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Gas Gauge Issue

Discussion in 'Technical Chat' started by Johnny Monsoon, Feb 15, 2009.

  1. Feb 15, 2009 at 7:36 AM
    #1
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    Well, I managed to find myself in a goofy situation. I pulled my dash apart and opened up the gauge cluster to get a dead bug out that had somehow gotten in there about 10 years ago. When I disconnected the cluster I discovered an issue: When you reconnect, it essentially 'resets' the gauge, thinking it's at empty, so now it reads 1/4 tank high and is empty when still reading 1/4 tank.

    I had hoped it was a mechanical slide that I could just physically move the needle, but no. It's capacitive.

    So...

    Anyone know the best/quickest method to deal with this? I know I'll have to find the correct points and drain off the capacitor, but I don't know which points they are. Any help here?

    Also, is there an easier access point to do this (maybe from the sender?) so I won't have to pull the dashboard all apart again?

    Thanks in advance for your help!
     
  2. Feb 15, 2009 at 9:22 AM
    #2
    sooner07

    sooner07 1/2 man 1/2 amazing

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    Could you not just empty the tank with a siphon, then unplug the cluster? You would then put the siphoned fuel back in the tank.
     
  3. Feb 15, 2009 at 9:34 AM
    #3
    mws4ua

    mws4ua I'll try being nicer if you try being smarter.

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    +1

    You might not be able to roll into the gas station at exactly empty, but could get a LOT closer than 1/4 tank.

    I've never taken my dash apart. If it's too complicated to do at the gas station, just go fill a 3 or 5 gal tank w/ gas then drive around until you're on fumes, go back to the house to re-set the dash, then fill w/ 5gal and head to the gas station.
     
  4. Feb 15, 2009 at 9:43 AM
    #4
    EnolaGaia

    EnolaGaia Well-Known Member

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    What model year Tacoma?

    I don't think you have a 'reset' problem. I suspect something's disconnected / loose / shorted as a result of your cluster removal / reinstallation.

    Your fuel gauge isn't 'dead' - it's stuck either because:

    (a) there's a constant voltage signal no matter what the fuel level is (wiring fault; maybe a blown fuel sender)

    (b) the needle is physically jammed against the clear face plate (or something else).

    Option (b) isn't far-fetched at all. People who've removed the face plate to install gauge face overlays have often had fuel or temp gauge (or speedometer) needles bind afterward ...
     
  5. Feb 15, 2009 at 2:05 PM
    #5
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    It isn't stuck at all, and it reads fine. It just 'reset' from a false zero position (I had a quarter of a tank in the truck, and when I unplugged it, then plugged it back in, the needle went 1/4 tank above full).

    I should probably mention that I'm an electronics tech; I know what's wrong, I'm just looking for a simple solution that doesn't involve tearing the dash apart again.
     
  6. Feb 15, 2009 at 2:10 PM
    #6
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    Nope, that's not the way the unit works. The sender and the actual gauge itself both have a part in telling you how much fuel is in the tank. The problem arose when I pulled the dash apart, disconnecting the gauge from the sender unit, electrically. When I did this, the gauge itself maintained its current reading (as it should so that you can turn the truck off and it'll remember it's last state). When I plugged it back in, the gauge didn't know it wasn't zeroized, so it just assumed it was already reading zero (empty) and moved up by the amount the sender unit at the tank read, which was 1/4 tank. This means that the needle simply reads 1/4 high (almost exactly) at any given point. It isn't really a big deal, since I know this, but I'd rather have it correct.

    The only way to do this is to discharge the capacitor at the dash, but I'm not eager to get back into the dash that far; it's just a time consuming process. If I could disconnect from a more accessible point and discharge the capacitor there, I'd be golden.

    I can't have been the only guy to experience this...
     
  7. Feb 15, 2009 at 2:22 PM
    #7
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    Doesn't work because the gauge at the dash itself is responsible for the error; there's no way around that aside from discharging that capacitor.
     
  8. Feb 18, 2009 at 7:07 AM
    #8
    JDCPA

    JDCPA Well-Known Member

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    Will the capcitor discharge if you disconnect the battery and leave it off for several hours?
     
