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Fuel tank sender resistance range conversion

Discussion in 'General Electronics Discussion' started by autowolf80, Dec 8, 2014.

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  1. autowolf80


    Dec 8, 2014
    Hello folks newb here and I am trying to find out if there is a way to convert a sender to a different ohm sweep. I have a fuel level sender that runs a ohm sweep from 0-90 0 as empty and 90 as full. The pcm of the conversion is looking for a sweep of 40-240 to read and send the correct signal to the gauge cluster to read correctly. Updating the fuel tank and sender unit is out of the budget atm. I was just wondering if there was an alternate way to get the current sender's ohm sweep up to a 40-240. Forgive me if I am not being clear enough still learning electronics.
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    Aug 27, 2013
    Hello autowolf80...Welcome to EP!

    First, some general advice about posting inquiries to technical forums....while you are intimately familiar with your project, we have to glean everything we know about your project from your post, and while some details may seem obvious to you, they are not obvious to us unless you give us some information about them....for instance I Ass-U-Me from the context you are attempting to "patch" a fuel sending unit in an automotive fuel tank by intercepting the signal between the sending unit and the fuel gauge/On board Computer thus "tricking" the fuel gauge/on board computer into displaying the correct fuel level.....but I am not 100% certain this is actually the case. Furthermore, it would be really helpful to know if the cause of the problem was a bad sending unit, or if the sending unit was simply not part of the OEM system. PCM may be a common term in automotive electronics, (and it may even be common in general electronics, but it doesn't mean anything to me)....I would assume if the fuel sending unit were connected to an analog fuel gauge that the fuel gauge would be a relatively simple current/volt meter and that if the sending unit were connected to a uControler or other on-board computer that there would be a simple ADC (Analog to Digital Converter) performing the task.....

    All that being said, without knowing if the sending unit is faulty or simply not the correct one, and not knowing if the fuel gauge is analog or digital or if it is current based or voltage based means that I can't offer any viable suggestions....perhaps if there is someone more familiar with automotive parlance AND automotive fuel sending units are "standardized" to the extent the information you provided is enough to go on then they might be able to make helpful suggestions, but from a strictly electronics point-of-view you will need to provide a lot more information.....BUT, if the sending unit is faulty, it is highly improbable it will be accurate regardless of how the signal is "read"....if it is simply the "wrong sending unit" and operating normally, then it might be possible to design/build an interface to make it "play nice" with your existing system....but I cannot imagine the time and effort required to design/build the interface would prove economically viable when compared to simply replacing the sending unit.....

    As an alternative, you might consider simply using the odometer as a fuel indicator....if you know roughly how many MPG the vehicle gets AND you know how much fuel the tank holds then it is pretty simple to keeping a running inventory of the fuel remaining/miles to go.....I use the odometer almost exclusively in my my car and truck both, I re-set the trip odometer when I fill-up and I know it is time for fuel @ [email protected] 350 miles I know I am very close to walking....there is a little deviation if all the miles are "highway miles" with no A/C, but not much...perhaps 10 extra miles before the Summer, if all the miles are around town then I might fill up @ 320 miles....the only real difference between the Truck and the Car is how many gallons it takes to fill up, LOL...

    Good Luck!

  3. autowolf80


    Dec 8, 2014
    Thank you for the reply and as stated I didn't know what all I needed to put out there for information. Here is some more information. The sending unit works fine, but that is the only gauge that currently works in the vehicle as it has a newer engine/harness and computer swapped into it.

    I want to put the dash cluster out of the donor car into the older dash. All gauges in the donor cluster will work by simply wiring up the serial data output and oil switch output from the powertrain control unit(engine computer). The problem comes in for the signal from the fuel level sending unit as it is a older unit and runs 0-90 ohms for its to gauge signal. The newer PCM needs a signal from 40-240 ohms into the pcm to send the correct information on fuel level to the gauges. There is a logic board in the gauge cluster that separates and sends the serial data information to the speedometer, tachometer, fuel level, oil pressure gauges.

    I added a quick diagram of the system as I understand it hope that helps some. I am really just wanted to explore if there is a way to step up the signal from the sender to the pcm that isn't going to be exceedingly difficult.

    Attached Files:

  4. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    Aug 27, 2013
    PCM ==> Powertrain Control Module....or so google tells me :) So I am going to Ass-U-Me that the gauges are electronic NOT analog....since you state that the gauges all receive serial data from the PCM, this implies that each gauge has either a dedicated serial line, or a unique serial address....most likely the latter.

