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Over the weekend I dropped my gas tank to replace sending unit and replace fuel filler and vent hoses.

I pulled the sending unit. And bench tested it. Empty 68 ohms. Slowly moving the float to full. The ohms went lower and lower then jumped to 85 ohms and at full position it was "open".

Okay.
Sending unit is junk. I tested the new one. Empty 73 ohms. Full 11 ohms. Smooth numbers while moving the float.

I have a old B body tank. I cut a 4 inch hole in the tank. I installed the old sending unit to see if there could be any adjustability in the float arm to tune it if needed?

A visual inspection. It is obvious that its a fool proof float setup. You can't even install the sending unit upside down by accident!

Now I have to weld up my old tank.

Okay.. tank went back in with all new items. I re tested the sending unit empty with it in the car and grounded!

73 ohms. Perfect!

Lower the car. I put in exactly 10 gallons of gas. (Half tank)

Start the car.
Fuel gauge says 1/4 tank.
The old sending unit with 10 gallons would register empty.

I have some improvement. I plan on running a new wire from terminal "C" to the sending unit.

I also have a new mother board with the intergrated voltage limiter.

What have I missed?

Thanks again
Joe

Also:
I have not yet tried the sending unit wire to ground test. But will be doing it today. The next test I will do is half tank ohm reading. Just a pain to jack up the "Whole" car to keep it level for that test..

Not sure what to tell you, but you could take the float out and put it back into the old tank with the hole to take a reading at half. You can remove the float with the tank in the car.

The sending unit needs to be grounded. The factory used a metal strap from the sending unit to the fuel line (jumps over the piece of rubber line connecting the tank to the line). If this is missing you will not have proper ground. If it's there, check to see if the fuel line is rusty and clean it if it is. http://images.search.yahoo.com/images/vi...=yfp-t-745

Update:
Sending unit is grounded! Chased blue wire up thru floor and along driver side sill plate. Found 6 x plug behind kick panel and emergency brake assembly.
Unpluged the connector and it was crusty!
Im going "Whos your Daddy"?

I cut and soldered in a by pass wire.

Now it reads empty! I grounded the blue wire with key on and it pegs the gauge.

Either im really out of gas? or WTF?
I dont wan to FILL it 100% incase I have to drop the tank AGAIN

I started at half tank with 37 ohms showing. fixed the crusty wires and im at 21 ohms. ( closer to full ) yet showing less than quarter tank now.

What is the ohm reading at the tank with the alleged 1/2 full gas?

37 ohms. I started with a level car. Had some idle time.

When I put my tools away and threw in the towel for dinner I was at 21 ohms. And the needle at 1/8 tank.

Okay, here is a little info on your issue, please refer to diagram above:

The coil in the tank unit is a variable resistor, or rheostat, which can be shorted out along its length by the action of the arm attached to the float, in effect when the tank is empty all the coil in the tank is in action (full resistance) but when the tank is 'Full' none of the coil in the tank is in action (little or no resistance). When the amount of coil in action in the tank changes, because it is connected across the coil in the dash gauge, it changes the effect that the coil in the gauge has in terms of magnetism. This change effect the balance of the magnetic fields in the gauge and the magnet swings moving the pointer. When the tank is 'Full' it effectively shorts out the coil in the gauge and the magnetic effect of the second coil is removed and the pointer swings to its fullest amount.

So in effect: the full resistance of 11 ohms is way too high. Remove the tank unit and readjust the float so that the resistance is as close to zero (or shorted) as possible. The wiring should also be checked from the tank unit to the gauge unit to verify that it is as close to zero as possible.

The proof: if you take your last reading of 21 Ohms and subtract 11 Ohms, you have 10 Ohms which is a much lower resistance and should in effect shows a higher (fuller) reading on your gauge because as you lower the resistance, the dash gauge shows closer to full.

Good Luck.

Thank you..

I agree with you Fratzog that I have a 11 ohm difference going by actual fuel level and the gauge reading. All research says these run a 9-11 ohm full and 68-73 ohm empty. If I was to adjust it to "Full" = 0 ohms? Will I ruin the gauge?

My new sending unit does have a set screw to adjust.