Technical Should I buy a fuel sending unit first?

I have a 1950 Ford Tudor V8. I filled it with gas but the fuel gauge in the dash does not move. Should I start by replacing the fuel sending unit in the tank? Is there other things I can check first to see it the problem is somewhere else? Thanks

 

A bit of diagnostic work might save you some time and money, instead of just throwing parts at it.
Try disconnecting, then grounding the wire at the sending unit first. With the key on, wire disconnected, the gage should go all the way full. Ground the wire, then it should go to empty. Some systems will be the opposite. At any rate, if the gage moves, then it is probably the sending unit. If it still doesn't move, look elsewhere.

 

Always diagnostics first, unless you like spending money on parts you often don't need. Especially important today when so many parts seem to be horribly bad quality and many parts come defective even new in box. You want to know what the problem is, not just throw parts at it hoping the problem goes away.

 

The '50 Ford will go to full when the gauge sending unit wire is grounded; empty when just disconnected. If that much works properly it may be that the tank isn't well grounded. I usually drill a hole in the outside edge of the tank (outside the perimeter weld) for a screw for a ground wire running to the frame. If none of that shines a light on the problem further diagnosis is required.

 

"Knowledge of operation is the first step to proper diagnosis"

 

this will sound real hokey,.. but here's where I always start... put a bunch of lucus in the tank and drive it.... lot of times the float is stuck, then Shazammm lucas free's it up, plus, you need to do that anyway...

 

These fine folk have you covered. I will add a couple things that should be part of any electrical diagnostic process.
Try NOT to disturb anything you aren't actively inspecting and checking. This means don't just reach under the dash and wiggle everything. If it causes the problem to go away, you have just moved the problem from something you can find and fix to the same problem some time later, probably at a bad time.
Understand the circuit. If you don't know what it does and where it is, it's like wandering in a forest lost. The fuel gauge gets power when the ignition is on (is not working all the time). Some vehicles have lower voltage for the gauges. This goes to one side of the gauge. The other side of the gauge goes back to the sending unit, which had a float attached to a variable resistor. The resistor varies with float level, which shows as the sweep of the gauge. The body of the sender must complete the circuit to ground.

So the whole thing is; power, gauge, signal resistor, ground. Mechanical parts are ignition switch, gauge, sender. Circuit parts are wiring between the mechanical parts, and often a fuse and the connectors in the fuse block.

When diagnosing and checking your work, ensure it doesn't work or does work repeatedly. Nothing is more frustrating than intermittent electrical problems. It fails, you find the time and place to check, you start to test things and suddenly it works. Now you can't continue to test, and the root cause isn't found and fixed. So make sure it really is failing, and keep checking as you go. Once you have found and fixed it so it works, keep trying the feature to ensure the fix did solve the issue. Comebacks stink, but not making it back to the garage stinks even more.

Confirmation! This may sound strange, but once you have found and fixed the problem, you want to confirm it's really fixed. At this point you DO want to wiggle things. Either manually, by grabbing the harness and shaking gently, turning on and off any switches, adding or subtracting fluids or temps, or just road testing.
Ford had a great feature on early computer controlled cars. Under normal operation, it would only set a code if it sensed an issue over time, ignoring little things. You could go in and set it to remove the time part, making it hyper vigilant. It was called the wiggle test.
Any shop worth it's sign out front will ask "When is it doing (the problem)?" It's so it can be tested under the conditions when it happens. Doing your own diagnostics should include rechecking under the conditions when the problem is seen.

 

One more thing for this exact situation. IF you diagnose the sender as the problem and have the new one in hand, connect a jumper wire from the sender body to ground, connect the sending unit wire and try out the sending unit out of the tank first and check the gauge readings BEFORE installing it.
Full, half and empty.
This makes sure it will work, will provide full sweep and the problem is resolved before you install it.
It's often not easy to return electrical parts, but it's even worse if it's been installed. Saves you time if it's not working properly, too.

 

Ford's gauges work differently than the more common resistance system. If the sender is the problem, you can take the cover off and carefully inspect and possibly clean the points and get it working again.

An aftermarket resistance sender will not work quite right with the stock gas gauge.

 

After what Rich B has said about resistance system and you have to to replace the unit have a look at Bob Drake one.

 

Mine stopped working.
Grounded the feed and the gauge moved. Removed the sending unit. The float was stuck against the tank side wall.
Bent the float rod a tad.
Done

 

Multimeter.