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'50 Dodge Fuel Gauge Operation


Gregarious13

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Hey,

I'm having trouble with the fuel gauge in the dodge. The sender is new with a range of 73 Ohms empty and 10 Ohms full. I tested and confirmed 6V to the gauge and at the sender wire when disconnected. I tested and confirmed continuity in the wire from the gauge to the sender. I tested and confirmed resistance of 18 Ohms through the sender to ground. I applied 12V to the gauge terminals and the needle jumps to full if the gauge is tapped on a little. I also grounded the sender wire with 6V applied to the gauge and the needle will jump to full if tapped on. But when connected to the sender it doesn't move.  Do I have the incorrect resistance on the sender or is the gauge bad? is there an adjustment for the gauge? 

 

Thanks for your assistance.

 

Greg

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carefully inspect and clean the grounds.....the guage itself must have a clean ground as the electromagnet circuits that pulls the needle toward empty requires this ground point.  The other pole, going to the sender in the tank gets it ground at the tank as the wiper moves across the wound resistor.  

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So one electromagnet pulls the gauge to full while the other pulls to empty? and as the resistance in the "full" circuit changes the pull from that magnet weakens or strengthens according to the resistance? are they constantly "fighting" each other to keep the gauge in the correct position? Is that how the gauge operates? 

All grounds have been checked and are good. The gauge will go full with grounding the sender wire direct to the chassis with everything else connected as it should be.

 

Thanks,

Greg

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this constant pulling in two direction is the stabilizing factor that results in a slow moving rock steady needle.  If you are pulling toward full...what change if any is there when you move the wiper arm (float lever)  if there is no swing to empty...you may not have an energized circuit.  You may well have burnt out the small winding within the gauge with application of 12 volt.  From here, you will need to test the gauge with your ohm meter for to verify the windings of each electromagnet circuit within the guage head to ascertain that the gauge is functional.  To me, your readings indicate an open circuit in the coil that is constant pulling to ground.

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Thanks, good information. I wasn't 100% sure how the gauge worked but no I have a better grasp.

 

I did check the resistance of the windings last night and found one a 63 Ohms and the other at 95 Ohms. (this was after checking with 12V) Not sure which was which or what the specs are, but the windings are intact and the ground path through the gauge housing is good.

 

I still find it interesting that the gauge wont read anything  until the sender wire is grounded, it has no problem going back to empty when the wire is un-grounded.  It seems like any resistance on that circuit keeps the gauge at empty.  I just don't want to buy a replacement gauge and have it do the same thing.

 

Greg

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when grounded there is no resistance and thus the electromagnet is at full strength to pull toward the full mark...you need the variable resistance to control the magnetic field and always some resistance to protect the circuit from going hard past the limits of the gauge, in each direction.

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many folks think you can run a 6 volt fuel gauge circuit on 12 volts...IF the gauge has a built in VR then yes...however you may well see a quirky flicking needle as the VR opens and closes at the 12 volt rate.  When the industry changed to 12 volts, the gauges remained the same and the application of a CVR was placed in the circuit to protect the windings in the gauge.  This CVR can be easily retro'ed into the circuit, a VR of ample current rating installed and regulated to 7.2 volts (nominal 6 volt charging rate) or the use of a zener diode with proper wattage resistors for heat related issues.  If you continue the use of 12 volts without a protective circuit on the input...you will continue to have issues and more than not permanent damage to your gauge head.

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Yes, that makes sense, so I would expect the gauge needle to move toward the full range when connected the sender but it does not, it stays pegged empty when the tank is full and the sender only has 18 Ohms resistance. 

 

Also when the gauge was energized on the bench there was magnetism on both pols, I could detect it with a pocket screwdriver.  It's like there is not enough current flowing to move the needle from empty.

 

I suspect the gauge must be bad. Perhaps the coil resistance is too high causing low current to flow.

 

Thanks for hanging with me!

 

Greg

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1 hour ago, Gregarious13 said:

Yes, that makes sense, so I would expect the gauge needle to move toward the full range when connected the sender but it does not, it stays pegged empty when the tank is full and the sender only has 18 Ohms resistance. 

 

Also when the gauge was energized on the bench there was magnetism on both pols, I could detect it with a pocket screwdriver.  It's like there is not enough current flowing to move the needle from empty.

 

I suspect the gauge must be bad. Perhaps the coil resistance is too high causing low current to flow.

 

Thanks for hanging with me!

 

Greg

 

In the condition above at the resistance you state your gauge should be reading approx. in the 3/4 full range (if circuit is linear) and tells me that you either have burnt out the electromagnetic coil winding in the gauge that pulls the indicator toward full, your wire to the sender is open or you have lost all ground at the tank sender...again however I must state that you should be testing with max of 7.2 volts...as for the loss of ground...ensure you have a known good aux. ground wire to the tank sender body, not just the tank.  

Edited by Plymouthy Adams
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Thanks!

 

I am currently using about 6.5V to the gauge for testing, I only used 12V once. 

