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I'm having issues with my gas sending unit in my 1950 3100.
Gas sending unit is new. When testing the sending unit outside the tank and moving the float arm it works as it should. When installed the unit sometimes shows empty or a little pass empty.
Outside the tank instrument fuel gauge works as it should When manually moving float arm up and down.i noticed when I'm tightening the screws I get no reading at all. I'm totally confused on what could be the issue. Yes there is gas and I traced the wires for loose or frayed wires there are none. Any suggestions?

You will get an empty reading if the wire attaching to the center stud on the sending unit is touching ground or if the stud or anything electrical from the stud down to the wire-wound rheostat is touching ground.

There should be two wires connecting to your sending unit. One connecting to the center (and this one is your signal wire going back to your fuel gauge) and another connecting to one of the 5 screws holding your sending unit in place and from there to your cab (the ground wire). Do you have both of those? If you don't have the ground wire your gauge will register only intermittently in all likelihood.

If you don't have the ground wire the gauge will either read right (because the tank is grounded elsewhere) or will read full, not empty like the OP reported.

Tightening what screws? Where is sender and tank and gauge when this happens? Explain carefully and completely.

That's what I have. When not installed in tank I hook up signal wire and tightened nut then I place the ground wire on the top of unit . The gauge goes to empty as I raise the float the gauge rises up to full. At wits end

When you test it outside the tank, are you holding it upright the same way it sits in the tank? If the pot is loose/wonky then orientation would matter.

Most fuel tanks in the AD trucks were isolated from the cab using either foam rubber, hard rubber or other material. This was to prevent the tank vibrating against other metal parts (safety issue since the tank was inside the vehicle). So it must have the extra ground wire. You've said your sending unit works when it is not inside the tank but doesn't work correctly when installed in the tank. Is there any way you can post an image of your sending unit? Was it a generic unit and did you have to adjust height and length of the float rod? Is there a possibility your float is rubbing against the side of the gas tank? There should be only one way the sender can be installed (so that the float will travel parallel to the tank sides but not touching them), but in 60 years of working on these I have seen situations where the sender was assembled incorrectly and could never work right because of interference with the walls of the tank. If it is working fine when it is not in the tank, then obviously something happens when it is mounted in the tank that changes things. The more information you can give us the better our answers will be.

Additional question: where are you getting your ground wire when you test it outside the tank? You say that you set it on top of the sender, but where is the other end connected to the truck?

Are you using that same wire when it is in the tank?

Sending unit is from summit racing and is correct for year and model. I tried 2 different gaskets 1 cork 1 rubber. I tried installing so the float arm facing drivers side and then float arm facing passenger side both have same effect. There are no obstruction in the tank as it is also brand new. No baffles etc. What I can't figure out is why it will register outside the tank and not register when completely installed. As I tighten the grounding wire to the top of sending unit I get a little bump just above the E mark in the gauge reading then goes back to empty.
Possibly a really bad ground when installing it to tank? But why does it work outside the tank using the same wires?

Ground wire is coming from the frame. Yes I'm using the same wires inside and outside the tank. Gas tank is new should I rough it up with sand paper to remove any coating? BTW it's about 3/4 full of gas so I can't use grinder

Try a different screw on top of the sending unit for ground.
I am assuming when you check it out of the tank ,you are holding the
ground wire on top of the sender. You might be the ground and the frame
needs to be scuffed up where the ground is attached.

Can you post a picture of the "Summit" sender?
And the top of your tank?

Will do

1. The idea of the green wire is to ground the sender metal to a good chassis ground. Which it seems you have done. There is no reason to worry about grounding the tank.
2. Your terminal crimps are sketchy looking and the center tap terminal should be away from the flange and bent up a little. Turn the red wire terminal around so that wire (insulation) comes straight into, and inside of, terminal sleeve.
3. The 5 screw pattern on the sender only allows sender to be mounted one way. Don't know how you are orienting the sender two different ways.
4. We have asked you to explain things several times carefully and completely because this gas gauge stuff is hard to catch. We see this question a 100 times a year. We are not there and trying to see what's up. So we keep having to ask a bunch of question about what you did and how you wired and how you tested.

