gas gauge reading when full but goes down quickly

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cudajim

cudajim

cudajim
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I replaced the tank sender in the 69 Barracuda a few years back and it's never read correctly. I do have a solid state 5v IVR and it's working just fine. Temperature gauge reads correctly too. Anyway, what happens is the gauge reads full when I fill the tank but goes down quickly to near zero when I still have 1/2 a tank or so.

So the question is what is the best way to get the calibration right or nearly so? Do I just have the wrong sender? I'm tired of guessing who much gas is left in the tank.
 
Newbie here - so take this as you will.

Usually using an incorrect sending unit will do this. Improper initial resistance (Ohms) can cause this issue if it's not within the correct operating range. Was it an ebay purchase? Is the gauge stock or after market? hopefully someone else with a little more know how can help you out! :)

I'd start by finding out what the gauge resistance is (I hope I'm using the right terms here) and then buying an appropriate sending unit from there that operates in that range of resistance. A quick test with a multimeter can tell you I believe.
 
I don't know of an "easy way." We know what the original resistances were................

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

but the thing is, even if you get the sender "there" the old gauge may no longer be "in cal."

But even if the gauge is OK, the "repop" senders have a reputation for not having enough "range." Add to that, you need to figure a way to "do this."

If you can get the tank near MT and near 1/2 and check sender resistance, you might be able to go from there. If I was younger, I might be interested in figuring a jig to calibrate these.
 
Maybe give this a try?

http://www.mgexp.com/article/fuel-sender-adjust.html

I really wish I would have found this sooner...just replaced a tank in my 66 fastback and it was a nightmare for a first timer. Probably won't even register >.<

Here's hoping you have one of these magical stop tabs...I guess you could adjust the sending unit to match what your gauge requires.

I think I would still look into testing that replacement and seeing if it can hit the range, replacing it if it can't be adjusted.
 
To answer your questions, yes it was a cheapie ebay repop. I'll yank it out this weekend and measure the resistance at all levels. It won't surprise me if it's way off.
 
I replaced my tank, sending unit with correct resistance and
installed a new aftermarket Auto Meter fuel gauge with the
correct resistance. Mine still drops after about 50 miles. When
it's a shade above empty it will hold 9-10 gallons.

I've just learned to live with it and fill up before it gets to the "E"
 
A few years ago the tech at Autometer told me this - note what he says about aftermarket or repop senders:

"Also, just for future "FYI", the gauges are calibrated to match a factory sender, which is very non-linear. 10 ohms = Full, 25 ohms = 1/2, 50 ohms = Empty, and 73 ohms = below Empty. Which this said, most after market, or reproduction senders are built linear which means that E & F will be correct, but everything in between will be skewed."

With my aftermarket sender I experience the same thing.
 
70Duster440 said:
A few years ago the tech at Autometer told me this - note what he says about aftermarket or repop senders:

"Also, just for future "FYI", the gauges are calibrated to match a factory sender, which is very non-linear. 10 ohms = Full, 25 ohms = 1/2, 50 ohms = Empty, and 73 ohms = below Empty. Which this said, most after market, or reproduction senders are built linear which means that E & F will be correct, but everything in between will be skewed."

With my aftermarket sender I experience the same thing.
Click to expand...


The problem I have however is that it reads empty (or below) when there's still lots of gas left in the tank.
 
cudajim said:
The problem I have however is that it reads empty (or below) when there's still lots of gas left in the tank.
Click to expand...

I seem to have a lot more fuel in the tank when it's "near empty" too. I checked my sender with an ohm meter before installing and it was pretty darn close to spec at full and empty. But I also had to bend the pickup slightly because it interfered with the bottom of my brand new tank - so who knows? I can pump 5 gal in when reading 1/8 or so and get a reading of 3/4 or more (Autometer gauge and repop sender).
 
