Fuel pressure switch question.

Discussion in 'Electrical Systems' started by nnyerges, Feb 7, 2012.

  1. nnyerges
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    nnyerges Nicolas62

    I am organizing and arranging all the wiring of my boat twin inboard diesel engines instrument panel and I have little doubt about the switched +12VDC power of the instruments.

    They are currently connected as shown in the diagram INS1.PNG. Obviously, the “fuel pressure switch” is only to feed the instruments main +12Vdc. The questions are:

    1. Is that “fuel pressure switch” is only installed, to feed the instruments?
    2. Is not enough to power the instruments form the +12Vdc of the ING.SW (See diagram INS2.PNG)?
    3. Maybe I have mistaken identified the complete route of the “fuel pressure switch”, but its goes somewhere else into the engine?

    Thanks in advance

    Note: The diagrams show the starboard side only. The port side is identical.
     

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  2. gonzo
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    gonzo Senior Member

    Are you sure it is a switch and not a fuel pressure sender?
     
  3. nnyerges
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    nnyerges Nicolas62

    Originally there is no fuel pressure gauge instrument on board, so, it most be a switch. By the other hand, if it will be a sender, how come is connected to the instruments +12Vdc switchen input?, so its a switch.
     
  4. CDK
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    CDK retired engineer

    Feeding the instruments DC+ from a fuel pressure switch is weird, exotic and probably wrong. Installing a fuel pressure gauge is even weirder.

    The only reason for a fuel pressure switch with external wiring could be safety, in which case it should be wired to a buzzer or a warning light. With the engine operating, a loss of fuel pressure could mean the tank is empty, the filter is clogged or there is a fuel leak (assuming there is a feed pump in or near the tank).
     
    Last edited: Feb 8, 2012
  5. michael pierzga
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    michael pierzga Senior Member

    Is this an electric fuel lift pump ???
     
  6. nnyerges
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    nnyerges Nicolas62

    ??????? :confused: ???????.
    I think you don’t mean to say that.. or ...you should try to feed the instruments with AC, and probably works better for you

    Is not weird. Is there original from manufacturer and I will going to suppose that the only purpose is for safety:

    Feeding the instruments true the "fuel pressure switch", guarantees you that batteries don’t drain in case you forget to turn off the ING.KEY at motor stopped. If no fuel, like tank empty or mechanical fuel pump damage, or any other reason, engine will stop, the engine oil low pressure alarm, alternator idiot light and gear/drive oil pressure alarm will do the job and there is no reason in having the instruments on (like feeding directly from the ING.SW), because there is nothing to sense from the engines.

    Is not a pump, is a “Fuel Pressure Switch” (it close when fuel pressure is low). There is a difference between engine SENSORS and ALARM SWITCHES. SENSOR will give gauges a analog or digital signal to the instrument in order to read pressure, temp, rpm, etc. The ALARMS SWITCH like the "fuel pressure switch", only change the state ON/OFF at certain pressure and temp.
     
  7. michael pierzga
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    michael pierzga Senior Member

    Ive never operated an engine with one. Oil pressure yes...fuel pressure ?
     
  8. nnyerges
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    nnyerges Nicolas62

    Here is a picture of the Fuel pressure switch
     

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  9. michael pierzga
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    michael pierzga Senior Member

    From the picture I cant tell what on part of the fuel system this sensor is located. Low pressure or high pressure?

    When the low pressure switch trips what is it telling you ? Fuel filters plugged ?


    What engine is it ? Normally a sensor is a NO or NC controlling the PLUS side. This output wire will supply PLUS to the buzzzer.

    http://www.westerbeke.com/OnlineManuals/13945_Rev_8.pdf


    Scroll to page 236 and you will see the wiring diagram for a fuel pressure switch on the low pressure side of the fuel system
     
  10. CDK
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    CDK retired engineer

    What type of engine are we talking about? Only if it was designed to be cable operated can the key switch be excluded from engine control.

    Is there no audible alarm to warn you?
     
  11. nnyerges
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    nnyerges Nicolas62

    Here is another picture that may be can help you. I dont know if is in the Low or High pressure side...

    Im sure that is Normally open, like the one in your diagram (close when fuel pressure is present). Also in your diagram the fuel pressure switch, feeds the instruments like in my case. Also feeds the optional alarm and the regulator (not in my case).

    The engines are two old detroit diesel 671N

    CDK, if you observe the diagram of Michel in page 236, you can notice that the fuel pressure switch is DC+ feeded directly from de circuit breaker also.
     

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  12. CDK
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    CDK retired engineer

    OK nnyerges, it is not wrong, just weird and exotic. Such solutions arise when people familiar with mechanical constructions are confronted with an electrical problem. An electrician/electronics designer would have used the oil pressure switch or the alternator's DF+ signal and a small relay to power the instruments, or -even better- a fuel cut off solenoid to stop the engine if the key switch is turned off.

    I see another issue with the present circuit: if the engine circuit breaker opens, there is no way to stop the engine!
     

  13. Frosty

    Frosty Previous Member

    It looks like a water indicator, I have them in my Yanmar fuel filters. A light will illuminate if I have water in the fuel. Most cars have them.

    Either that or a blockage indicator, indicating either a dirty filter or a blockage.
     
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