 

The gauge still goes to full if I ground the sender wire, that tells me the electromagnet is still intact and that the wire to the sender is also intact. 

 

I have a ground wire from the top of the sender body to the car body.

 

If 18 Ohms is about 3/4 full, what is the resistance for empty and full? Perhaps I have the incorrect sender.

 

Greg

Edited by Gregarious13
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empty and full is the upper and lower limits of your sender...10 prevent overdriving the gauge to full (overdriving is not good for the unit...it is a quick test but should never allow the needle to move past the full mark into the upper nether region...and the 73 should allow the gauge to indicate empty at the E line for accuracy without falling below there by more than maybe just a shadow of the needle.

 

get a 10 ohm resistor and place on the wire at the sender and ground....should read full...get a 35 and it should indicate approx. 1/2 tank...etc etc...you must ensure that the post of the sender will read to the body of the sender across the internal resistor in a linear fashion...indication is that you are not completing this leg of the gauge through the sender and its ground point.  On the 18 ohms, I looked at my drawing wrong and it should read 3/4 not 1/4 as I previously stated.  I will correct this so not to mislead anyone.  At this point, I believe you need to remove the sender and verify the swing of the lever and linear action of the resistor.

Edited by Plymouthy Adams
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From the sender post to the car body is 18 Ohms, telling me the path through the sender to the chassis is good.  So if I apply voltage through the gauge and sender (18 Ohms)  to the chassis I should be reading 3/4 full on the gauge, but I get a reading on E instead. This further enforces my suspicion of a failed gauge. Thank you for all your information, it has helped immensely!

 

Time to procure another from Ebay.

 

Thanks,

Greg 

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ok...back up.....test the components outside the system...this is the only way to say...this is good this is bad and the ground point here is good and or ground point here is bad.   outside of this you will be guessing till the cows come home.  I would suspect you have the repair manual.  If not, odds are you can go to a library and get your hands on a Motors manual of this era.  I can never say this often enough, to get it to work you must first know how it works, with that concept engraved, the next truism is the fact that if you do not know what is wrong, prove what is right.....there is nothing mystical at all about this circuit and the few components that make it all operational.  Step back, get a cup of coffee....wrap you head around the how and test/record/verify your knowns.  

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9 minutes ago, Plymouthy Adams said:

ok...back up.....test the components outside the system...this is the only way to say...this is good this is bad and the ground point here is good and or ground point here is bad.

 

I appreciate that wisdom very much! a lot of technicians in the automotive industry have a hard time with that concept, rather assuming something is as it should be.

 

I do electrical diagnostics on a daily basis where I work.  I came here  looking for some confirmation of my findings and making sure I didn't forget something. I came to the experts in these old Dodges and you have helped greatly!

 

  I fully understand the system, especially now that you have confirmed how the gauge itself operates with the electromagnets.  I am confident that the other components of the system are operational. I fully understand how the level sender operates and what it does in the system and know it is functioning properly, I tested it before installing it in the tank.  :D

 

I do indeed have the Dodge manual and the Motor's manual. I like to have the books for my cars. 


I'm confident the gauge is the faulty component in the system.

 

Thanks again,

Greg

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 I have used a light bulb such as may be found on a front fender marker light  in series with the gauge ( between the ignition switch and the gauge).  It will glow dimly and reduce the voltage to about 5.5.  Gauge linearity does not seem to be affected.  If you need a replacement gauge for a 49 or 50, send me a PM.

Edited by dpollo
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2 minutes ago, dpollo said:

 

 

 I have used a light bulb such as may be found on a front fender marker light  in series with the gauge ( between the ignition switch and the gauge).  It will glow dimly and reduce the voltage to about 5.5.  Gauge linearity does not seem to be affected.

 

I too used a test light to see if the resistance of the light bulb would affect the gauge but it was still reading empty. I only ever got a full reading with full voltage and no resistance.

 

Greg

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a shop manual once told me that the resistance about what you might get from a test probe between the gauge to tank unit  and ground would cause the gauge to read about 1/3.

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26 minutes ago, Gregarious13 said:

 

I too used a test light to see if the resistance of the light bulb would affect the gauge but it was still reading empty. I only ever got a full reading with full voltage and no resistance.

 

Greg

Greg, stop and think...if you touch the wire to ground that is either the tank, sender body or the aux wire to chassis and as there is no resistance in line, gauge is pegging to full indicates that the coil in the gauge is working as the magnetic field is now greater than the magnetic field pulling it toward ground and that the wire to the sender is good....you seem to be loosing it at/within the sender.  Go back to your electrical background you speak of and THINK completed circuits....again, I cannot emphasize enough the need to have a couple known resistor values when testing the gauge.  Put a 10 ohm resistor between the wire from the gauge ground and gauge should deflect toward full but YET not peg past the full line into the upper stop/nether region.  You almost there.... don't ever at any time assume because the device you have is NIB that it is good functionally.  You could have a simple problem like the wire is broken off or poor connection nut or cold solder joint on the wiper where it is attached on the sender itself thus not going to ground.  You alone are the only guy in this loop that can make that measurement and place on the check list, KNOWN GOOD   or   KNOWN BAD

Edited by Plymouthy Adams
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24 minutes ago, dpollo said:

a shop manual once told me that the resistance about what you might get from a test probe between the gauge to tank unit  and ground would cause the gauge to read about 1/3.