My rally fuel gauge works like new. Some number of other owners may or may not chime in to say theirs does too. But then after it is renewed it is supposed to work like new. Replacing a mechanical limiter with a solid state regulator does not renew or calibrate the gauges. A regulator than can never, even during initial warm up period, send more than 5 volts through the gauges helps extend the life and long term accuracy of the gauges.
The mechanical aspect of a gauge is what so many ignore or don't comprehend. Once the heat is not properly transferred to the metal or the metal no longer responds properly to the heat, you can calibrate, bend little thingys, alter voltages, line resistance, whatever. It's still not going to work right.

I like to think most of yall can read between the lines and start to understand why so many TEMP' gauges still work perfectly. By the way... Most oil and temp senders are aftermarket reproductions too.
 

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I have the exact same issue, once I drop to half a tank it bottoms out QUICKLY to empty
 
I'll yank the sender out this weekend and give an update. I expect I'll find a bad sender.... should have bought the spectra fg69a in the first place instead of the cheap Chinese clone.
 
cudajim said:
I'll yank the sender out this weekend and give an update. I expect I'll find a bad sender.... should have bought the spectra fg69a in the first place instead of the cheap Chinese clone.
Click to expand...

Get a picture of that one before you buy it. Very good chance it's the same as the cheap "chinese" ones.

Just look at your old sender, next to the new ones. They are designed very different. Much beyond the ohm reading.
 
autoxcuda said:
Get a picture of that one before you buy it. Very good chance it's the same as the cheap "chinese" ones.

Just look at your old sender, next to the new ones. They are designed very different. Much beyond the ohm reading.
Click to expand...

The difference in the senders design or shape is in year models. If the OEM sender is 1973 or later It probably will be identical to aftermarket. Well almost. The very cheapest aftermarket sender has a plastic float.
 
I know that the steel that they are making the lines from is really hard, unlike the originals, and the ground strap has a hard time getting a grip on it for a good ground! Its hard to do, but you need to bypass the cheap eBay units and go for the better OE reman'd units, or the really expensive brass tubed ones! Geof
 
You do know the arm on that sending unit bends right? Drop the tank, fill the tank with a metered amount of water, adjust it till it reads right. No big deal.
 
RedFish said:
The difference in the senders design or shape is in year models. If the OEM sender is 1973 or later It probably will be identical to aftermarket. Well almost. The very cheapest aftermarket sender has a plastic float.
Click to expand...


Problem I saw was the new float arm was way shorter. That changes the ratios. That makes it so at first the gauge reads Full to 3/4 full for quite a while. Then goes from 3/4 to 1/4 full fairly steady. Finally it goes from 1/4 to E VERY fast.

Now with the right circuitry that situation can be overcome. But they don't. You can see this float/rheostat assy is used is lot of different Repro Sending Units. Dodge, Ford, Chevy. One size fits all. :violent1:

Here's a comparison picture. This is a side view of the pickup as they would be installed in a gas tank.
 

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I'm not going to argue the difference in the float arm length.
Draw 2 clocks, a box clock with a pendulum window 12 inch square, and a grandfather clock with pendulum window 12 inches wide and 36 inches long.
Recon both clocks would keep time ? I think there would be 60 swings per minute in both LOL Back to the senders
The arm stops and that first 1/2 inch or so of the arm that hits the stops, along with distance from bottom to top of tank, all constant and unchanged
Yall believe what you want to believe.
 
Update... I jacked the car up and pulled out the sender. It looked good and tested out ok through it full sweep. Approx. 10 to 70 ohms. I did notice however that the plastic float had a bit of gasoline in it so I swapped it out with a brass one I had from a bad sender. Anyhow I hooked it up again while it was sitting on the floor while the wife checked the gauge. It was still not showing correctly so I decided to yank the cluster. I quickly realized I never updated to a solid state IVR on this car so I'll do just that. Hope to have er done in an hour or two.

Update of the update... Ok, so I yanked the dash out, pulled the gas gauge out, modified it so it doesn't put out voltage and installed it back into the dash. I also added a 7805 regulator which I had built up on a small pc board along with a couple of filter caps and a teeny LED to show 5vdc output. Turns out the gauge was bad so I replaced it with the last one I had in stock and it worked just fine. It's now back in the car and the test drive showed the temp. gauge right the first fat mark and fuel gauge reading correctly.
 
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