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Coolant Sending Unit

Discussion in 'Early CJ5 and CJ6 Tech' started by tripilio, Oct 18, 2021.

  1. Oct 24, 2021
    timgr

    timgr We stand on the shoulders of giants. 2021 Sponsor 2020 Sponsor

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    Resistance is resistance, whether it's your tester or the sender. Current, voltage, resistance - nothing more. How much does the voltage drop at the sender when you connect the gauge wire? How much does the voltage drop when you connect your tester and the gauge works? Should be the same, if the resistance and the path to ground is the same. Has to be your ground from the sender to the block. Or you are comparing performance through the Jeep wiring to performance on the bench. Where are you grounding your tester? Try putting a clip lead from the sender body to somewhere else on the engine that's a good ground.

    What do you mean, "outside?" With the cluster out of the dash?
     
    Last edited: Oct 24, 2021
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  2. Oct 24, 2021
    tripilio

    tripilio Proud American! 2020 Sponsor

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    Ok. Let me see if I can answer. The resistance between the body of the sender and the negative post if the battery is about 1 ohm or less. Outside means cluster fully assembled and tested outside of the dash. I used initially a dying car battery that produced about 5.5 volts to verify that it worked and it was not my power supply. I am not sure but I believe the voltage drops to about 3.2 vdc. Don't know if that is too much and I need to look for another voltage source. I bought a cheapo that might not be the right one.
    My tester when using it outside with the old battery got grounded to the negative post. Same as on the vehicle.
    Screenshot_20211024-162632.jpg
     
  3. Oct 24, 2021
    Twin2

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    Caution .12 Volts on gauges . without instrument cluster grounded
    will produce that magic smoke . or is that why your showing the external voltage regulator
     
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  4. Oct 24, 2021
    timgr

    timgr We stand on the shoulders of giants. 2021 Sponsor 2020 Sponsor

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    Use a 7805. Simple. Works.
     
  5. Oct 24, 2021
    tripilio

    tripilio Proud American! 2020 Sponsor

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    I guess I will have to fabricate a voltage regulator with 7805. Is there anyone who sells a finished product?
     
  6. Oct 24, 2021
    timgr

    timgr We stand on the shoulders of giants. 2021 Sponsor 2020 Sponsor

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    It's three parts. The 7805 and two capacitors. And some wire. You can build it with one capacitor.

    Also, if it works on the bench, it should work in your Jeep. I think you must be overlooking something simple. That little switching supply is overkill, but not wrong as such.
     
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  7. Oct 24, 2021
    tripilio

    tripilio Proud American! 2020 Sponsor

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    I will remove the bridge between the two gauges and try again!
     
  8. Oct 24, 2021
    timgr

    timgr We stand on the shoulders of giants. 2021 Sponsor 2020 Sponsor

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  9. Oct 24, 2021
    Ol Fogie

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    That is what I did, simple fix and cheap too.
     
  10. Oct 24, 2021
    timgr

    timgr We stand on the shoulders of giants. 2021 Sponsor 2020 Sponsor

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    Do you understand this diagram?

    upload_2021-10-24_18-53-20.png
    The dotted lines represent measurements that Jeep added to this diagram to make it less clear. The bridge sends 5V from the fuel gauge to the temperature gauge. When you put 5V into this bridge, and disconnect the green wire from the ignition switch, it feeds both the fuel and temperature gauges with 5V.
     
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  11. Oct 24, 2021
    tripilio

    tripilio Proud American! 2020 Sponsor

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    Yes, I understand it (I think). I left the green 12vdc out and placed the 5vdc lead in one of the bridged posts. I will start checking continuity on all the wires. Something should turn up as a culprit.
     
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  12. Oct 24, 2021
    tripilio

    tripilio Proud American! 2020 Sponsor

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    To isolate each gauge, maybe there is an underlying issue with one or the other I am not seeing. Part of troubleshooting is to eliminate variables. Right now am at a loss. Since you think the power supply I am using is fine I don't know what to say. I will buy the 7805 and try it.
     
  13. Oct 24, 2021
    timgr

    timgr We stand on the shoulders of giants. 2021 Sponsor 2020 Sponsor

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    When you are stuck, measure what you can. Change what you can. Get more data. I would measure every terminal with the complete assembly in the Jeep. As I mentioned, measure the voltage with the sender connected, and disconnected. If you 5V supply is in there, it should be ok with and without the sender connected.

    As the resistance changes, the current changes too. The gauge deflects based on the current. Have you tried using your tester under the hood, with the cluster in the dash?

    Sometimes it's helpful to quit, have a drink or soda or dinner (your choice), and come back to it in the morning.
     
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  14. Oct 24, 2021
    tripilio

    tripilio Proud American! 2020 Sponsor

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    That is probably the smartest thing (and I think you are frickin smart) you have ever said, sr! I'll drink to that.
     
    Last edited: Oct 25, 2021
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  15. Oct 25, 2021
    tripilio

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    Sooo, today after thinking about how to simplify my troubleshooting I removed the wire from the sender and applied it to ground. The needle moved as expected. This is the second sending unit I have installed that does not work as expected, one fron O'Reilly's and this one from Advanced. Could it be? What could be wrong?
     
    Last edited: Oct 26, 2021
  16. Nov 10, 2021
    Ol Fogie

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    Humm, engine to battery ground clean and tight, ground to body/cluster clean? probably not the problem though of every thing else is working properly. No sealer or tape on sending unit threads? If so clean it ALL off. I have had problems with those little L/shaped boots that plug onto the sending unit getting streached and not making good connection.
     
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  17. Nov 10, 2021
    Walt Couch

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    Did you make sure the hole for the sender is not plugged and not allowing the water to reach it?
     
  18. Nov 10, 2021
    tripilio

    tripilio Proud American! 2020 Sponsor

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    I might have found the cause of (some) of my headaches. Ol Fogie, all the ground points are ok, and since grounding the wire coming from the gauge made the needle work I narrowed it down to the sender. After dealing with it I realized I was using an adapter bushing that kept the sender way too much outside of the water passage. I will correct the issue and report back. Walt, the hole is not plugged as far as I can tell. I will check later on, but maybe the sender that much away from the fluid is the cause.
     
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