Bench testing rally gauges

Electrical and Ignition

  1. RedFish

    RedFish Well-Known Member

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    Steve, Fuel gauge test at A and S. Others are I and S (there's no post at A).
    QUOTE="Mattax, post: 1972844181, member: 29156"]So it is safe to put 3 volt power source directly across the gauges with no additional load?[/QUOTE]
    Yep, 2 double As to make up a 3 volt supply.
     
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    • 4spdragtop

      4spdragtop Well-Known Member

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      Tested the fuel and temp gauges as mentioned above, no needle movement so I believe they are toast.
      Thanks all for the input.
       
      Last edited: Feb 17, 2020
    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      Steve.
      I think what Redfish and Dana are saying is that the oil and temperture gage can be tested by connecting to a low voltage supply - 3 volts would be fine.
      But the Fuel gage can't be tested that way. The stud connects to the IVR first, and if that doesn't see something close to 12 volts the fuel gage probably won't get anything from the IVR.
       
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      • Mike69cuda

        Mike69cuda 64 is the new 17 FABO Gold Member

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        You might measure you gauges with an ohm meter before you pass judgement. You should get about 13-20 ohms if they are good. This might referee a set up problem with applying voltage. They are pretty hard to kill. Even if they are a little toasty looking they will still work ok.
         
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        • 4spdragtop

          4spdragtop Well-Known Member

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          I could be wrong Matt, but I remember a while back testing a fuel gauge with a 6v battery? Lol maybe Ill put 12v to it anyway, seeing as it no workee.
           
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          • RedFish

            RedFish Well-Known Member

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            Wrong, Fuel gauge is tested same way but at posts A and S. If the gauge is on a workbench, the limiter isn't chassis grounded, back feed voltage into that limiter cannot go anywhere or have any effect at all.
            That's why nobody ever really needed to open this fuel gauge and bend a little thingy. Just isolate that limiter from ground. Move the the 12 volt supply to post I from circuit board to external limiter or regulator. 5 volts in at post A.
             
            Last edited: Feb 17, 2020
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            • 67Dart273

              67Dart273 FABO Gold Member FABO Gold Member

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              !!DAMMIT RED!!! You did it again. Came in here and explained the whole darn thing!!!
               
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              • 4spdragtop

                4spdragtop Well-Known Member

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                I'm electrically fd, not sure which posts on fuel to measure resistance....lol it's taken 2 pages for me to figure out 6v -ve and +ve. This is the stuff that makes me wonder why I dont sell it all and get a nice boat....
                 
              • RedFish

                RedFish Well-Known Member

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                Steve, If you have a ohms meter.... the fuel gauge should measure 20 ohms, the limiter should measure 50 ohms.
                 
              • RedFish

                RedFish Well-Known Member

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                I've seen crusty gauges that did work but not OK. Crusty build up ( carbon? tar and nicotine from the ash tray below? whatever it is) restricts movement much like a ace bandage on a limb. A bi metal beam should be arrow straight. If it has a bow at room temperature has been overheated. The needle position has already changed some, and that beam wont respond properly to temperature changes. So for these 50 yr old thermal gauges, passing ohms tests says no more than still electrically complete. What once was fairly accurate and fairly fluid like needle movement is wrong and/or sticking then jumping needle movement.
                 
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                • Mike69cuda

                  Mike69cuda 64 is the new 17 FABO Gold Member

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                  Don’t disagree with you at all. Just hate to see a guy throw away a good gauge because he didn’t hook up his test setup right. Measuring with a meter isn’t a test, but maybe it could tell you if you were at least hooked up to the right places.

                  Certainly there is a wide variation in crusty or burned appearances. Just hate to see a guy throw away a functional gauge just because it looks a little toasty.
                   
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                  • 4spdragtop

                    4spdragtop Well-Known Member

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                    Thanks guys.
                     