    There are four approaches you might consider, each with its own difficulties:

    1) If the PCM is programable, then it might be possible to simply alter the PCM firmware....this approach assumes you have access to the PCM source code firmware, that the PCM firmware is updatable AND that you have the requisite coding skills to alter the code and update the PCM....I am going to assume that at least one of the conditions is false....

    2) If you know the serial protocol used by the fuel gauge AND have information about the serial data corresponding to the various levels of the display you could isolate the fuel gauge from the PCM and use something like an Arduino module (or virtually and stand-alone uC with a built-in ADC) to read the fuel sending unit and update the Fuel Gauge....this approach assumes you have intimate knowledge about the serial protocol used by the fuel gauge, a good understanding of electronics and the skills to write the firmware.....Again, I am going to assume at least one of the conditions is false...

    3) If you know the voltage curve of the sending unit to the PCM that the PCM is expecting then you could use a micro-controller with an ADC to read the existing sending unit and translate that to a DAC output corresponding to what the PCM is expecting.....This approach would again require good coding skills and some fairly decent electronics skills....

    4) Finally, with some really good analog electronics skills you might be able to devise an analog interface that used the 0 to 90 ohm sending unit to mimic the output of the 40-240 ohm sending unit....this has the potential to be both the easiest AND the most difficult would still require knowing what voltage range the PCM was expecting....for instance 2.5V to 5V, or 2V to 10V etc, etc....once that was determined it would be a matter of biasing an Amplifier to translate a reading from the sending unit to the voltage expected by the PCM for the corresponding fuel level....the basic design is not really that complicated, getting the details right could prove extremely tedious/time consuming.....

    So, before you get too excited about any particular approach, I should state that ALL 4 approaches are going to take a considerable time investment to implement.....I would guess, with the right skills and all of the requisite information available you should budget a minimum of 100 hours to design/debugging/testing before there would be anything ready to test in the vehicle....actualizing a device ready for implementation in the vehicle could take as much or more time if you observe safety and reliability protocols. I cannot see this endeavor being a good use of time for a one-off hack....I would guess that purchasing/installing a new sending unit would be considerably faster/ might even be able to replace the entire fuel tank with one from a salvage yard (with the proper sending unit already installed) for less time/money....

    I should add, if this type of problem is common in the automotive industry, there may be an existing product that is designed to be a "universal solution" would be easy enough to design a micro-controller based interface that would solve this quickly and easily....but justifying development would require a fairly large market.....I would think >>1k/year....and honestly I can't imagine it really comes up that frequently.....but I have been wrong before, lol. You might also research the particular PCM and see if there are existing "hacks" for it....if there are, it may be as simple as updating the firmware on the PCM with existing code which would preclude any coding skills on your part....You might even look into PCMs in general and see if there is a "semi-universal replacement" that comes with an easy to use programming interface....I really don't know squat about the automotive industry, but electronic design in general tends toward a single piece of hardware that can be programmed for multiple situations...if this is the case with PCMs, then there might be an "easy" answer waiting in some hot-rod hacking forum....

    Good Luck!

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi and welcome to Electronics Point :)

    I'm jumping in here a bit late I'm afraid. If you're still looking for an answer, can you tell us a bit more about the sender and the PCM input please.

    Does the 0~90Ω sender have one wire or two? In other words, is the return path through the chassis?

    What input connections does the PCM have for the sender? Are there two connections? If so, can you check continuity to the 0V rail (chassis connection) of the PCM to see whether either of the PCM's sender input connections is grounded?

    It should be possible to make a circuit to allow a 0~90Ω sender to work with a 40~240Ω input, but you can't simply amplify and offset resistance; I will need to know how the PCM is measuring the resistance so I can design a circuit to create a signal that it can deal with.
  6. Bluejets


    Oct 5, 2014
    Here's a mechanical/electronic workaround.
    Take your 0-90R and feed with a 5V supply and feed result into Arduino (or Picaxe) analog input.
    Output this to a servo 0-255 to give 60 degree output and mechanically drive a 1K pot.
    40R about 11 degrees of pot travel and 240R about 65 degrees of pot travel.(65 minus 11 approx 55 degrees so adjust servo drive accordingly) o_O
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