Temperature gauge does not work on my 1970 340 Duster ?

Discussion in 'Electrical and Ignition' started by DaveM, Oct 6, 2018.

  1. Mike69cuda

    Mike69cuda 63 is the new 17 FABO Gold Member

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    Dave, I think I gave you bad instructions on #1. If you have the original regulator the voltage will vary like you have seen. Sorry, I was thinking about the replacement that is fixed at 5v. My mistake.

    It looks like the voltage regulator is at least cycling and maybe working correctly.

    Try step 2. If you ground each of the sensor wires the gauges shoul each go full scale.

    Sorry for the lack of precision in the instructions. This kind of stuff is confusing in person, much more from a distance.
    .
     
  2. DaveM

    DaveM Active Member

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    No prob Mike, thanks for the update and confirmation the voltage readings appear to be good. I did ground the temp sensor wire and the temp gauge goes up to high/hot. I will have to ground the gas sensor wire ...but if that also works then ??
     
  3. Mike69cuda

    Mike69cuda 63 is the new 17 FABO Gold Member

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    Where are you grounding the wires, at the gauge cluster?
     
  4. DaveM

    DaveM Active Member

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    no, near the sending units, i.e. engine block for the temp and gas tank for the gas sensor lead.
     
  5. Mike69cuda

    Mike69cuda 63 is the new 17 FABO Gold Member

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    What I a. Trying to do is to divide and conquer. If we can see that grounding the sensor right at the gauge cluster works, the we move father away ( like maybe at the sensor connector on the block)and try it again.
     
  6. Mike69cuda

    Mike69cuda 63 is the new 17 FABO Gold Member

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    Ok, if the temp gauge moves when you ground the wire at the block, looks like the circuit is good back to the dash.
     
  7. Mike69cuda

    Mike69cuda 63 is the new 17 FABO Gold Member

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    It has been awhile on this, let me re read the thread & see what you have done so far.
     
  8. 67Dart273

    67Dart273 Well-Known Member

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    Let's go back to square one..................see the photo?
    c-3826-jpg-jpg-jpg.jpg

    Those resistance values in yellow, someone edited that to reflect the factory tester values. If you can heat your old sender and get it close to one of those values, then stick it in the circuit from block to sender wire, and if the gauge reads somewhat close (of course it will change--------the sender is cooling down!!!

    THEN THE ENTIRE GAUGE CIRCUIT is likely OK, or at least "OK enough" that you can move forward

    I tried to tell you earlier..........with the engine "somewhat warm" take your meter and measure the resistance from the sender stud to the sender brass. That should give you some idea based on temperature (see the photo) of whether the sender is good

    Measure resistance from the sender brass to some known ground........LOL....LIKE THE BATTERY NEGATIVE POST. It should be close to zero

    Try subbing in the old sender and see if these values change.

    ANOTHER THING you can try is to connect the temp sender (jumper clip wire) to the oil sender wire. THIS WILL TURN THE OIL PRESSURE GAUGE into a TEMPERATURE gauge. You can also do the opposite............jumper the temp sender wire to the oil sender and that should cause the temp gauge to read OIL PRESSURE
     
    Last edited: Dec 6, 2018
  9. DaveM

    DaveM Active Member

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    I did measure the voltage of the temp sensor right at the gauge cluster using the dash ground earlier so the dash ground s/b good. Not sure grounding the sensor with the dash ground will buy us anything?
     
  10. Mike69cuda

    Mike69cuda 63 is the new 17 FABO Gold Member

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    Yeah, you’re right. I have forgotten a lot of the details of this puzzle.

    One thing that I remember from my youth (I think) is that the temp sensors can be damaged by too much heat. If you are trying to simulate engine temp with a torch or a lighter, the sensor can be damaged and will go open I think. Don’t know if you have done this but worth mentioning.
     
  11. 67Dart273

    67Dart273 Well-Known Member

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    To put this another way a standard dash with no oil pressure, or a ralley dash with oil pressure, both temp and fuel (and oil pressure) gauges ARE THE SAME. The same resistance at any sender will cause any of the gauges to act the same
     
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    • 67Dart273

      67Dart273 Well-Known Member

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      I believe that's true, and they can also be ruined by over tightening
       
    • Mike69cuda

      Mike69cuda 63 is the new 17 FABO Gold Member

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      Very confusing. It seems like that when nothing makes sense it is often a bad ground, as currents can go ways they normally don’t go.
       