So now I will ask what everyone wants to know.
1. One time you test with tank mounted in truck and sender mounted in tank and gauge mounted in dash, right?
2. Then you test with tank mounted in truck, sender loose and gauge in dash, right?
3. Both tests, wires are as we see. Wires not disconnected from sender or gauge or ground, just lift sender out, right?
4. When all are mounted properly, gauge reads near empty with lots of gas in tank. Right?
5. When sender is held outside tank with all wire still hooked up, gauge shows correctly on gauge when arm operated by hand, right?
6. You said..... "i noticed when I'm tightening the screws I get no reading at all." I asked you what screws you are talking about in that sentence, YOU DID NOT ANSWER. We are looking for clues here so help us out.
7. You said....."When not installed in tank I hook up signal wire and tightened nut then I place the ground wire on the top of unit". What does "place the ground wire on top of the unit mean? How are you attaching the ground wire to the "unit" when it's hand held ????????....and let's don't call things "units". I assume you mean sender.
I am focusing on this being a clue. Like good ground when sender is out and bad ground when sender is in. So need to know how you are grounding the green wire when sender is out verses installed on tank and putting terminal under screw head and tightening the screw down on tank sheet metal. Be sure ground terminal is touching tank flange directly and not on top of a washer. The washer could be rusty or corroded or not conducting. Ground should still conducting thru the threads but you never know.
All we have is questions and your answers and your detailed explanations to go by. We know how it's wired, we know about ground. We are trying to catch a mistake or malfunction. please notice all questions and answer EACH ONE. We will get it solved that way. We have lots of experts here but the key is we need to visualize what exactly you are doing so we can suggest tests.

The whole problem with grounds of any kind is that the motor has rubber motor mounts, the body has rubber mounts, the tank has rubber pads, the bed has rubber sometimes. The gauge frame needs to be grounded. the needle sticks, the sender has a gasket. The sender ground relies on screw threads. Light housings have gaskets. Rust, corrosion, grease and paint interfere with grounding........So all those braided ground straps on our trucks are real important as well as clean, bare metal ground spots.

Are you sure that your float floats? I have had a world of trouble with bad floats on brand new replacement sending units. Three in a row at on point. What float suppliers who have been making floats for sending units suppliers are calling "solder" is a joke.
I ended up using plastic floats designed for Jeeps.

Very good point. That hits the jackpot for why it works hand held and not in tank. Sinks and shows empty. Man I wish I would have thought of that. Oh, wait, I did but just didn't have time to post it. Had to do my hair.

Surely he would have noticed the float being full of fuel when he took it out for the test.

When someone has a problem and seems to be doing things right......and there still exists a problem, there are a lot of surelys we can't eliminate.

Ah vey

Why does it always happen to me

Cheapskate, was that the problem?

I appreciate your feed back to help get this sending unit working , so I ll explain 1 more time in as detail as I can.
1 tank is new
2 sending unit is new
3 wires that connect to sending unit are new
4 wire that comes from gauge to new sending unit registers 30 ohms when checked
5 ground wire is grounded to underside of cab
6 red wire is connected to top of stud protruding from gas sending unit
7 green wire is attached to any of the 5 hold down screws to secure sending unit to top of tank.
8 when the gas sending unit is outside the tank and in upright position, with red wire secured to stud and holding the green wire and touching the top of sending unit I raise and lower the float arm with the key on the instrument gauge shows empty when float is all the way down and shows different levels of gas as I raise it up.
9 when installed in tank and wires are connected it reads slightly above empty when I turn the key to on position.
10. There is no corrosion as everything is new.
I tried with both cork and rubber gaskets and still come up empty no pun intended.
11with the key on and installing the sending unit I get a bump to about a quarter of a tank showing on gauge , I have at least 3/4 of a tank of gas.
12 so there it is in a nut shell
13. Installing a new unit in a new tank with new wires and gasket should have been a cake walk but no it's the nightmare of my week

No apparently there is something ridiculously wrong with either the grounding. I'll try hooking up to negative ground on battery and see if that does anything.
To be honest I may just say ##### it at this point and carry a can of gas with me.

Hi Cheap,
Please make certain (using your handheld meter) your ground wire is making a good connection and then please bend upward (enough that it clearly does not touch the mounting part of the sender) the center (red wire) connection. It may simply be the image you've sent, but it appears there is a chance it is making contact with the part of the sending unit that will be grounded. That (as has been pointed out here several times...maybe several hundred) will cause your gauge to read incorrectly. The whole fuel gauge and sender works on the principle of a balanced magnetic scheme and not to bore you or anything but variable resistance to ground and two differently wound electromagnets (along with a small hourglass-shaped permanent magnet) are what makes it all work. You are getting an incorrect variable resistance amount once your sender is installed it seems. Since it is new, I'm going to assume your float is good. It ought to be. At empty your sender should be sending zero resistance (resistance to ground that is) to your gauge. At full, 30 ohms. Naturally at 3/4 of a tank around 22~23 ohms. Our chore here is simply to find out why the resistance you're getting is more like (what I'd expect) to be maybe 8 ohms with 3/4 of a tank.

What happens if you run a second ground wire from the wire on the sending unit screw directly to the battery?

I`d move the ground to the frame. Or BATT.

