What should the resistance values be for the Sending Unit?

[McGyver]

I have followed the directions on the sticky topic for restoring the fuel gauge and am in the testing phase before I seal it up with new glass and the bezel. The gauge is trying to work, but the needle doesn't want to go down past half mark. If I disconnect the sending unit from ground the needle seems to sit at a good "E" position (I quit putting the face plate on after I started having issues).
As for some numbers:
My fuel gauge reeds about 35 ohms between the two pins, 50 ohms from the switch pin to ground, and 85 ohms from the sender pin to ground.
Sending unit goes from about 1 ohm at full to 65 ohms at empty.

Playing around with some resistors, I put about 200 ohms of resistance in place of the sending unit, and the gauge still jumped up to the 1/2 (straight up) position.

[YLG80]

Your fuel gauge coil resistance are in line with what I have found. You have measured the two coils in series.

This is how I test the gauge. The 150 ohms variable resistor allows for a full deviation

(Ford type)

Willys type

Do not forget the ground connection as shown on the schematics.
Fuel gauge ground path via the gauge housing

Yves

[YLG80]

I have followed the directions on the sticky topic for restoring the fuel gauge and am in the testing phase before I seal it up with new glass and the bezel. The gauge is trying to work, but the needle doesn't want to go down past half mark. If I disconnect the sending unit from ground the needle seems to sit at a good "E" position (I quit putting the face plate on after I started having issues).
As for some numbers:
My fuel gauge reeds about 35 ohms between the two pins, 50 ohms from the switch pin to ground, and 85 ohms from the sender pin to ground.
Sending unit goes from about 1 ohm at full to 65 ohms at empty.

Playing around with some resistors, I put about 200 ohms of resistance in place of the sending unit, and the gauge still jumped up to the 1/2 (straight up) position.

See also here for grounding problems : viewtopic.php?t=306780

[McGyver]

Yves Thanks for the reply. I'll look around for a potentiometer of the appropriate range to do some more bench testing. Your note says use one with a range from 0-150, my sending unit only maxes out at 63 ohms. Should it be able to go higher?

I also appreciate the inductance values for the two coils (although I don't think I have an instrument that will allow me to measure that easily. I have also been fiddling with the metal fork that is bent inside the one coil. In one of your comments in the "how to rebuild" thread you made mention that adjusting this would affect the deviation, but I have not been able to see any major difference from my adjustment. That same piece is connected to a curved piece that is at the center bottom of the gauge, and a flat piece that is next to the other coil but does not go inside of it. Should moving these two pieces around cause any adjustment of the needle? I have done some minor experimenting with this but still see no promising results.

[YLG80]

Hello,
The sender resistance value is one of the most discussed topic !
There is no real need to measure the inductance. It wan be done with some multimeters with capacitance and inductance options.
It can confirm that there are no windings in short circuit.
Yes you can adjust the needle deviation but it has a minor effect.

The sender resistance was a problem for me because my jeep was not equipped with a sender before I started to restore her.
There was only a "magic wooden ruler" stored under the driver seat

However, I have restored a few original fuel gauges and to test them I had to find the MIN-MAX sender resistor values.
This is why I ended up with a 150 ohm potentiometer value. (above 100 ohm for the EMPTY tank value)
This was to test the fuel gauge.

When I have restored my jeep, I have bought a Ford Vintique fuel sender to adapt it to my fuel tank.
It was a 120 ohm sender and the original fuel gauge on the dashboard was happy with that value.

However I have found information on G503 about a much lower sender resistor value.
Also I have found that many Ford trucks were equipped with 10-70 ohms senders

According to a paper that you can find here, https://www.cckw.org/pdf/fuel_gauge_problems.pdf , your sender has the correct value.

You should check if the sender resistor value is really very low when it should show a FULL tank
It seems that the current cannot increase above +/- 200mA with your sender.

Some senders have a FULL resistor value much higher than a few ohms.

Yves

[McGyver]

My jeep had an incorrect CJ style tank shoved in it when I got it, and I feel like the sender was rusted beyond repair.

I did find an old resistor pot in a box of junk electronics, and had to crank it up to about 350 - 400 ohms to get the needle to come down to the empty location. That seems way too high to me. I might just give up and know that when the needle gets close to half it really means almost empty. I just wanted to check and make sure I wasn't missing something simple before I sealed up the gauge.

Thanks for all the help,
Paul

[Wolfman]

I will agree with Yves last post. The original sending unit resistance range was around 5 to 70 ohms. 5 ohms to ground on the sending unit side of the gauge would make the gauge read full. 70 ohms empty.
So, if you set the gauge up with a 70 ohm resistance and put the needle on the gauge at empty, you should be close.
Enter repop sending units. I have seen the resistance range on these all over the place. Last repop sending unit I came across had a range of 65 to 180 ohms. The OEM gauge read full most of the time and by the time the tank was empty, it still showed 1/2 a tank. I hate walking home !!
As I recall, I ended up putting a 54 ohm resistor to ground from the sending unit terminal to bring the gauge in line. Gauge read empty while there was still a couple inches of gas in the tank.
This was a small, low watt resistor. Not a big ceramic resistor like an ignition resistor.
No math involved. I have been around electronics my whole life and have a box full of old resistors. I just experimented with resistor values until I found one I liked.

[McGyver]

As far as I know the gauge is original. (If not original it was replaced in the 50's)

So, if you set the gauge up with a 70 ohm resistance and put the needle on the gauge at empty, you should be close.

So is there a way to reset the needle's position on the shaft? It seems very delicate so I haven't wanted to risk tearing anything up.

Paul

[YLG80]

That is good.
With one fuel gauge I also had to add a resistor in parallel with the gauge.
This was the only solution for the needle to go down to Empty .
Yves

[Wolfman]

I was of the opinion, since you were restoring the fuel gauge, you had the needle and face removed and now were trying to put the gauge back together and get the needle in the correct position.
Maybe the gauge was not disassembled that much ???
First, power up the gauge on the bench with a 70 ohm resistor connected to the sending unit terminal to ground and see what the gauge reads.
If the gauge is original OEM, it should read empty or close to it.
The needle is just pressed on the shaft. removal & installation is a delicate operation. Check it out first !!

[McGyver]

I was of the opinion, since you were restoring the fuel gauge, you had the needle and face removed and now were trying to put the gauge back together and get the needle in the correct position.

Yes I took the bezel and glass out. I had wondered if the needle was just pressed on, but was scared to mess with it until I knew for sure, because I didn't want to tear anything up.

I'm out of town for work right now, so won't be able to work on it again until possibly the end of the week.
Thanks,
Paul

[Wolfman]

Go up and look at a series of pictures Yves posted. You can click on the picture and blow them up, so they are easier to see.
One of the pictures shows the gauge with the glass, bezel and can removed. The face with the level markings and the needle are still in place. The needle goes through an arced slot in the face plate and is fastened on the shaft behind the face plate. ( Note: some gauges have the shaft sticking out a hole in the face plate, with the needle pressed on the shaft in front of the face plate ).
Another picture shows the gauge assembly with the face plate removed and you can see how the needle is on the shaft. The shaft is on small jeweled bearings in a frame and the frame would have to be disassembled to get the needle off the shaft, in the above picture.
Unless you have a real good reason to go this far, I wouldn't. The face plate can be removed without disassembling the rest of the gauge frame or removing the needle.
Once the needle was removed from the shaft, it would have to be put back on the shaft, in the same location for the gauge to read correctly. This is pretty fragile work.
I don't know you personally or your skill level. This might not be an issue for you, but I wouldn't go there.