Testing water temp sending unit or gauge JD 425
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Thread: Testing water temp sending unit or gauge JD 425

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    Testing water temp sending unit or gauge JD 425

    Is there a way to test the sending unit that powers the temperature gauge on my 425? The temp needle on my gauge never moves. Or is there a way to test the gauge? Thanks

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    JD4044M's Avatar
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    First I would make sure it is connected to the wiring jack. If it is then un/hook it and see what the gauge does. When you say it does not move do you mean that literally no movement at all even when warmed up good? A manual would tell the ohm specs on the switch you could check it that way to see if it is open or shorted a simple ohm meter can do that.

    Just found this;
    Re: JD 455 temp gauge not moving
    That is correct. If you ground the wire that goes from the temperature sensor to the temperature gauge, the gauge should read "hot". Not sure about your machine, but on mine that wire is yellow/blue.

    If grounding that wire gives you a "hot" reading, then the sensor is bad.

    If grounding that wire does not give you a "hot" reading, then test the gauge. (1) One pin on the gauge should have 12V battery voltage (key on engine off). (2) Another pin should be connected to ground (near zero resistance to ground) always. If either of these fails, check the wiring.

    (3) The other pin goes to the sending unit. If you ground that pin while the other two wires are connected (whether or not this pin is connected), then you should see a reading of "hot". If so, check the wiring from that pin back to the sending unit. If not, the gauge is bad.

    If you have an ohm-meter, you can measure the resistance from the sensor output to ground. The resistance should go down as the temperature goes up ("negative temperature coefficient").
    Last edited by JD4044M; 05-23-2019 at 03:30 PM.
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    sennister's Avatar
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    I don't have a scale for what it should read but they are basically a variable resistor where the resistance changes based on temp. So even if you don't know that _____ Ohms of resistance = ____F simply looking for a change would be a starting point.

    Disconnect the sending unit and put your multimeter on Ohms (may look like a horseshoe) and take a reading with the engine cold. Run it for a bit to warm it up and repeat. Is the resistance reading the same? If so it is shot. If it changed it is a sign that it may not be the problem.
    Last edited by sennister; 05-23-2019 at 08:15 PM.
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    Testing water temp sending unit or gauge JD 425

    Often that’s a sign of a failed dash.

    My tractor forum has a few threads about fixing it, but it’s very fragile and easy to break the needle off.
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    I checked the ohms on the sending unit and it checked out ok. I grounded the wire from the sending unit and the gauge did not move. I did a search on this web but nothing came up about repairing the dash. Could someone direct me to the posts about the dash repair. Thanks

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    sennister's Avatar
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    Maybe someone will be around with a link on dash repair for the 425. I had to repair mine on my X585 which is a slightly newer version of a 425. I just don't know how much a like they are. In the past I have repaired a few GM clusters so I have worked on similar systems.

    In the case of my X585 it was more of a ground issue. That is why I don't know that they are the same. I used the schematics for the x585 which has a wiring diagram to make a bonded connection to the ground connection right next to the connector that plugs into the dash. I then dissembled the dash. A trick I found is to put a bit of blue painters tape under the needle and mark the position of the needle on the tape before removing. For the GM clusters I was typically replacing a stepper motor so I would move the needle to 0 and mark there before removing. To remove them you slip a fork under the base of the needle and use that like a pry bar. Once the needles were out I could disassemble the cluster to get to the problem on the X585 cluster. For me it was broken soldier joints. I used a desoldiering iron. It is an iron with a soldier sucker built onto it to clean the connections. Then I fluxed them and used my digital soldier station to resoldier the connections. Between this and the bonding of the ground it fixed my issue.

    Where my issue was different than yours is my temp gauge worked if the tractor was running. When I turned on the PTO it would go crazy from time to time and the temp and eventually other gauges like fuel would peg out. I do have a small write up on some of the work I did to my cluster as part of the thread I did on refreshing my X585. Here is a link. Not sure how much help it will be as I didn't document it in detail and my issue might be different than yours. In your case you may have a bad stepper motor if you don't get any movement. You might have to remove the dash to get to the stepper motor and see what it is so it can be ordered.

    Link to thread.

    Refreshing a X585

    Here is a link that will take to you the point where I was starting to get into the dash.

    Refreshing a X585 - Page 3

    Again, not sure how much help it will be for you as you have a different model and I don't know your symptoms are the same.
    Last edited by sennister; 05-24-2019 at 08:16 AM.


    JD Z950R 60" Deck with DFS Collection System

    JD X585, 54C deck,
    CTC Model X4750 F.E.L - Modified Imp Pressure Relief from 900 to 1175PSI, Power Flow and MC519 cart, 54-inch Quick-Hitch Front Blade, 47-inch Quick-Hitch Snow Blower, 3-pt hitch, HF Quick Hitch, Heavy Hitch, 60" box blade/rear blade, Dethacher, Sprayer cart

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