fuel tank sender reading full all the time

I have removed the fuel tank and the sending unit. I switch on ignition and fuel gauge still reads full. With ignition on, I put a test light on the sending wire at back of car and it does not light.. With ignition off and a 9V battery supplying power to the test light, It still does not light.
So I am about to check under the hood. I plan on finding the wire that goes from gas gauge to sending unit. need to know how ignition switch powers dash gauges and turn signal. At this time the green wire at the blinker switch is not connected because the blinker is electronic. Turn lights do work but dash light indicator does not. If I hook up the green wire to flash unit, then it only part way works. Does that green wire that is disconnected cause any problems witfuel sender? I think the fuel gauge was working this way for sometime. I think one of the marked wires goes to fuel sender.

The sender applies a resistance to ground to the gauge. when the sender wire is completely shorted to ground it makes the gauge read full or empty depending on the model of gauge and sender you have. At some point they swapped it around.
The gauge movement is not enough to light a light. If you have the sender out, connect an ohmeter to the output going to the gauge and the other lead to the housing of the sender. You should see the resistance changing as you lift the float up and down. I have worked on a bunch of these. In order for the the sender and the gauge to be accurate, they must be calibrated together. There are some poorly made senders out sold new that are just not right without doing some internal work on them.

Kim

Sim, are the sending unit and gauge from a Bugeye (as shown in your avatar)? If so, the gauge will read full when it is not provided a path to ground via the green/black wire and sending unit.

The early cars had a different gauge system which operate from roughly 10 Ohms = Empty to 90 Ohms = Full. (The exact values can vary a bit). You'll notice that as resistance increases the gauge goes to full. With the sending unit wire disconnected, the resistance appears infinite so the gauge goes to full.

As Kim said, when you are ready to put this back together you'll get the best results by calibrating the gauge to work with the sending unit. You can do that by following the steps on the MGA Guru website.
MGA Guru Link

dklawson said:

Sim, are the sending unit and gauge from a Bugeye (as shown in your avatar)? If so, the gauge will read full when it is not provided a path to ground via the green/black wire and sending unit.

The early cars had a different gauge system which operate from roughly 10 Ohms = Empty to 90 Ohms = Full. (The exact values can vary a bit). You'll notice that as resistance increases the gauge goes to full. With the sending unit wire disconnected, the resistance appears infinite so the gauge goes to full.

As Kim said, when you are ready to put this back together you'll get the best results by calibrating the gauge to work with the sending unit. You can do that by following the steps on the MGA Guru website.
MGA Guru Link

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Yes 1958 bug eye so I guess this is an early car? I just got my hands on an ohm meter. will get back Thanks for your help

so it could be as simple as where the wire connects to the sending unit. Like it needs cleaning. I will check
Thanks
rod

The fuel gage on my 60 BE has a separate ground to the gage housing. The ground is shown on the wiring diagram but you need to look closely. Gage will not work if not grounded.

Agree with Joe. Mine on my BE wouldn't work till I noticed another car had the ground going to the stud on the back of the gauge that clamps the gauge in the dash. Moved the ground wire to that spot and it works fine.

nomad said:

Agree with Joe. Mine on my BE wouldn't work till I noticed another car had the ground going to the stud on the back of the gauge that clamps the gauge in the dash. Moved the ground wire to that spot and it works fine.

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Just now getting the tank back in, Will check later today Thanks
Rod

Thanks for all the help. you all saved me a lot of time going in the wrong direction. I was looking for a short in stead of a bad ground. I think the ground was bad at the tank or the connector under the hood was not tight. Again thanks Fuel gauge is now working.

Great ! I'm glad you got it working again.