Testing Fuel Gauge

[IIA]

My fuel gauge has been acting funny - reading full when full but empty when there's still fuel in it. So, after checking the sender and the voltage stabilizer, I tested the gauge today. The results were off so I want to make sure my test was reliable before I order a new one. Does this sound right:

I removed the gauge and connected the terminal nearest the F side to the positive terminal of a reliable 10v power source. I connected the E terminal to one side of an adjustable resistor (to simulate the sender) and the other side of the resistor to the ground of the power source. I ran 10v through the gauge at various resistances from 0 to 260. I got those numbers from a chart in another post. Below are the results I got. All the numbers are rough. I estimated what I think the gauge reading should have been on the percentages in the chart and a 12gal tank.

Ohms	Gauge	Should Be
260	Below E	E
207	E	~1/4
135	~1/8	1/2
72	~3/8	3/4
42	~5/8	7/8
10	F+	F
0	F+

So, assuming my test setup was OK, the next question is whether the gauge can be fixed (it's a Smith - probably original).

[lumpydog]

Mark!

[mearstrae]

Did you check the sending unit for OHMs? Or make sure the float itself actually, floats in a bucket of water? It may well be the sending unit that's at fault.

[SafeAirOne]

Mark!

I only have a ohms vs. temp reading for the temp gauge, I think.

Since the variable resistance of the sender is linear, if you know the ohm range of the fuel sender, you can just plot this on a graph (or just do the simple math) and figure out how many ohms= 1/4, 1/2, 3/4, etc. on the gauge.

[SafeAirOne]

Oh...Make sure the + and ground wires are on the correct terminals on the sender. Reversing them will give you wonky readings. If you only have 1 wire/terminal on your sender, make sure the sender is properly bonded to the tank and/or body/chassis and that the tank is properly grounded to the chassis

[lumpydog]

I only have a ohms vs. temp reading for the temp gauge, I think.

Found it - see Mark's reply here.

This was the first (of many) time that Mark bailed me out where I got stuck.

[IIA]

Thanks. I've seen Mark's post and tried all the other suggestions. I think the gauge is the only place the problem could be.

[SafeAirOne]

Found it - see Mark's reply here.

Huh...I really don't remember making that graph, though it seems like something I'd do if I had a brand new sender in-hand (like I did when I installed my long-range aluminum fuel tank a few years ago).