  9. Feb 18, 2009 at 7:08 AM
    #9
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    Depends on the type of capacitor and how it is in circuit. It may be a few minutes; it may be a few years.
     
  10. Feb 22, 2009 at 5:42 PM
    #10
    bass mechanic

    bass mechanic Well-Known Member

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    i would fill the tank then wait about 5 minutes with the engine running to make sure its reading full and then install the needle at the full mark or slightly above since thats where most gauges read when full.

    obviously, what you have done was install the needle on the needle shaft at the wrong position.
     
  11. Feb 23, 2009 at 2:01 PM
    #11
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    Obviously, what you have failed to do is read the post at all.

    The needle never came off the instrument at all; the dash instruments never came apart, just the dash cluster was removed from the dashboard, intact.

    The problem is an electronic one. There's a sender at the tank that tells the needle what it should be indicating. There's a capacitive circuit at the needle itself (in the dash cluster) to retain the last state information. When I de-coupled the instrument cluster, I had to remove the sender input from the gas gauge. When that happened, the needle remembered where it was at the last fill. So, there sat the actual gauge, doing a good job as it was supposed to at reading 1/4 tank full. When I reconnected the dash pod the tank sender told the needle that it had a quarter of a tank of gas. So, the needle responded accordingly and moved the needle up a quarter of a tank higher than it had been sitting. Thus, my gauge reads 1/4 higher than it should.
     
  12. Feb 23, 2009 at 6:12 PM
    #12
    bass mechanic

    bass mechanic Well-Known Member

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    i have actually replaced the sending unit on my 96 and am quite familiar with the circut using a capacitor and a resistor to slow the movement of the needle so that it does not react to the constant movement of fuel in the tank. i am not aware that the capicator maintains its voltage because the resistance of the meter (needle) would drain voltage from the cap over time, i suspect the needle has no return spring therefore mechanically it stays at the same reading and has no connection to the cap while the ignition is off, i am unaware of any meter that draws 0 current out of a cap to maintain its position and i am aware of no capicator that doesnt have a small amount of leakage either. so i am not 100% sure of your theroy, not saying its not possible however.

    that said and considering the circuit is nothing more than a simple series circut with the sending unit, the resistor and the cap that changes voltage across the cap and the cap likely drains or looses voltage through the meter circut as a high value resistive load.

    i suspect that in newer tacoma they use a stepper motor, so the circut that drives the motor knows wether to step it forward or backward depending on voltage. this means it does not have a reference point to count steps from. that said, if you either move the needle without removing from the shaft or remove the needle and reinstall it in a new position does exactly the same thing. i think in your case you need to fill the tank drive home and with the ignition off move the pointer to the full position. if it is a stepper motor it should reference the new position, if not then i would suspect you have in some way altered the voltage reading to the meter itself.
    i do know that with my sending unit the potentiometer on the sending unit in the tank goes to ground. if memory serves me it should be reading 12 volts on the sending wire with a full tank. you could probably trace the circut in the cluster across the cap and read the same 12 volts across the cap. that will tell you if your getting the signal your supposed to. you could install a POT on the same sending unit wire to ground and vary the resistance to verify the operation of the gauge when you think you have the problem sorted out. this way you dont need to drive around thinking you have a full tank for 50 miles before you see any movement.

    from what your telling me i think your theroy is correct, somehow it remembers the position (im guessing its a stepper motor) and untill you removed the power it knew how many steps from empty it was, so it advanced the needle the same number of steps based on its initial reading.
    i think you can get there in 1 of 2 ways, either remove the cluster as you did previously and set the needle at just below the empty mark. or without removing the wireharness from the cluster put the needle in the full position with a full tank. i would use a full tank in either case because its a lot easier and logical to test its position knowing its totally full verses finding empty the hard way.
    i think if you have to disconnect the wire harness again from the gauges on a full tank and setting the needle at below empty as most cars are as long as the needle returns to just above full on a full tank your good to go. if not you have to repeat the process moving the needle to back to empty in a new spot to get it to go to the expected full position. i suspect it will move the same number of steps from where you started each time you connect and power it up.