 

yes a lamp in series will act as a resistor if the value falls within the upper lower limits of the specific ohm range empty to full, you need only to measure the impedance and wire it into the circuit...based on value placed in a circuit and assuming linearity, the value will easily tell you what your gauge should indicate.

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49 minutes ago, Plymouthy Adams said:

Greg, stop and think...if you touch the wire to ground that is either the tank, sender body or the aux wire to chassis and as there is no resistance in line, gauge is pegging to full indicates that the coil in the gauge is working as the magnetic field is now greater than the magnetic field pulling it toward ground and that the wire to the sender is good....you seem to be loosing it at/within the sender.  Go back to your electrical background you speak of and THINK completed circuits....again, I cannot emphasize enough the need to have a couple known resistor values when testing the gauge. 

 

That's exactly right, I have confirmed the coil is intact and that the wire to the tank sender is good.

 

What I don't know is if the coil resistance is within range or not. If the resistance of the coil is too high it will limit current through the coil, weakening the field, thus not moving the needle.

 

I think the key clue here is that I have to tap on the gauge to get it to peg at full because the magnetic field is too weak to move the needle all on its own, but once the needle is moved the field will hold it there. Once the resistor (fuel level sender) is added to the circuit the magnetic field is weakened further thus the needle wont move at all.

 

I have tested the entire range of the level sender and confirmed no drop outs or glitches in the resistance values, thus I can use the level sender as my known resistor. 

 

 

Greg

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ok....lets back up....you said that you could ground the wire and it will read full...there was nothing in that statement indicating that you tapped on the gauge for this test...if so, it should well have been mentioned...   As for using the sender as a know resistor....yes...but only if you have confirmed its linear action is going to a clean ground.  Again, as I indicated from the beginning...the gauge itself MUST have a clean ground....perhaps your tapping on it is allowing it to touch a clean ground at that time.  Until you stop what you are doing and verify each component outside of the system.....I am not sure you will make any headway.  IF your intermittent operation is only when you tap the gauge....this screams of an area needing proper checks to ensure its value.   There is often no real short cut to proper testing.   There is very little one can do to help if all facts are not given in one briefing.  

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as a bit of further assistance based on a unit I have in my hot little hands, I took down a cluster I had on a shelf and the input of the gauge to its own body ground that is the coil pulling toward empty is approx. 70 ohms....the coil that is from input to the output that goes to the sender via the single wire to the rear is approx. 27 ohms.   IF you overheated these during your testing with the 12 as you stated, you could well have overheated/melted the insulation on the windings allowing them to touch somewhere in the middle of the coil and yes changing your value and resulting magnetism.  

 

 

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6 hours ago, Gregarious13 said:

I tested and confirmed resistance of 18 Ohms through the sender to ground. I applied 12V to the gauge terminals and the needle jumps to full if the gauge is tapped on a little. I also grounded the sender wire with 6V applied to the gauge and the needle will jump to full if tapped on.

 

3 hours ago, Gregarious13 said:

From the sender post to the car body is 18 Ohms, telling me the path through the sender to the chassis is good.  So if I apply voltage through the gauge and sender (18 Ohms)  to the chassis I should be reading 3/4 full on the gauge, but I get a reading on E instead. This further enforces my suspicion of a failed gauge.

 

1 hour ago, Gregarious13 said:

I have tested the entire range of the level sender and confirmed no drop outs or glitches in the resistance values, thus I can use the level sender as my known resistor. 

 

I am confident I have fully and properly tested the entire system and that the gauge is indeed the fault.

I will let you know how the replacement, hopefully NOS, gauge checks out compared to the original. 

 

Thanks,

Greg

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2 minutes ago, Plymouthy Adams said:

as a bit of further assistance based on a unit I have in my hot little hands, I took down a cluster I had on a shelf and the input of the gauge to its own body ground that is the coil pulling toward empty is approx. 70 ohms....the coil that is from input to the output that goes to the sender via the single wire to the rear is approx. 27 ohms.   IF you overheated these during your testing with the 12 as you stated, you could well have overheated/melted the insulation on the windings allowing them to touch somewhere in the middle of the coil and yes changing your value and resulting magnetism.  

 

 

 

THAT is very helpful information. I think it supports my theory of the high resistance in the coil going to the tank sender.

 

Greg

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not that you confirmed all testing included a manual intervention of a physical shock tell a complete different story.....might be folks following this, if any, will heed the very caution that these cannot be operated on 12 volts like the two wire system that HAS an internal voltage regulator though I do not recommend that action.  VR's are just too cheap and easy to place in the circuit...

 

Compare your gauge coils to that of what I read on mine here at the house.  Report your findings.

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