It reads too low. It seems consistent, but low. Full it is just above 3/4. And at 1/4 it only takes about 5 gallons. This should be above half at this point. This is an aftermarket gauge, w/stock sending unit. The after market is the correct one for Mopar. I don't remember details but I think it goes from 0-90 ohms resistance with zero being full. Maybe I have that backwards. This is not my long suit. There is an in tank pump, which would displace maybe a gallon of fuel, so the total is now less than stock. However, this is a replacement tank for a 72 California car with the four vapor vent tubes in the corner. I have no idea what the capacity of these tanks is supposed to be.

I have checked the grounds and they seem fine. I could bring a ground wire up to the ground block in the trunk where the battery resides. Problem is there is no good ground connection to the sender itself. They relied on the silly clip-on strap to provide it, and pick up tube is going to get corrosion on it.

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When I first saw that clip-on strap its function and durability seemed really questionable. However, they seem to work pretty well for lots of folks - much to my surprise. I have used a bit of ground strap material and screw type hose clamps successfully. Whatever method you use, the contact areas must be nice clean bare metal. Using some grease to coat the whole area inside and out will keep corrosion at bay pretty well. You might have issues with the grounding of the metal fuel line to the frame. Cleaning and greasing those clips and the fuel line at the clips won't hurt.

Did your tank readings change with the aftermarket gauge, or gradually over some time period? Are you sure the aftermarket gauge is correct, not just the box it came in? Sometimes parts get into the wrong boxes. How old is that stock sending unit? You didn't say what car you have, but another thought occurred. What is the supply voltage to the gauge? Remember,Mopar used that instrument voltage regulator to provide approximately 5 volts to the gauges. Does your aftermarket gauge have the voltage supply it needs??? (Probably 12 volts switched) Your stock fuel sender might not like 12 volts. It used to run on five.

Of course, as always, I might be wrong.

BC

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'64 Dart GT convertible, 64 Dart 170 2-door post sedan in faded blue and rust (the Az Dart) & a sixty THREE Dart 170 2-door post sedan in faded blue and rust. (future project)
Early Dart Disorder (EDD) is real, and I've got it!

Capacity is about 16 gallons in the 1967-1976 A-body tanks. It was determined that varies based on the fill neck and it's baffling causing the pump to kick off sooner or later (i.e. in the case of the feather duster the baffle causes it to kick out sooner so it's listed at 14 gallons, but the tank is physically the same as the 1967 single belly band tanks, but with the beads for the suspender bands).

I have found that it takes some bending of the arm on a replacement sender to get it right, but also found that the wiring can also contribute to the extra resistance and cause the gauge to short cycle somewhat. Typically I take the new sending unit and hook it to a jumper wire so I can see how it reacts on the gauge in the car, then bend the arm as appropriate to get the correct range within the gas tank. Before life got complicated I was going to take one of my older spare gas tanks and cut the end off to allow me to mock up and correct the float level and arm so it reads 'E' and 'F' correctly....

-D.Idiot

Those are good questions I do not know the answers to. I think the voltage to the sender was 12V and still is. This gets fed back to the gauge with varying resistance, but I do not know if the voltage from the sender to the stock gauge got stepped down to 5. Would applying a smaller voltage to the sending unit make the gauge read higher? It seems the logic of this setup is for the gauge to read more full as the voltage goes UP. If I have this right, then lowering the voltage would make it read even less full.

If I am right about this, and I am not at all sure, then DI's mechanical approach seems like the easiest way to get it to read more full. In order to take this gauge out, I must use up half the fuel in the tank, or fuel will run all over the place. NO? I suppose I could siphon it out.

I am away this week, so can do nothing on the car, but will continue to pester you guys with questions and comments from my I-pad.

Sam

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Don't suck on your siphon hose! (Don't ask how I know what a bad idea that is!).

With the original gas gauge in the cluster, the instrument voltage regulator dropped the 12 volts to the cluster to 5 volts at the gauge.
Then that voltage was supplied to the sender and the current to ground was limited and varied by the resistor on the sender.

If I understand the aftermarket gauge setup they are supplied with 12 volts directly to the gauge and the sender gets the full 12 volts for the resistor to play with.
If the sender resistor stayed constant, the higher voltage would cause higher current through the gauge and a higher fuel level indication.

"Sender" is kinda misleading when dealing with auto gauges. The sender doesn't send anything, it reduces the current in the circuit.
Since the current is the same everywhere in the circuit, the sender reduces (and varies) the current through the gauge.
More resistance in the sender = less current in the circuit = lower gauge reading.
Higher voltage applied to the circuit (at the gauge) will cause higher current in the circuit and a higher gauge reading if the resistance of the circuit stays the same.

Bending the float arm might be the only way to correct your reading. D.I.'s cutaway tank idea is great if you're gonna calibrate a bunch of senders.
The only way I see to calibrate just one is to leave enough gas in the tank so it doesn't run out and decide what that gas level should read on your gauge,
then bend the arm to make the gauge agree.

BC

_________________
'64 Dart GT convertible, 64 Dart 170 2-door post sedan in faded blue and rust (the Az Dart) & a sixty THREE Dart 170 2-door post sedan in faded blue and rust. (future project)
Early Dart Disorder (EDD) is real, and I've got it!