yet another fuel gauge thread - reads full all the time

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str12-340

str12-340

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1970 Dart with a standard (not rallye) dash

So I installed a new fuel sending unit in my tank. I got it set the way I wanted it by bending the arm and it worked fine for a couple months, though it wasn't driven much, and now it reads full all the time (like on the F). The car has a new style IVF and an original style gauge that was tested before installation. New circuit boards and new dash wiring harness (M&H) within the last year or two. The temp gauge works fine and is on the same IVF. The sending unit has a solid wired ground to the frame.

My best guess is the sending unit is defective, but wanted to see what other ideas folks might have before going through the whole, drain the tank and replace the unit thing...
 
How many gallons are in the tank.

You might need to remove 5 plus gallon s for the tank to read anything g other than full.

Aftermarket senders are far off.

Look at the chart

Product Review: A100 Fuel Sender For 67 Dart


Also your bending of the arm might have moved the sender into a place where it reads full for an extended time.


Btw
You can not bend the sender arm to make the sender more accurate


The after market senders are too liner for the shape of our tanks and the curve the gauge is expecting
 
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Disconnect wire at the sending unit and see if the gauge still reads full. If it does your sending unit wire is shorted to ground somewhere. Not a common problem but its possible. My guess would be an issue with the sending unit. The new sending units are garbage. I gave up and just keep track of my mileage. I had mine working perfect and then it started doing what yours is doing. Reads chock full for a long time then drops to around a quarter.
 
Take the sending unit out and leave it hooked up and then ground it. Operate the float manually and see if it is on full when raised all the way and lower it to half and then empty while having someone watching the gauge or use a long jumper wire and watch it yourself.

I had to bend my float holder wire to calibrate it. Sometimes they get stuck in the full position. Just something to check if the electrical circuit is not shorted to ground.
 
I ended up using a sending unit from a D100 van/truck. Seems to work better then the aftermarket for A bodies...
 
1711944276850.jpeg
This one
 
str12-340 said:
Source?
Click to expand...
Amazon has it........A100 truck. Fuel sending unit. It will come up....Mine works OK, not perfect but better then the one with the resistor in the middle...Goes to half in about 4 gallons but after that pretty accurate to empty. About 2 gallons left at empty...
 
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str12-340 said:
Source?
Click to expand...
Before you spend money on another sender see if you can get a real A Body sender and have it repaired. It will be costly but your satisfaction will be MUCH greater.

Lets say you get a usable used one for 75.00, then pay 200 to have it refurbished. You are in 275.00 and have a gauge that will be very accurate.

Now lets say you have already spent 50.00 on an aftermarket sender, then another 50 on a different aftermarket sender, both provide the same inaccurate level so you buy a meter match 90.00, now you are in it for 190 bucks and your tank MIGHT show a form of accurate.

If you look at the wrightup I did in the post I attached you can see the issue with the aftermarket senders. They are linier vs provide a curve. Our gauges need a curve AND our tanks need a different curve. The A100 tank is a rectangle. 1 inch down = 1 gallon of fuel removed from the tank (just numbers for this example.) Our tank has the spare tire cut out in the top, so the first gallon drops the level by 2 inches, the 2nd by 2 more inches, the 3rd by 2 more inches. Now the level is below the cutout and the tank is basically rectangular. So the next gallon drops the level 1 inch and it goes that way for 5 more gallons. Now the bottom of the tank has a taper so 1 gallon is 1.1 inches then 1.2 inches then 1.3 till it is empty. The shape of the tank REALLY matters.

My numbers are way off but you get the point.
 
Dana67Dart said:
Before you spend money on another sender see if you can get a real A Body sender and have it repaired. It will be costly but your satisfaction will be MUCH greater.

Lets say you get a usable used one for 75.00, then pay 200 to have it refurbished. You are in 275.00 and have a gauge that will be very accurate.

Now lets say you have already spent 50.00 on an aftermarket sender, then another 50 on a different aftermarket sender, both provide the same inaccurate level so you buy a meter match 90.00, now you are in it for 190 bucks and your tank MIGHT show a form of accurate.

If you look at the wrightup I did in the post I attached you can see the issue with the aftermarket senders. They are linier vs provide a curve. Our gauges need a curve AND our tanks need a different curve. The A100 tank is a rectangle. 1 inch down = 1 gallon of fuel removed from the tank (just numbers for this example.) Our tank has the spare tire cut out in the top, so the first gallon drops the level by 2 inches, the 2nd by 2 more inches, the 3rd by 2 more inches. Now the level is below the cutout and the tank is basically rectangular. So the next gallon drops the level 1 inch and it goes that way for 5 more gallons. Now the bottom of the tank has a taper so 1 gallon is 1.1 inches then 1.2 inches then 1.3 till it is empty. The shape of the tank REALLY matters.

My numbers are way off but you get the point.
Click to expand...
Like I said, first 1/2 on gauge is about 4 gallons. Good from there to empty. On empty 2.5 gallons left. Really the empty part is what is really important....
 
For what ever its worth...:realcrazy:

I just CADed up an A body gas tank based on ROUGH measurements from a real tank.

There are many variables I did not account for. But I came out only .2 gallons greater than a theoretical 18 gallon tank!

The gallons per 1/2" of level drop are far from linier. There are sections of the tank where they are linier but the tank overall is not. If the tank were rectangular each slice would be the same gallons.

(10 is the top of the tank slice 0.5 is the bottom of the tank slice)

1712011009747.png


1712010157037.png
 
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