Same thing as if (as a youngster) you stand on the corner and throw rocks at passing vehicles. You get double grounded.

Key in Off position I get a reading of 8.2 on multimeter
Key ON . I get a defaulted 1 reading and fuel gauge moves from E to almost a 1/4 tank.
I tested float by placing in a 5 gallon bucket of water and it works as it should.
Is it possible the fuel gauge is bad? Even by raising the float arm manually?

1. I don't know what trying cork or rubber gasket has to do with anything but we were asking if you have tested the float on the sender in a container of water to see if it stays afloat for awhile. You now say you tested it.
2. I asked if gauge was mounted in dash on all tests. Never moved from there and wires not changed.
3. I said be sure the ring tongue terminal on the green wire is against the sender metal, not a washer. Like when you held it against the sender by hand.
I will repeat ......
I am focusing on this being a clue. Like good ground when sender is out and bad ground when sender is in. So need to know how you are grounding the green wire when sender is out verses installed on tank and putting terminal under screw head and tightening the screw down on tank sheet metal. Be sure ground terminal is touching tank flange directly and not on top of a washer. The washer could be rusty or corroded or not conducting.
4. Don't know what

You have shown the sender to be working when out of tank. So it's not faulty. We are looking for something different between testing out of tank and in tank.

So far these things
1. Float??????????????
2. Ring tongue touching directly to sender when sender mounted in tank.??????????????
3. Wiring left in place except the green wire. When testing, was ALL wiring, except the green wire on the sender kept in place. not moved or disturbed??????????
4. Gauge or it's wiring not disturbed.??????????????
5. Float or arm not binding inside tank.????????????

We understand all is new. We understand your wiring. We, including you, are trying to see what changes between hand test and installed test. THIS IS THE KEY. Focus on that with us.

https://tinyurl.com/yvk8mskb

That is an article I wrote about the AD fuel gauge a while ago. Your gauge probably isn't broken although if you've opened up the gauge cluster there is a chance the needle could be binding on something. If you were an electronics guy, you could go get a common 33 ohm carbon film resistor, remove the sending wire from the sending unit, turn on the ignition and connect that resistor between the sending wire and ground. Your fuel gauge should jump quickly to Full and try to go a bit further. Here is something you can check: remove both wires from your sending unit. Using your handheld meter, set the thing for resistance and measure the resistance of your new sending unit by holding one lead to ground and the other to the center post of the sender (where your signal wire will connect). Please do that and let us know. At 3/4 tank it should be somewhere in the 20 ohm range...maybe 22~23.

Ok as I explained this several times.
You are not helping me
The fuel gauge and instrument cluster has not been removed.
Green wire is mounted directly on top of sending unit no washers with screw.
Red signal wire sitting on top of insulated washer not touching anything but the screw post securely tight.
Tried running a ground to sending unit from negative side of battery SAME results.

Well, we're all trying to help. What will help us help you is to know (with the 3/4 tank of gas you have) what the resistance being fed to your gauge is. You can tell us that by disconnecting both wires, setting a handheld multimeter to resistance (there may be a low setting and a high setting or it may decide automatically for you), touching one of those leads to the center post on the sender and the other to ground.

Wanted to bring your image back up .... maybe it'll help.

Jon G
Please explain
If I understand you correctly you want me to remove both wires from the sending unit and see what the ohms are?

If not please explain on exactly what needs to be accomplished

Thanks Peggy,
Let me take that image and add the points between which we need a resistance number. Hope they show okay.

Yes, exactly. I need the resistance between the red wire and any good ground point...or you could use the resistance between the red wire and the green wire (if the green wire is a known good ground point). I'm looking for 20 to 25 ohms with 3/4 tank of gas. That will be a large clue.

Jon
With wires disconnected I get a reading of 7.7

Thanks! If 7.7 ohms is the reading, that will equate to almost exactly 1/4 of a tank of gas. Either your sending unit doesn't work when it is submerged in gas (a possibility) or the float isn't floating (but you say you tried cork and other types of floats) or I suppose it is possible there is something in the way the rod is set up that might be causing your sender to be in the wrong spot when it is installed. Was that rod very long or very short? Some of the senders require you to cut the rod to length.

Jon, he said cork and rubber GASKETS and I mentioned that in my last post. He did say he "tested" the float.

I'm not sure on how to attach a picture to this posting but I'll attempt it.
When I submerged the sending unit in a bucket of water it floated to the top as I raised it out of the water the float went totally down
So I assume that the arm and float are working correctly.
Is it a possibility that the wires connected to the gas gauge are reversed? Asking for a friend
Do you have any suggestions on how I can raise and lower while just sitting inside tank without being secured? Maybe a string?