    also FYI the low fuel light uses a thermistor, it gets warm when the fuel is low because there is no gas touching it to keep it cool, as it gets warm due to its own internal resistance it drops in resistance value, it essentially goes into thermal runaway creating its own heat. this turns the light on via a voltage regulator so that when the light comes on its full brightness verses on the older trucks and cars will glow as it heats up and resistance lowers and voltage increases across the internal load resistor.

    and yea i have a pretty good background in electronics too :)
     
  13. Feb 23, 2009 at 7:17 PM
    #13
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    I'm an electronics tech and work on aircraft avionics (among a lot of other things that aren't related to avionics that live inside the aircraft) for a living, and have done so for the last 16 years now. I hold a degree in electronics and another in IT systems.

    For your sake I'm going to spare you the electronics lessons. I'll just recommend that you do some more study if it is a field of interest.

    Any capacitor will eventually discharge, but the fact that there may be a resistor in the circuit has nothing to do with how or why it discharges; heck, the wiring of the circuit itself is resistive. Until you ground a circuit, you don't have a difference of potential, and a difference of potential is required to move electrons. If you don't move electrons, you won't discharge a capacitor. In fact, the electrical system has no ground; it just has differences of potential.

    However, we're belaboring points that I have an extremely firm grasp of. What I was after in this thread was a simpler means of discharging that capacitor without tearing into the dashboard. There doesn't seem to be a more readily available disconnect or feed-thru available, so it looks like I'll just have to suck it up and make some time to get back into it.
     
  14. Feb 23, 2009 at 8:13 PM
    #14
    bass mechanic

    bass mechanic Well-Known Member

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    i think you missed my point, the fuel gauge is a meter, and it reads voltage across the capicator, since a meter is a coil of wire and a magnet it has its own resistance, it may be a high value but some leakage will occur, to assume a capicator will not show some discharge through the meter itself over time would be a miracle. i am sure a man with a resume like yours would easily see that.
    in any case i dont want to give you an electronics lesson either as i am sure with your background and the confidence you show with it trying to explain this any further to you would be a waste of typing. i simply wanted to offer you a solution to your problem and a few things to consider trying. since your already the expert on the subject im sure if it was a simple as discharging the capacitor you would have already done that!
    so if you dont want the advice then so be it, i offered my suggestions and apparently you want to be "right" more than you want to take the advise you asked for, which if i understand was a solution to your problem,
    my fuel gauge works perfectly, so to spend any more time trying to help you sounds like a waste.
    ive got 27 years in electronics and choose another field i am sure you would have to agree that since the first week of electronics covers simple circuits like this it doesnt matter what your resume says, you either know the material or you dont capicators and resistors work the same way no matter the level of your education.
    good luck
     
  15. Feb 24, 2009 at 5:40 PM
    #15
    Johnny Monsoon

    Johnny Monsoon [OP] Member

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    LOL, if anyone put a resistor into this circuit it would be to PREVENT capacitor discharge! :rofl:

    'smoothing' the needle reading would be accomplished with the charge time of the capacitor vs the output of the charging circuit (the sending unit in this case).

    I can see why you changed jobs. ;)
     
  16. Feb 25, 2009 at 6:52 AM
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    JDCPA

    JDCPA Well-Known Member

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    You guys both need to chill out. This is not a contest to see who is the biggest and baddest.
     
  17. Feb 25, 2009 at 4:43 PM
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    sliverworm

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    It seems the only way to fix this Johnny, is to take the dash apart again.
     
  18. Feb 25, 2009 at 4:55 PM
    #18
    bwood_usmc

    bwood_usmc Wiskey Tango Foxtrot....

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    Good lord ppl.... its the flux capacitor located next to the muffler bearings right under the blinker fluid.... i dont see how yall couldnt figure that out
     
  19. Feb 25, 2009 at 5:46 PM
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    Monkeysuncle

    Monkeysuncle My Cat's breath Smells like Cat Food

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    NERD FIGHT NERD FIGHT!!!!!!! This is a "Friendly" forum, someone can and probably will help you out. And, it's not a pissing contest either. You both seem to have some experience with electronics, work it out, put your heads together. Two minds are better than none.:) Ha Ha Nerd fight.
     
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