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Discussion Starter · #1 ·
For YEARS, my gauge would hang at full until I consumed a quarter + tank of fuel, then drop to the appropriate / correct indicating level. I routinely filled to just under full to keep the hang-up from happening.

Well, I messed up and topped it off and now it's stuck. Worse yet, I drive it so infrequently that I'm not sure how much is REALLY in the tank to feel safe as to not run out of fuel.

Question is: has anyone the experience to point me toward either the dash gauge or the tank sender as the culprit?

Question 2: If a hung float arm level in-tank issue, can I access it via a 3 foot long, 1/8" diameter TIG rod via the tank fill neck to jar the float loose maybe? :sneaky:
 

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My first guess about the problem is the float. You could try slapping the gas tank with your hand to see if you can dislodge the float. Trying to reach in the tank with the filler rod would be difficult unless you are extremely lucky.
 

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Discussion Starter · #3 · (Edited)
I'll try that. Thanks.

FWIW...I've started purposely running over speed bumps with a bit more gusto, but nothing yet. :(

Maybe slaloming a bit will cause a sloshing / wave effect? I'll try that too.

Oh, I did pound on the gauge panel a bit to try and shock the needle loose (if that is even the problem source).
 

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welcome to the club with that fuel Gauge issue. I have a new float indicator and fuel tank and have the same issue.
Sometimes it reads correctly and other times reads full. So if I go for a drive I just stop and top it off.
 

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On my 68, the senders ground wire is broken off at the tank and mine stays on full until I get down to about the vapor level and then it starts dropping.

I document the mileage to where I know when I need to get gas but every now and then "think" I need gas only to find out I can only put in a couple of gallons.

You need to look under the car and make sure the ground wire is solidly attached and maybe clean the connection points of the wire terminal end and the spot it is screwed down to the trunk floor.

If this is good, you can do the basic unplug the sender and then ground the sender wire in the trunk area but this gives VERY limited info and one really needs to do further tests with specific resistor values. With the unplugged and then grounded sender wires you can get a false positive reading so ideally get some resistors from a local electronics repair place.

A video I put together years ago (it's a silent movie but it was how I tested things on my 68):


Jim
 

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Discussion Starter · #7 ·
So if I understand this right, for starters, I can test for gauge movement (at least) by...

1. shorting the sending unit gauge sensing lead (brown lead accessed in the trunk, at least in the video 68' Nova) to ground and the fuel gauge will move to "E" with the ignition turned on? :)
 

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So if I understand this right, for starters, I can test for gauge movement (at least) by...

1. shorting the sending unit gauge sensing lead (brown lead accessed in the trunk, at least in the video 68' Nova) to ground and the fuel gauge will move to "E" with the ignition turned on? :)
Yes, this basically tests the wire going up to the gauge and then shows the gauge is getting power and ground with the key in the run position.

So how this works is if the gauge is getting power and ground and the sender wire is connected to ground, this resistance will be 0 ohms which is the same resistance the sender should have when the tank is empty. This test would get the gauges needle to go to the empty or below mark.

If you then leave the key on and then unground this wire in the trunk going to the gauge, then the gauge needle should swing up to FULL or above. An open or ungrounded wire has infinite resistance which is higher than what the sender in the tank would be if the tank was full (and the sender ideally would show 90 ohms to get the gauge to show FULL).

The problem with these two tests is possibly it can give you a false positive result because these two tests do not take into account the gauge itself working properly with different resistance values that would be BETWEEN 0 ohms and 90 ohms.

Built into the gauge is what they call a shunt resistor and the basic test of grounding and then not grounding the sender wire will get, or should anyway, have the gauge needle go between empty or below and then to full or above BUT this will not tell us if the gauge will read right for a 3/4 full reading, a half full reading or a 1/4 full reading. This is why the resistors need to be used as additional tests. A gauge with a defective shunt resistor or one that is fine will both read the same with a short or an open on the sender terminal of the gauge.

Jim
 

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Discussion Starter · #9 ·
I'm curious now. What is the default position of the the gauge needle with the ignition off? Does it reside at its last powered up tank leveL, OR relax to some prescribed level indication? :unsure:
 

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I'm curious now. What is the default position of the the gauge needle with the ignition off? Does it reside at its last powered up tank leveL, OR relax to some prescribed level indication? :unsure:
You need to ignore what it reads when the ignition is off. Some gauges may stay at the last point before power gets removed or some might move due to some other thing like an stray electrical signal being sensed or even from a vibration or shock to the gauge assembly that gets the movement to change it's pointer position.

