Gas tank level monitor

Discussion in 'General Electronics Chat' started by kyle8ton, Dec 2, 2008.

  1. kyle8ton

    Thread Starter New Member

    Dec 2, 2008
    4
    0
    Hey guys first off I think this is really cool that you spend some of your time to help out rookies like myself...

    What I am trying to do is monitor the fuel level in my truck. I would like to build a simple circuit that will turn on a fuel pump in an auxiliary fuel tank to fill up the primary tank and shut off the aux fuel pump when the primary tank is full. Most vehicle sending units are simple variable resistors that change a voltage or resistance to ground. I am struggling with the specific components to make this work. maybe a voltage comparator that energized a relay at an adjustable value and released it at another adjustable value? but the circuit would have to be powered separately by the vehicles 12vdc

    Any ideas?
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    kyle8ton,

    I'll start this off with a few questions...what happens if:

    ...the transfer pump does not turning on?
    ...the auxiliary tank is empty when the main tank needs fuel?
    ...the transfer pump fails to turn off when the main tank is full?

    Why not "Y" the fuel lines from the two tanks, with check valves between each pump and the "Y". Then select which pump is supplying fuel to the engine. I have this in a '93 Ford F150. The selection switch on the dash selects both the pump and the sensor that is being used. Your envisioned circuit could just switch tanks when needed. Your circuit could just sense the low tank and switch pumps/sensors as needed.

    Or am I missing something important?

    Ken
     
  3. kyle8ton

    Thread Starter New Member

    Dec 2, 2008
    4
    0
    You are correct if it was that easy the AUX tank is a 50 gal in bed toolbox type with no sending unit. This truck is a 2004 dodge diesel with the pump/strainer/sending unit in the tank and all of the lines are OEM. I want to be least invasive as possible, cutting into OEM lines with all their new connectors and seals seems like a pain to bring back to OEM when I sell the truck. Also the aux pump has the volume but not the pressure needed to feed the injector pump. Besides that this way I have an excuse to come up with a handy device that could automatically handle this situation. there would be an "empty" float switch installed on the aux tank to prevent running the pump dry.

    The aux pump would kick on when the primary tank is half empty (or full :) ) and stop when it reaches full (via oem sending unit voltage or resistance i have not interrogated the truck yet...) this process would continue until the "empty" limit switch opened killing power to the pump. The order to fill up the truck would be primary tank first aux tank second. Sorry if I have been rambling...
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Since the secondary tank is above the primary, why not just use a gravity feed and forget about an electrical/electronic solution?

    Running fuel through a pump will heat the fuel, and require energy to move it.

    Gravity doesn't cost you a thing, and has been reported to be quite reliable - for the last few thousand years, at least.
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,672
    899
    If you are set on an electronic solution, I would consider an capacitance method. Is it a metal tank?

    John
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Can't the vehicle's gas sender be used - just pick off the voltage with a high impedance buffer?
     
  7. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    You had and go and say that, now your sure to jinks it :rolleyes:
     
  8. kyle8ton

    Thread Starter New Member

    Dec 2, 2008
    4
    0
    yes gravity could work but again the AUX tank is 50 gal the main tank is 30 gal both are vented (by emissions law),,,, fuel spills everywhere... I guess you could only fill the aux tank to 30 gal then have a ball valve that you open manually when the primary tank is empty and there you go. I was also thinking of a float switch that had a mercury switch in it that could close the circuit when it was low and open it when it was full... yes the Aux tank is steel but the primary tank is plastic (most new cars are safety thing...)
     
  9. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    "I guess you could only fill the aux tank to 30 gal then have a ball valve that you open manually when the primary tank is empty and there"

    That really sounds like the most reliable and safe means to proceed. I love building electronics things, but when it comes to mixing hydrocarbon molecules and electrons, I think a simple non-electrical solution would be a better solution. ;)
     
  10. kyle8ton

    Thread Starter New Member

    Dec 2, 2008
    4
    0

    Could you ellaborate on this thought? I think this is what I was envisoning, This circuit will just controll the existing pumps and will not be in contact with any fuel.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The sender already in place has wires out to the instrument panel. You should be able to grab the hot one and monitor it. It should be possible to tell from it when the tank is low enough to want to trip a valve to fill it from the bed tank, and also when the fuel level is high enough to stop filling.

    Grabbing that signal line with a DVM should demonstrate if that is correct. Read voltage when close to empty and again when full.
     
  12. TanTJ

    Member

    Mar 6, 2008
    21
    0
    Beenthere, you are absolutely correct.

    Would it be possible to send the outputs from the fuel level sensors from both tanks to the inputs on a comparator. When the fuel level in the main tank is lower than the secondary tank, the comparator would detect this and energize a relay to turn on the transfer fuel pump thereby transferring fuel to the main tank.
     
Loading...