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Thread: New Fuel Gauge Circuit

  1. #1
    Senior Member Bitsyncmaster's Avatar
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    New Fuel Gauge Circuit

    After prodding from a few owners, I decided to start a fuel gauge circuit design. I’ve come to accept that car fuel gauges are inaccurate in that when you fill up, the gauge shows above the full mark and most stay at that point for many miles of driving. I then tend to full up the car when the gauge reads at the ľ full mark or a little below. I have the TankZilla which has not caused me much problem other than a bad connection one time.

    Note the fuel gauge and most other car gauge circuits use a two coil design to prevent car voltage variations from affecting the gauge reading. One coil is always powered and the other coil counteracts the first coil as it conducts current via the fuel gauge sender varying the current with its resistance. This does work very well to prevent car voltage variations changing the reading but really uses a lot of power at 333 ma. At 14.5 volts.

    So I started designing a driver for the gauge by just driving a transistor to provide an analog current sink. This method turned out to be very problematic in that it provided very unstable gauge readings. The cars voltage would change the gauge reading over 1/8 when going from 12.5 volts to 14.5 volts. Also as the transistor heated the reading would change. Then it hit me to try driving a MOSFET with a pulse width signal to change the duty cycle rather than using analog. This turns out to be the way to drive the gauge since it keeps the driver “transistor” cool and also returns the stability of the duel coil gauge and voltage changes have no effect.

    I have not decided how to make the unit adjustable for different sensors yet. I leaning to using different software and maybe a few component values to change the unit to use any sensor resistance.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  2. #2
    President, DeLorean Industries
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    I have a lot of thoughts on this. Email me when you have a chance.

  3. #3
    Senior Member Bitsyncmaster's Avatar
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    I ordered a few prototype circuit boards for my design. What I have come up with are adjustments (trim pots) to calibrate the gauge and tank sender. Things may change after testing and software development. I have two spare fuel gauges for bench testing and of course the one in my car. I however have the Zilla sender in my car.

    I'm planning on one adjustment to set the full scale of the gauge. This calibrates the gauge tolerance it self. A second adjustment calibrates the gauge empty reading. A third adjustment calibrates the sensor full value. A fourth adjustment calibrates the sensor empty adjustment. I added another adjustment which is not assigned yet for it's use. My plan is to have the FUEL light turn on at 1/8 tank level. Not sure if I should have that low fuel light flash or just stay on.

    Of course the software will "filter" the gauge reading to prevent fuel slosh from moving the reading.

    I have not even started the software yet but will have to get that going so when my boards come in I can start testing.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #4
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    When this project is done I will be awaiting to purchase for sure. The fuel gauge accuracy has always been...annoying

  5. #5
    Senior Member Bitsyncmaster's Avatar
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    It would be nice to get resistance readings of senders that are installed. The full reading is normally what the manufacture states since the float is maxed out (unless the float arm is bent). The low (empty) reading is also subject to a bent arm if it does not hit the sender stop range. I need these readings to make sure my adjustment has enough range to compensate for correction.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #6
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    Thanks to our odd shaped tanks, I've noticed the full reading varies on integrated senders depending on the position or angle of install. I've seen the DMCH GM Style sender read about 10 Ohms below full extension while hitting the tank ceiling. On tube style senders you could get similar variance as the float will rarely make it all the way up (even to 83ohms) on a full tank.

    What are you thinking for slosh reduction? My tank slosh usually acts up when less then a 1/4 tank.

    Quote Originally Posted by Bitsyncmaster View Post
    It would be nice to get resistance readings of senders that are installed. The full reading is normally what the manufacture states since the float is maxed out (unless the float arm is bent). The low (empty) reading is also subject to a bent arm if it does not hit the sender stop range. I need these readings to make sure my adjustment has enough range to compensate for correction.

  7. #7
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by dmcnc View Post
    What are you thinking for slosh reduction? My tank slosh usually acts up when less then a 1/4 tank.

    I will put a long software timing to average the sensor readings. I may think of some ways to average that work better. When you first turn the car on it will quickly show what the sensor reads and then start averaging. So if your car is parked flat you should get an accurate reading right away. Maybe I can adjust the averaging since if the car is not shut off the reading should not increase (unless you fill the tank with engine running).

    Another thought is where to get 12 volt power for the unit. If you take power from the fuel pump the unit would only work with the engine running. If you mounted the unit under the dash you could get power easy, not sure if you mount it over the tank.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #8
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    Agree on the 12v power concern although I think Houstonís just runs off the pump wiring. I would think mounting the unit inside the car is a better option if the module isnít water and heat resistant (albeit with longer wiring). An easy spot might be through the AC rubber gasket on the passenger footwell? On the other hand keeping it outside by the tank would reduce installation complexity.

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