Temperature gauge does not work on my 1970 340 Duster ?

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

  1. DaveM

    DaveM Active Member

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    Here is what I have done so far during troubleshooting this prob.

    1) grounded the sending unit wire, gauge moves up to 'H' (means the gauge should be good)
    2) replaced sending unit with a new unit, gauge does not move??
    3) connected my orig sending unit to the sending unit wire and grounded the threads of the unit. Then heated up the sending unit, gauge registered/moved accordingly towards 'H' !

    Why doesn't the gauge work with the original sending unit installed in the manifold??
    Could the gauge still be bad even though it moves when grounding the sending unit?
    Not sure if it is related but my gas gauge reads empty. I will try grounding its sending unit wire as well....any help would be appreciated.
     
  2. Mike69cuda

    Mike69cuda 63 is the new 17 FABO Gold Member

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    Do you have a good ground from engine to frame? That gets left off in a lot of engine swaps.
     
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    • DaveM

      DaveM Active Member

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      Not sure, I didn't check that but did check the ground from the battery to the block.
       
    • DaveM

      DaveM Active Member

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      It is the original engine, never removed. The only thing replaced were the engine mounts.
       
    • Mike69cuda

      Mike69cuda 63 is the new 17 FABO Gold Member

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      It was just a guess on my part, but seems like when wierd things happen that don’t make sense, a lot of times it is a ground issue.
       
    • DaveM

      DaveM Active Member

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      I agree, and that is what I suspected a bad ground...I will pursue the gas gauge prob and maybe it will lead me to the rootcause of both probs? thanks for your input.
       
    • Logan

      Logan FABO Gold Member FABO Gold Member

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      Using pipe dope or teflon tape?
       
    • Plymouth 65

      Plymouth 65 Floorable Deplorable

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      There should be a ground strap from the motor to the body also. I suspect that may be the issue. 65'
       
    • Dana67Dart

      Dana67Dart Most undignified way to get to Colorado!

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      Quick check would be to run a dedicated ground wire temporarly from batt negitave to the body of the sending unit in the manifold. If temp guage works you have an engine to ground or sensor to manifold grounding issue
       
    • DaveM

      DaveM Active Member

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      No pipe dope or teflon tape used on the threads...thx
       
    • DaveM

      DaveM Active Member

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      I will check today. I know there is a good ground to the front of the block from the neg terminal off the battery and also goes to the front of the chassis.
       
    • DaveM

      DaveM Active Member

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      I will also try this "test" today as well. thx
       
    • halifaxhops

      halifaxhops It's going to get stupid around here! FABO Gold Member

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      I had like a vapor lock once and the coolant did not hit the sender, drove me nuts.
       
    • Alaskan_TA

      Alaskan_TA Well-Known Member

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      An air bubble can do it.

      Also, all 1970 cars had a dual negative battery lead, large wire to the block & small wire to the radiator support ground.

      You can check to see that the connections are tight & clean.
       
    • Mike69cuda

      Mike69cuda 63 is the new 17 FABO Gold Member

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      Seems like you could loosen the sender while 5be engine is running & see if antifreeze seeps out. That might also bleed it. Maybe coat hanger down in the hole if there’s any buildup in there blocking flow
       
    • RustyRatRod

      RustyRatRod Just another dumbass. FABO Gold Member

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      Might check the instrument cluster voltage regulator.
       
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      • Alaskan_TA

        Alaskan_TA Well-Known Member

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        Are you testing it with the cluster out of the dash?

        If so, make sure it is grounded too.
         
      • 67Dart273

        67Dart273 Well-Known Member

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        You can easily check the ground between sender and block without a big dance

        Install the sender. Get a meter and learn to set it up for "low ohms" scale. If you post the model/ photo of the meter, we can help with that

        Stab one probe HARD into the brass on the temp sender shell
        Stab the other probe into a good ground point on the block. Make sure to stab through paint, chrome, rust, etc, and it should show "close to" zero ohms.

        If it shows say, .2--.5 ohms, this is likely probe/ lead resistance. Mash the probes together HARD to see what the meter reads that way.

        I have NEVER had trouble with either tape or sealant, but I can see how rusty threads in the block might be trouble.

        IT IS ALSO POSSIBLE by overtightening to warp and damage the internals of the sender. You can also test it "in the block." Measure resistance from the sender terminal to the block, cold. Warm up the engine and measure again. This will be subjective as you have no way to calibrate it. But here are sender resistances for cold, middle, and hot:

        c-3826-jpg-jpg.jpg

        ALSO do not dismiss the idea that you might have a poor connection inside the temp sender connector.........either loose, corroded at the stud connection, or "broken" inside at the wire connection.
         
      • DaveM

        DaveM Active Member

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        I tried grounding the sending unit body, nothing. But did notice after the engine was warmed up, when I would turn the ignition in the "on" position, (just before energizing the starter) the temp gauge moved to the middle. When I started the engine, it would slowly move back to the bottom or cool position? What would cause the gauge to stop working when the engine is started or running? I shut the engine off and turned the key to the "on" position and sure enough it moved back towards the middle. What would cause this to occur, the instrument cluster voltage regulator?
         
      • halifaxhops

        halifaxhops It's going to get stupid around here! FABO Gold Member

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        Low coolant.
         
      • DaveM

        DaveM Active Member

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        Thanks but I doubt that the reason the gauge is not working when the engine is running, is because of low coolant. I checked coolant level, also that the thermostat opens and that it is not "vapor locked". Again when the engine is warmed up and running, the gauge never moves. But when you shut the engine off, and turn the key to the 'ON' position it will read the proper temperature. Start the engine and it immediately moves to the bottom or cool position.
         
      • 70dustah

        70dustah Well-Known Member

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        follow 67dart273 diognostics
         
      • 67Dart273

        67Dart273 Well-Known Member

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        This indicates some electrical problem to me. Check block to body grounding, and CLUSTER TO BODY grounding. That is a known problem. The cluster is (poorly, OEM) grounded via the cluster mounting screws. Attach a ground pigtail to a ground point on the board, and bolt that to the column support.

        One good way of insuring the block-to-body ground is good is to use a short "starter" cable, that is, eyelet to eyelet, bolted to the rear of the driver side head, and bolted to the firewall, or master cylinder mounting stud.
         
      • DaveM

        DaveM Active Member

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        70Dustah, please note...I basically already followed the diagnostics from 67dart273 diagnostics with no success. If you follow all my entries, the gauge is good, the sensor is good, the wiring to/from the sensor is good, the sensor ground is good. I will add a good ground from the cluster to the column and from the body to the block as suggested, another redundant ground can only help. If it doesn't fix, not sure where to go next? thanks again for all your input and suggestions. No wonder aftermarket, mechanical gauges are so predominant :)
         
      • Mike69cuda

        Mike69cuda 63 is the new 17 FABO Gold Member

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        The block to body ground is very important. The negative battery cables hooked to the frame and the block both carry current, so there can be ground loops and voltage drops that are not obvious. The ground from the block to the frame will not necessarily carry a lot of current, but will hold the block and the frame at the same potential. That is why it is usually a large gage wire or or mesh braid. As someone mentioned above, I used a battery cable with two ring lugs. I tied one to the block and the other to a firewall bolt on my power brake booster. Even if it doesn’t solve your problem it is good practice. Usually no fun chasing electrical gremlins.....