                  • KudaKid

                    KudaKid FABO Gold Member FABO Gold Member

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                    So if you have a bad IVR, I guess you are changing the FUEL gauge. I have an issue with some of the gauges losing power and then coming back. Since I have my bezel out for re chroming I was looking to change the voltage regulator and now found out with the Rallye gauge setup it is par of the gauge. Just asking, what is the capacitors function other than the obvious ?
                     
                  • 4spdragtop

                    4spdragtop Well-Known Member

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                    Ive no idea lol.Not sure if intermittent power would be ivr or fuel gauge sender possibly? Or is the voltage reduced before the fuel sender? Lol
                     
                  • KudaKid

                    KudaKid FABO Gold Member FABO Gold Member

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                    Already replaced sending unit years ago for another issue. Just seems to be an intermittent problem and while everything is apart will be going thru it as much as possible.
                     
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                    • 4spdragtop

                      4spdragtop Well-Known Member

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                      I am not sure, but sounds like either a loose "feed wire" or ivr. Carefully open the fuel gauge for a peek?
                       
                    • Mike69cuda

                      Mike69cuda 64 is the new 17 FABO Gold Member

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                      Since the gauges function by heating wire inside the gauge, switching the power off and on relatively fast doesn’t cause the gauge to move with the voltage switching. The gauge responds to the average power, which would be about the equivalent of applying 6 volts all the time, assuming the power was off and on for equal times.

                      This is the same principle as incandescent dimmers in your house. To dim your lights, the dimmer turns the power off and on very fast. So fast that the filament does not have time to cool down and flicker. The light bulb integrates (averages) the voltage. The longer the dimmer is on (compared to the time it is off), the brighter the bulb gets. The technical name for this is Pulse Width Modulation (PWM)

                      The capacitor is there as a noise filter, so you don’t hear it switching on your am radio.
                       
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                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

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                        The IVR powers the Fuel, Oil Pressure and Coolant Temperature gages.
                        It's not a true regulator. It rapidly switches on/off creating the equivalent of 5 Volts as far the instruments are concerned but there are some sublte differences compared with a constant on 5 Volts.
                        Anyway its an electrically noisy process. The capacitor makes it less so. We could see what Chrysler says if it also helps steady the power supply out. Redfish probably knows.
                         
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                        • Mike69cuda

                          Mike69cuda 64 is the new 17 FABO Gold Member

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                          You might ask why mother mopar would go to all this trouble. I have heard it said that the gauges were originally designed for 6 volt cars. By reducing the average voltage to about 6 volts, they didn’t have to redesign the gauge system.
                           
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                          • Mattax

                            Mattax Just the facts, ma'am FABO Gold Member

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                            Last edited: Feb 23, 2020
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                            • Mattax

                              Mattax Just the facts, ma'am FABO Gold Member

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                              It seems plausible. It also seems likely they wanted some operational voltage that was sufficiently lower than a low battery would ever get so the gages would not be effected by the main system. So maybe a combination of factors.
                               
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                              • Mattax

                                Mattax Just the facts, ma'am FABO Gold Member

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                                Are those three gages going out together?
                                If so, yes they share the IVR and in turn they share the power supply connection.
                                 
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                                • Mike69cuda

                                  Mike69cuda 64 is the new 17 FABO Gold Member

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                                  • KudaKid

                                    KudaKid FABO Gold Member FABO Gold Member

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                                    One of the other posts referred to a YouTube video for 69 Valiant instrument troubleshooting. Watched the video and it definitely seems like a power issue due to all the gauges looking like the power is removed and the gauges Slowly going dead and then recovering to there original positions or like they were powered back up. That is why I think the IVR is what I have an issue with.
                                     
                                  • 4spdragtop

                                    4spdragtop Well-Known Member

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                                    I'm not sure if it makes any difference, you have a rally setup(all 2nd gen cudas did), and the valiant one you are watching is more than likely a standard setup with an external plugin style ivr. Whether or not that makes a difference I dont know.
                                     
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