Motorcycle Fuel Gauge Interface

Discussion in 'Automotive Electronics' started by MikMaher, Oct 16, 2016.

  1. MikMaher

    Thread Starter New Member

    Oct 16, 2016
    Hi Guys,
    I've been tinkering with very basic electronics for a couple of months now and need some help with a project.
    I've read the other posts on this forum about similar issues but none of the answers were really what I want to do.
    Like many others, I have a fuel gauge/sender incompatibility issue and need a circuit to resolve this. The gauge is 0-100 ohm and although I don't have a sender at the moment, I'm finding it difficult to find one that suits both the Ohm range required and the limited size hole that I would want to drill into my tank. so far it's looking like a 0-190ohm sender would be closest option.
    I've found a great gadget online that essentially changes any resistance measurement into any other by way of custom calibration. It's a bit out of my budget and I would love to try my hand at replicating it through reverse engineering.
    Luckily the site offers a picture of the circuit so finding most of the components hasn't been hard.
    So far I've got:
    2x 1k ohm resistors
    1x 220 ohm resistor
    1x 1000uf Cap
    1x LED
    1x L77805CV Voltage Reg
    1x MC7815CT Voltage Reg
    1 8way dip switch
    1x Potentiometer (unknown resistance)
    1x Resistance network (unknown resistance)

    The rest of the components IC, Potentiometer, various caps etc, I have no idea about along with the circuit required interconnecting them.
    So far I've deciphered (perhaps incorrectly) that the two Voltage regs must be to get the voltage from 12v down to 5v, presumably for the IC.
    I've attached a photo of the circuit board and the instruction manual to look over. If anyone has an easier way of doing this custom conversion procedure (ie. setting empty and full manually) then I'd love to hear your thoughts or see a circuit diagram. Please note that this is purely for my own use and reverse engineering this module is only to further my understanding of electronics and give me some hours tinkering in the shed.
    Look forward to hearing from you all,
  2. crutschow


    Mar 14, 2008
    If the gauge is designed for 0-100 ohms and the sender is 0-190 ohms then the easiest is just to add a 210 ohm resistor in parallel with (across) the sender.
    That will make the maximum sender equivalent total impedance equal to 100 ohms.
    The limitation with that simple fix is it's non-linear, thus when the sender is reading half full (85 ohms) the equivalent resistance will be 60.5 ohms, instead of the desired 50 ohms, giving a meter reading about 10% off.
    But that's usually not a big problems as it's the end points that are normally of more interest and those will be correct.
    Anyway most gas gauges I've encountered are rather nonlinear with respect to the half-full reading.

    A simulation of the total (parallel) resistance (vertical axis) versus sender resistance (horizontal axis) is shown below.

    Last edited: Oct 16, 2016
  3. MikMaher

    Thread Starter New Member

    Oct 16, 2016
    Hi Crutschow,
    Cheers for the advice but that doesn't really solve my problem. I would really like to be able to program any sender unit in just as this interface module can. I'm a bit of a perfectionist and would hate having a gauge that sort of almost reads correctly.
    Any thoughts on the PCB in the picture or other circuits that could do the same thing?
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Does your fuel guage work on resistance or voltage range,

    in other words if the sender is just varying the voltage input to the guage, it can be done with an op amp,

    if its a resistance divider then a resistor in parallel across the sender as said will do.
  5. crutschow


    Mar 14, 2008
    Sorry but I'm not.
    10% off at midpoint is close enough for me (I doubt that many vehicle systems are better than that anyway) so if you want perfection you may have to buy the device.
    I'm not much into reverse engineering someone else's work anyway. :rolleyes: