT: Told you wrong, see achris's explanation below.
    Don S.


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  22. #22
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    Default Re: Low Voltage At Fuel Pump

    Thanks Don. That sounds so much more simple than my current configuration and I'll have a little more peace of mind for those longer trips offshore (more simple design = less that can go wrong). I'll let you know how it turns out. I just hope NAPA has one in stock so I can pick it up on my way home from work and give it a try.

  23. #23
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    Default Re: Low Voltage At Fuel Pump

    Don,If he does that, when the oil pressure switch closes it'll also re-engage the starter. He'd need to use a diode in the Yellow/Red wire to stop current flowing back to the starter soleniod.Jeff,Check if you have an unused terminal on the back of the starter marked 'I', if you do, that's where you run the wire from. That terminal is only live when the starter is engaged and was orginally designed to feed the ignition coil with full voltage when cranking (old points type system). As the ignition systems are all electronic now, it's unused. Merc use it to run the fuel pump during cranking. So can you.Chris..........
    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)
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  24. #24
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    Default Re: Low Voltage At Fuel Pump

    have you checked for spar? start with the basics. good luck

  25. #25
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Low Voltage At Fuel Pump

    Drat!, forgot about that yellow/red wire coming from the wrong terminal on the start solenoid. Good catch, hope Jeff sees your instructions.It needs to be hooked up like the Merc drawing I posted earlier. It shows that Purple/yellow wire coming from the I terminal on the starter solenoid.
    Don S.


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