    • DaveM

      DaveM Active Member

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      Please note...I did not use a torch to heat up the temp sending unit. I heated up a cup of water and placed submersed it. I also have tried a brand new temp sending unit. I dont think I damaged the sending unit. So far I didnt touch the gas tank sending unit. At this time the oil pressure gauge works fine, but the gas & temp gauges do not work. All voltages at each sending unit appear to be good. By grounding the temp sending unit the gauge also seems to work (moves all the way to hot), I will try grounding the gas gauge sending wire, if the gauge works then it would suggest I have two defective temp sending units and a defective gas sending unit. As the dash ground checks out, and limiter seems to be putting out the proper voltage cycling.
      Before replacing the sending units, I will perform the resistance measurements mentioned earlier by 67dart273. If they do not change (lower resistance) when heated, that will confirm the sending units are defective.
       
      Last edited: Dec 6, 2018
    • Mike69cuda

      Mike69cuda 63 is the new 17 FABO Gold Member

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      Anything is possible with cheap Chinese crap nowadays. I would suggest that you follow 67 darts instructions exactly in order. I think he has a superior knowledge of how this works. I think I am just confusing the process trying to help.
       
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      • RedFish

        RedFish Well-Known Member

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        Is it possible that your engine hasn't made hot water enough to move that gauge needle? If the engine is up to normal operating temperature there should be somewhere between 40 and 25 ohms resistance between the senders contact post and chassis ground.
        You don't absolutely have to go under the car to ground the fuel sender wire. It can be found in rearward harness connector behind left kick panel. Then again... To verify the ground jumper over the fuel senders hose connection and check all of the blue sender wire run, you must go under the car.
        So why the service techs needed a 3 position tester... The sender wire direct to ground zero ohms test doesn't always prove good gauge. I've seen some that were soooo toasted inside that zero ohms could cause movement while within normal range ohms could not. Looking at those, the needle doesn't go all the way home at off. It'll stop very near the hash mark scale.
        This 4th/off/home needle position isn't on the tester.
         
      • 67Dart273

        67Dart273 Well-Known Member

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        Exactly why I'm trying to get him to check sender with an ohmeter
         
      • RedFish

        RedFish Well-Known Member

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        I just re read this entire thread. I read the oil gauge works but have no clue how it works, what it reads.
        The above post along with a couple others suggest a working but faulty mechanical limiter. It should produce a pulse voltage that can operate all 3 thermal gauges but it does put out a higher current during warm up period before leveling to the circuit demand. So maybe its' leveling to a very weak pulse voltage and that takes the path of least resistance to ground. Thus 2 or 3 volts going through the oil gauge at start up, leaving notta for the temp gauge.
        There would need to be a serious fault in the oil gauge circuit for that to draw away all of a good limiters output but... maybe that is possible. I haven't seen every condition, every way a oil sender can feck up.
        So just for knowing... disconnect the oil sender and isolate that wire terminal. Now lets see if the temp gauge responds differently. If it does, we know the fault is in amount of limiter output going to where? and why? to replace the oil sender wouldn't break the bank. If that yields no joy...
        Next question is... Is the limiter truly bad or is it only lacking a good chassis ground? It gets its chassis ground via a small piece of metal on its back pressed against the inst housing and inst' housing screws to dash.
        It is possible that all of these instrument problems are due to 3 loose mounting nuts on the fuel gauge.
        I must add that if I determine the fault is inside the car, and I must pull the inst' panel and disassemble it, I will install a solid state inst' voltage regulator while I'm there.
         
        Last edited: Dec 7, 2018
      • DaveM

        DaveM Active Member

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        Ok thanks for all the input...I
        Ok I verified one of the temp sending units is good (new one) by heating it up within a cup of water and the gauge moves accordingly, i.e. mid way at about 40 ohms. The resistance of the new sending (cold) is around 200 ohms. The old or original was about 2x, (400 ohms). I did heat it up as well and the resistance lowered but never hooked it up to the gauge. I also grounded the gas sending wire and the gauge did move all the way to full. I cannot start the car and bring it up to normal temp at this time because I am rebuilding the p/s unit. Once complete, I will start the car and check the temp gauge. It should work, if it doesnt??? I will isolate the oil pressure gauge from the circuit to see if it is somehow consuming all the voltage from the limiter. If it still doesnt work??
        Note: I used the ground off the battery to check the temp sending unit and gauge circuit. This ground cable splits and is tied to the chassis as well.