I have seen some gauges when off like a tach show an rpm reading above 0 but the motor is off but then when power is applied to the gauge, the needle goes to 0 and then once the motor starts rotating the needle comes up off of 0 RPM.

On a side subject a Lincoln my mom wants to keep has an instant MPG reading and it's laughable when I'm going 60 and then start going down a decent sized hill and I'm off the gas the instant MPG goes to almost 100 MPG. Like this will actually happen.

Jim
 

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Discussion Starter · #11 · (Edited)
Good news, bad news.

Grounded the sender brown lead...


Got this! :)


Recoupled and turned on the ignition and (video) .... ;)

Stick gauged the tank via the filler neck. 3/8" wooden dowel, lands at tank bottom at an adverse angle to discover MAYBE an inch of fuel in the tank. :cautious:


Summation?
Gauge and circuit up to at least the connector...GOOD!

Near empty tank pegs the gauge still. BAD!

Next? Inspect the accessible connector to tank sending wire length for a short to ground (probably none, but).

If good there...drop the tank and check / replace the sender.
 

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Discussion Starter · #13 ·
In fact...the existing sending unit is original to the Nova, so I'll just replace it. Even if I find a tweakable correction to this one, why take the chance! New hose too.

The tank I replaced once due to pin-hole rusting on top. It'll be OK, but I'll inspect, of course.
 

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Discussion Starter · #14 ·
And oh!
What is the default position of the the gauge needle with the ignition off? Does it reside at its last powered up tank leveL, OR relax to some prescribed level indication? :unsure:
It resides at the last powered up position. ;)
 

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Check the ground wire from the sender to the body to make sure it has a good connection.

If you get a replacement fuel sender (I went with eBay, but a local vendor had one for not much more), make sure you check the ohms on the float/sweep before putting it in the tank. I did that, and the module was dead out of the box.
 

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I have the same problem with my 70 Nova. Gets stuck at full or above then comes down at some point and reads fine after. I replaced my sender with a NOS float thinking that was the problem, but still does the same thing. I just think the needle in the gauge gets stuck at the full mark for whatever reason and breaks loose at some point. Seems to work fine after that. So, when I go to fill up, I try and not fill the tank to the top.
 

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Discussion Starter · #17 · (Edited)
Ground the open sender wire as I did and you can check for proper gauge integrity / sweep to rule that end out.
 

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Discussion Starter · #18 · (Edited)
make sure you check the ohms on the float/sweep before putting it in the tank. I did that, and the module was dead out of the box.
OK. Bench tested my newly received Spectra sending unit with my Fluke. In the OHM position (one lead to the sender pin connection, one to the sender frame) I get all sorts of odd readings and continuity beeps throughout the range of float movement. :unsure:
What am I doing wrong? Type of meter issue? Shouldn't I be seeing 0-90 ohm readings only??

I now intend to wire in the sending unit and test it to the dash gauge BEFORE I land the sender in the tank.

Rectangle Font Screenshot Number Parallel


Tool Office supplies Cable Gas Measuring instrument
 

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Off memory when I tested some senders, it seems to me sometimes the resistance would not lock into one solid value but if the sender was working it should never show a direct short or 0 ohms resistance unless the sender arm is all the way down like it would be in a gas tank to indicate empty.

It seems to me that my fluke when on the ohm scale the beeper would sound at values above 0 ohms to let's say something like 30 ohms.

I found this from Fluke and they say different meters will vary but the beeper will sound between resistances of 0-50 ohms SO with that said, best to ignore the beeping.


The reason you may see some swings with the sender is the contact pad should be hitting more than one wrapping on the wire and a slight movement of the arm can get the resistance values to change.

I do remember one sender on a car is that the wiper arm did not have enough pressure being applied to make solid contact with the wrappings of wire on the other half and some slight bending of the arm to create more pressure cured the problem BUT while I do not know how this "tweak" worked long term,I have to think the added pressure would wear things out quicker.

I think you are on the right track wanting to do a test wire of the sender outside the tank.

Also too, maybe work the arm up and down a bunch as the wire wrapping where the contact arm hits may have corrosion on it just from sitting dry. Once in the tank it might be in a higher humidity and then work as designed.

Jim
 

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Discussion Starter · #20 ·
Fixed! :)

Tested the new sender attached to the Nova wiring / gauge before inserting it in the tank.
Landed the tank and all good now!

Thanks to those who fed back.
 
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