Fuel gauge not reading properly

[mike price]

Need help.  My 1970 Gran Coupe/Rally gauges has problem.     Fuel gauge read 1/2 tank when FULL, quickly drops till 1/8 tank then drops to EMPTY.
I replaced sending unit and still only get 1/2 full reading on a FULL tank.  Next replaced voltage limiter with Charger Specialties CSVR2 unit and told NOT to connect Suppressor.  Dash is not back together but gauge cluster is mounted in car and battery power on and turned key.   FUEL GAUGE READS EMPTY !!
Is there something I am missing (as this car has overhead consolette) or what?  I HAVE NOT REPLACED THE FUEL GAUGE AS OF YET.
Any help is greatly appreciated.  Mike Price

[Roadman]

   First, WELCOME @mike price              Now, I would check the ground at the sending unit first. Then, since you have overhead console, possibly the low fuel relay. 

[fc7cuda]

Try by-passing the low fuel relay going to the overhead and see what it reads.  I got different readings both ways on mine. 

[Katfish]

How's the float? They can get pin holes that make them sink and always read low.
Check by shaking to hear the gas sloshing inside.
A shorted line (grounded) should make it read high.
You're reading low, so I would suspect the sender or gauge.

[Chryco Psycho]

 Welcome @mike price
Sounds like the resistance of the sender is not correct , I would far rather have a gauge that reads the bottom 1/2 than the top half , bending the float arm may put it closer into range but may also limit travel at the bottom or top .

[Cuda Cody]

Yes, I'm pretty sure you'll need around 10 ohms to make the gauge read full.  Around low to mid 20's should give you about a 1/2 tank reading.  Trying putting about 10 ohms of resistance to the gauge with the about 5 volts and see what it reads.

[shawge]

The Miller calibration tool lists these ohm ranges for the fuel gauge
Low = 73.7 Ohms
Half = 23.0 Ohms
Full = 10.2 Ohms

[Cuda Cody]

Sounds like the gauge is working correctly.  If you run a straight 4.69 volts to it then it should show full pretty quick.  It needs some resistance (ohms) to work correctly.  If you had 5 volts with the ohms you tested at (10, 23, and 73) it most likely would work perfectly.

Hi again,
I took an old fuel sending unit and got it to read 10.3 (full) and 73.4 (low).  Did the test as described in video with 4.69 volts on batteries and sending unit.
Gauge will move and read up to 3/4 on the gauge at 10 ohms, will read empty at 73.4 ohms and 1/2 tank at 23 ohms.  I then did the straight 4.69 volts and got a FULL needle reading.

[Cuda Cody]

What does your fuel sending unit read in ohms at low middle and high?  You need it to put out about 10, 23 and 73 for it to register correctly on the gauge.

[Cuda Cody]

And this old unit will only shows 1/2 a tank of fuel? 

Did the fuel gauge ever work and if so when did it stop working?

The OLD unit on the bench is putting out 10.2 near the full position.   Putting out 23.5 near the half position.   Putting out 73.1 at empty position.
             (All of the above arm is eye balled)

These numbers are based on the fact that at each of these 3 positions the arm is taped and I then test the gauge.  Trying to do a sweep, (ohms) all over the place in numbers.  could be 15 - 32 very touchy and bouncy. (at the same spot)

Fuel gauge movement is good till half tank reading - then gets very slow to move toward full.

Mike

[Chryco Psycho]

& you have a pulsing signal averaging approx 5V correct ...

[Chryco Psycho]

the bklue feed wire to the sender should read 5v approx , it will pulse if you are using the factory reg to supply voltage to the gauge

[Cuda Cody]

See my comments in Red...

New sending unit bench tested.  Arm up HI 8.7 ohms  Arm at MIDDLE (visual) 44.0 ohms   Arm down LOW 77.5 ohms Regardless of the middle, the high and low should mean the sending unit is capable of producing a high and low signal so that is a good sign that it works.  As long as you have a stock fuel tank it should give the proper ohms to the gauge.  We know the works because you have testing it with about 5 volts and the correct ohms.  It might take 2 people, but you could try to hook the sending unit up to the vehicles wires and have someone read the output behind the dash (at the gauge) then you will know for sure that the correct ohms are either getting to the gauge or not.  Just another thought on how to test it.

I have had car for 1 yr.  and fuel gauge has always read 1/2 Full and NO higher.  Now I am trying to repair problem.  This is helpful because we know that it's not worked as long as you have had it.  Is all the wiring factory stock?

I've never owned an E-Body with an over head console.  Maybe someone will be able to chime in and let us know how the might work through the overhead console.

[dodj]

Not sure if you got this sorted out or not, but I'll add my 2 cents.
First, I would not recommend putting the full 5vdc on the fuel gauge again. It is possible to damage it that way. It needs some resistance in the cct to limit current through the heater (the one inside the gauge that causes the needle deflection).

IMO, for troubleshooting you should hook up a 100 ohm mutli-turn potentiometer in place of the fuel gauge. With the key on, sweep the pot between 10ohms and 70ohms and check the reaction of the gauge. If it goes full sweep (don't think it will with the info you have posted) then the problem is in the sending unit/ground/5vdc connection etc.
If it does not go full sweep, it is one of. 1/gauge  2/LFR wiring in oh console  3/ wiring condition between tank and interior.

If you had a spare gauge it would be handy but I suspect you don't. So test the gauge with the pot you have and wire it up directly to the gauge. Remove the wire that goes to the sending unit and connect the pot in it's place and connect the other pot connection to ground. Power it up and vary the resistance and check the sweep of the gauge. If it goes full sweep, gauge is good. Wiring/LFR connections are the problem. If it doesn't go full sweep. Buy a new gauge.
Not the only way to check it out, but that's the steps I would take to determine the problem.

[screamindriver]

I'll throw my thoughts in on this one... First, if your fuel gauge is reading 1/2 tank the overhead console "low fuel" light shouldn't be on... If it is you need to verify the fuel relay /rallye dash wiring is correct {it's different than a standard rallye dash for the fuel with the OH console}. The low fuel light should only be illuminated at 1/8th of a tank and less...
   I haven't read through all the responses but I'll throw this out there...
     With all the wiring as it should be....Turn the key on, have a buddy watch the gauge and climb under the car and ground the sending unit wire to the chassis...The gauge should start at empty with the "low fuel" consollette light illuminated having the sending unit wire off the tank and not touching anything ...After grounding the wire the gauge should have a steady sweep  to full{stop when you get there}...This will verify the wiring,relay and gauge are working as it should and the tank's sending unit ohm value is the problem...
If not check the gauge itself by doing the old flashlight battery trick...Two D or C batteries connected{to the gauge studs} to give you 3 volts should bring the gauge to 1/2 {+ or -}..
  If this checks out most likely the low fuel relay has issues...
   Lastly, sometimes with problem child/aftermarket fuel tank sending units I don't even install them in the tank initially... I run a two jumpers, one from the vehicle's sending unit wire to the sending unit and a second jumper grounding the unit...Then swing the unit through it's range of motion and have a buddy watch the gauge..Bend the float arm and possibly the stops to get the reading where it needs to be full and empty...Then install it..