55.2-59 59 Fuel level sender Ohm values?

[BlouDon]

My 59 has a brand new fuel level sender but when full the gauge only reads a needle width above 1/2. I need to establish if the sender is wrong or if the gauge is wrong.

What should the correct Ohm values be for empty and full respectively?

I have electronic components with which to simulate the correct sender.

[mr48chev]

Original sending unit to go in the original tank should be 30 ohms according to this on Classic parts
Gas Tank Sending Unit for Chevy Truck

Replacement style fuel sending unit assembly. Fits in gas tank. Comes with gasket and pick-up tube. 30 ohms. 5/16" Line size.

1949-53 Chevy / GMC Truck
1954-59 Chevy / GMC Truck


30 ohm sending units work with original style fuel gauges up thru 1966 trucks. 90 ohm sending units were used on most 1967 and later trucks.

That is for this sending unit https://www.classicparts.com/1954-59...ctinfo/23-525/

The sender for the 67/72 V8 has a 3/8 line rather than 5/16 and is 90 ohm. https://www.classicparts.com/1967-71...ctinfo/23-530/

I'm thinking that the later ones do go right in the early tanks.

Best way is to make sure your gas level is down below half and then pull the sender out and hook it to an ohm meter.

[BlouDon]

Originally Posted by mr48chev

Original sending unit to go in the original tank should be 30 ohms according to this on Classic parts
Gas Tank Sending Unit for Chevy Truck

Thank you !!

Is that 30 Ohms when full or 30 Ohms when empty?

[BlouDon]

Originally Posted by BlouDon

Is that 30 Ohms when full or 30 Ohms when empty?

30 Ohms when full. Tested it today.

[mr48chev]

You had me on that one as it has been a while since I tested a fuel gauge sending unit.

The last time I did it was checking the "universal" sender on the 30 gallon tank in the front of the bed in my 71 GMC about 15 years ago.

[dsraven]

A fuel gauge is basically a needle held between electromagnets. One magnet is a constant and the other is variable depending on its ground provided by the sending unit. If you remove the wire from the sending unit the manetism from the variable side will be gone so the needle will be drawn to the manet that is constant. That means the gauge will read all the way over to empty or full, depending on which side the constant magnet is placed when thegauge was built. If you ground the sending unit wire the gauge should pin thd opposite way. Doing this would test the gauge opetation. Now, with that said, you can see the importance oc having a dedicated ground to the cab and also from the tank sending unit to the cab. Otherwise the ground from the sending unit to the cab relys on the tank touching the cab somewhere. It is supposed to be sitting on some rubber or othet kind of mount so getting a good ground to the sending unit could be a challenge, relying mostly on the tank straps. Possibly a dedicated ground strap from the sender to the cab sheet metal and also from the cab sheet metal to the truck frame, engine block or battery would help you get the gauge to read accurately. Of course there are other reasons why the gauge could read wrong, like a poor float or a bad sending unit, but dont forget the othet part of the circuit. The circuit is only as good as its weakest link.

[BlouDon]

Originally Posted by dsraven

The circuit is only as good as its weakest link.

Or as good as the installation....

In my case the sensor was reading 21 Ohm with a full tank of fuel which corresponds with the 2/3 reading of the gauge. It would seem therefore that the sensor arm was possibly not set correctly.

[dsraven]

I would pull the sensor out and swing the arm through a cycle like it would see in the tank. leave the connector attached and install a ground jumper wire. turn on the key and see how it performs. ensure the hole in the tank is covered and sealed as good as possible first of course. a quick sheet metal patch with the holes in the right spots would be a good idea.don't wanna burn down a nice looking truck like that one. or any truck for that matter.
while it's out check the float to ensure it isn't water logged with fuel. while the sender hole is open you could also check inside the tank, visually, to see if it is clean.

[BlouDon]

It would matter more to me if I get burned :-)

[dsraven]

yeah, there is always that.....
remove the sender and plug the tank hole. use test wires to check the movement from a distance? pull the tank right out?