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gauge repair

Discussion in 'Troubleshooting and Repair' started by mw., Jun 26, 2011.

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  1. mw.

    mw.

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    Jun 26, 2011
    I'm looking for some information to repair a gauge on a fire engine that measures the water level of the tank. It is from 1977 so I would think it would be pretty simple but I'm having problems identifying some of the components. The light marking full stays on all of the time and the 1/4 tank light never comes on. The 1/2 and 3/4 lights work.

    The lights are activated by water completing the circuit between the tank and four wires suspended in it.

    The resistors I have identified as 4.7k and 10k 10% although I'm not sure what wattage to use.

    The transistors (I think) are all the same and have the markings NSPN 4356 on them.

    One part I'm not sure what is is brown, looks lik a resistor and is in line with the 12 volt input. It has a gold band and 002 in gold.

    I have not been able find the transistors (if thats what the are).

    I would be willing to make the whole thing new using leds but would need help designing the circuit. If you can't tell I'm in over my head here but that's never stopped me before.

    Thanks for any help and I'll be trying to put some pictures up. Let me know if and more info is needed.
     
  2. mw.

    mw.

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    Jun 26, 2011
    pictures......
     

    Attached Files:

  3. mw.

    mw.

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    Jun 26, 2011
    The blue wires go to the tank to read levels, one of the yellow wires is power in, the other four in a row go out to the lights.
     
  4. kwoolsey94

    kwoolsey94

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    Feb 14, 2011
    It seems to be either a short in the actual sensors or possibly a crossed wire. You know the actual lights work and you know that it senses when its down to a quarter of a tank there is just something wrong with half and 3 quarters. Check the contact point wires and resistors for them and make sure they are wired correctly.
     
  5. mw.

    mw.

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    Jun 26, 2011
    The bulbs are good and sockets are clean, making good contact. The wires to the tank are not broken or shorted. No resistors or other components are present except on the board.

    The full light stays on all of the time,
    The 3/4 light works
    The 1/2 light works
    The 1/4 light never works

    Will the resistors need to be pulled off the board to check?
    If I could identify the components I could replicate the whole thing. I would rather do that than tear this one apart. (very expensive to replace and hard to find)
    I would think it could be built inexpensivly from radio shack or digikey parts.
     
  6. kwoolsey94

    kwoolsey94

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    Feb 14, 2011
    All resistors have a color code. ( the color bands around the resistor) to test them find a color code chart , find the resistance test the input and output and see if they match up with the resistance if not then that is what has failed. try re creating on a bread board or proto board if you wish to solder.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    Your photos are not the best, but it looks like the 4 lights are controlled by 4 totally identical circuits. I presume that there's a sensor for 1/4, 1/2, 3/4, and full.

    What I suggest you do (if you can) is have the tank between half and 3/4 full and try each wire connected to the 1/2 and 3/4 sensor. Obviously the lamp should be lit when connected to the 1/2 sensor, and not lit when connected to the 3/4 sensor. I imagine that it should also be off if the wire is not connected to anything.

    Tell us what you see, for each wire, 3 results disconnected, 1/2 and 3/4.

    Any fault should be quite easy to fix. Better photos of the underside of the board may be helpful.

    Have you ever done soldering before? Do you have a multimeter?
     
  8. mw.

    mw.

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    Jun 26, 2011
    t1893 bulbs are used for the lights which are 5 watts.
    Like I've said I looked up the resistors as 4.7k and 10k 10% but I'm not sure what wattage to use. Will 1/4 or 1/2 watt do? I'm not sure how to tell what the current ones are.
     
  9. mw.

    mw.

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    Jun 26, 2011
    Yor correct about 4 totally identical circuits and the sensors (which are only a bare wire that extend to different lengths in the tank.

    It is off of the truck now and I have tested by grounding the wires to the case individually and all at once with the results above.

    I have soldered before. not pretty but it worked. and I do have a multimeter.
     
  10. mw.

    mw.

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    Jun 26, 2011
    I hope these pictures are better. Please let me know if you need and more.
     

