Fuel Gauge problem

Discussion in 'General Electronics Chat' started by Lanz, Apr 19, 2010.

  1. Lanz

    Thread Starter Active Member

    Oct 30, 2008
    I open the gauge on my car dashboard just to see how it was built n works.
    I try to turn the needle with my finger and found rpm needle are very easy to spin n loose.
    But the fuel needle are diff.its quite hard n not loose.It cannot be resetted to zero.
    But Rpm needle can drop to zero.
    After fix back everything to normal...i went to gas station to fill up the fuel tank.
    I fill up full tank,first it shows 3/4 full...so i tought it will turn to full in a while.
    But for my surprise..its still in 3/4 scale even the tank was full enough.
    So i dono how to fix it back.
    Another thing..the low fuel indicator has been malfunctioned...i had checked the light..its still ok but the indicator isnt working.
    So..i think want to do a separate circuit with LED indicator for that n dono how to do it.
    Hope u guys can help me.
  2. SgtWookie


    Jul 17, 2007
    Let's see - it WAS working, right?

    And then you messed with it, and now it is broken.

    Taking things apart might be useful to see how they work, but it's a good idea to only do that with things that you won't need anymore.

    We have no idea even what kind of "car" you have. How can we possibly help you to fix it when we don't even know what kind of car it is?

    Repair of automotive instruments is really a bit beyond the scope of the forum. Unless you can quickly find a connector that you left unplugged, a blown fuse, or something like that - your best bet is probably to have someone experienced (like a mechanic) to replace/repair it for you.
  3. Lanz

    Thread Starter Active Member

    Oct 30, 2008
    Yeah.Im just curious abt how the gauge works...thats why did like that [​IMG]
    The low fuel indicator has been malfunctioned a long time ago.
    Im using Toyota Corolla.
    Last edited: Apr 20, 2010
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    RPM uses a very sensitive meter, and they a very light to touch
    But fuel gauge may have a heavy coil and it responds to the sensor in the tank.
    Either the meter is faulty or the sensor
  5. Lanz

    Thread Starter Active Member

    Oct 30, 2008

    Yeah...i can see RPM meter has something like spring to make it return to zero.
    I think as my car quite old,it has float switch instead of sensor.
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    Many speedometers and tachometers use weighted needled to measure accurately.

    Many fuel gauges use a float in the tank that changes a POT.
    The Gas gauge is then basically a volt-meter. When the gas tank is full, the pot is adjusted so most or all of the vcc is passing. As the level drops, more resistance is applied to the signal lowering the voltage.

    If you have a real old car, you may have a needle that is mechanically attached to the float or even one that uses vacuum pressure. So if you fiddled with it, you likely moved it out of calibration.

    It wont pass inspection like that. You may be interested in purchasing a OEM replacement gauge that can go on your dash, or even replace the dash meters with a digital plug-in replacement.

    Also, If you touched the odometer, you can be charged with fraud if you try to sell the car. They are very sensitive meters and are very easy to tell if they have been tampered with. So have a mechanic replace them before you sell it.
  7. someonesdad

    Senior Member

    Jul 7, 2009
    Lanz, I had to laugh when I read your post -- not at your misfortune, but because I suspect all of us scientists/engineers/technicians have similar experiences. We are curious about the world and how it works. My parents told me I took a clock apart when I was little (I don't remember doing this) and of course couldn't put it back together. I've also broken my fair share of things that were working just because I was curious about them. It's embarrassing to have to admit this to others -- they almost always say something like "If it ain't broke, don't fix it" or ask "Why in the hell did you mess with it?". But they don't have the need to understand how things work.

    If you pay close attention to the things you disassemble and understand their principles, at some point in life you may fix more things than you break. :) You'll also develop experience that helps you decide when to proceed and when to take it to a pro.
  8. retched

    AAC Fanatic!

    Dec 5, 2009
    Someone has a tag-line on this forum that says something along those lines..

    "If it ain't broke, fix it until it is."

    I have played that role MANY times.
  9. t06afre

    AAC Fanatic!

    May 11, 2009
    I think the correct diagnose is the "The Knack"
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Some automotive fuel and temperature gauges use a bimetal strip heated by a resistor to move the needle, rather than a conventional moving coil or moving iron meter.

    If it's that type, the internal mechanism has probably been bent or broken.

    It's easy to recognise that type as the gauge rises slowly to the correct reading over a few seconds when the ignition is turned on.
  11. Lanz

    Thread Starter Active Member

    Oct 30, 2008
    First,im not a kid.Second,im curious how gauge works.Third,im not a pro.Thats all.

    The gauge isnt spoilt but the reading not accurate..thats all.
    I think its moving coil type gauge n definately not the bimetal strip type.
  12. gerty

    AAC Fanatic!

    Aug 30, 2007
    I'm not familiar with your particular car, but most fuel gauges require the fuel tank to be properly grounded. If you have a high resistance ground to your tank it will affect the accuracy.