    Attached Files:

  11. daddles

    daddles

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    Jun 10, 2011
    Those look like simple circuits, so I'm guessing that it will be easy to fix. One thing to find out is what the load is (the lamp). If it's a standard lamp number, give us the number, as what we need is the current the lamps run at (even better, measure a working one if you can). Then that and some voltage measurements across the resistors will probably be enough to identify what's needed. Those are probably some TO-92 transistors with current ratings around 200 mA. Oh, take a picture of the other side of the board and we'll know most of the circuit.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    Yep, they're much better.

    Now if you can do the same for the other side of the board.

    And if there are any markings on the transistors, can you tell us what they are.

    I presume the blue wires are the sense wires? and they connect to ground to activate the light? Can you indicate where power is supplied to the circuit and which wires are for the bulbs.

    If we have all that then we can draw out the circuit and suggest where to test the circuit to determine what has failed.

    Since all the 4 circuits are the same, you could use the multimeter on an ohms range to test the resistance between the transistors leads. You may be able to spot some readings that appear similar for the 1/2 and 3/4 circuits, but substantially different for the 1/4 or full circuit. Note that the transistors are polarised devices, so if you are comparing readings between 2 of them you must make sure that the leads are connected the same way.
     
  13. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Test by shorting transistor pins C to E on the PCB

    Could the sensors in the tank have become broken, insulated (oxidised), or shorted?

    The corresponding transistor could be shorted, or the in-tank sensor shorted to ground. Try shorting the sensor wire to +12V.

    The corresponding transistors might have a break, or the in-tank sensor is broken or oxidised. Try gounding the sensor wire.

    No, not usually, and besides these resistors "never" break without a fight.

    Yes, it can be easily replicated with just about any parts you can find, but it's even easier to repair.
    Disconnect the tank and do comparative measurements as Steve suggested.
     

    Attached Files:

  14. mw.

    mw.

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    Jun 26, 2011
    More pictures.
    The yellow wire in the last two pictures is 12 volt +
    It connects through the resistor labled 002 with the gold band.

    The blue wires do go to the tank to read level.

    The third picture with the transistor shows it labled NSPN 4356. I can usually find something trying to look the numbers up but no luck on these.
    Not sure if they have discontinued these or changed the numbers.
     

    Attached Files:

  15. Resqueline

    Resqueline

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    Jul 31, 2009
    Didn't you notice my post and the circuit diagram I made for you??? All the information neccessary was already provided by you..
    NS means National Semiconductor and is not part of the part number btw..
     
  16. mw.

    mw.

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    Jun 26, 2011
    No I didn't:eek:
    The info on the NS helped a great deal and I think Digikey also doesn't have them. I did locate some at a place called Mouser though or I'm open to sugestions on places to order also

    Two more questions if I could.

    What type of resistors would be best? Carbon, ceramic, metal foil, metal oxide, ect.

    If I use a 12 volt led in place of the bulb are any other modifications to the circuit needed?

    I think I can get this done with the help from you. Thanks.
     
    Last edited: Jun 26, 2011
  17. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    So you're set on building a new, and not finding the existing fault? Without testing you won't know if the problem is with the sensors or the circuit.
    Anyway, like I said you can use almost any PNP transistors if you're going to use bulbs, and absolutely any if you use LED's as indicators.
    No mod's are neccessary. The resistor values & types are also absolutely non-critical.
     
  18. duke37

    duke37

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    Jan 9, 2011
    I do not know what the PN4356 is but it lies between PN4304 and PN4391 which are n channel junction transistors, these could be connected to the sensors, they are likely to be delicate.
    More powerful transistors would be necessary to drive the bulbs.
    Are all eight the same number?
     
  19. duke37

    duke37

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    Jan 9, 2011
    I have had a Google, the PN4356 is a general purpose transistor, 60V 500mA. EBC.
    I agree with Resqueline circuit
     
  20. mw.

    mw.

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    Jun 26, 2011
    The sensors are only wire that get grounded by water touching them. I have tested them for continuity, shorts, ect and they are all ok. Unless I'm missing something the problem can only be on the board.

    Mechanical things I'm good at but electronics I'm still struggling with.
    The reason I would rather make a new one is these things are extremely hard to find working or not and very expensive if you do. I would rather not risk damaging this one more when I can make another for less than $10. If I learn to build them I might can help one of our neighboring departments with their trucks also.

    Thanks for all the help. I'll get the parts ordered and update this when I get some results.
     
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