Fuel guage question

[LM44S]

After replacing the fuel gauge in my 38 pickup it registered full when it only had a couple of gallons in it. New tank and sending unit. Understand that low voltage could be the problem. Checked it out and have 6.3 volts to the gauge going in and 3.8 coming out. Does that indicate a faulty gauge or do I need to add a ground wire somewhere? Thank you for any advice.

[19Fordy]

Is your gauge compatible with your sending unit? King Seely gauge will only work accurately with a matching King Seely sending unit. They don't work on the Ohms principle.

[JayChicago]

Agree. Modern sender will never give accurate reading with an original gage.

But yours seems extreme.(needle always moving full scale) That’s not a lack-of-ground problem. Just the opposite, indicates possible short to ground. To test, remove the wire at sender and keep it from touching ground. Needle should then stay bottomed. If it does that, gauge and sender wire probably OK, problem is with the incompatible sender.

[LM44S]

Ok, the gauge will not work with the sending unit but I do not uunderstand why there is 6 volts going to the gauge and only 3.8 volts going out of the gauge?

[JayChicago]

Originally Posted by LM44S

I do not uunderstand why there is 6 volts going to the gauge and only 3.8 volts going out of the gauge?

The gauge itself has some internal resistance, so it's output to the sender would be expected to be something less than the 6 volts going in.

I have never put a voltmeter on a gauge's output to the sender, but your 3.8 volts seems reasonable to me.

[19Fordy]

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On my 40 I get a reading of about 6.3 V coming from the gauge wire that runs from the gauge to the gas fuel tank sending unit.
Carefully check your wiring with this 40 Ford wiring diagram.
https://www.fordbarn.com/forum/showt...wiring+diagram

Just for the heck of it try using only a 6V dry cell to supply Voltage to your gas tank sending unit. Make sure your gauge is not grounded against your dash etc. Look here:
https://www.fordbarn.com/forum/showt...ford+gas+gauge

I also ran a separate ground wire from the gas tank sending unit to the frame to make sure it has a good dependable ground. Normally, the gas tank sending unit is grounded by being attached to the metal tank which rests on the frame. A separate ground wire is good insurance.

[JayChicago]

Deleted. Not sure if what I was saying is correct, so better to not say it.

[LM44S]

Finally got a chance to work on my truck, grounded the sending unit to the frame with a separate wire and grounded the frame that holds the gauges to the dash with a separate wire also. The gauges read full and according to the manufacturer of the sending unit that indicates a break in the wire to the unit. Ran a jumper wire from the back of the gauge to the sending unit and stiil read full. Not sure what to try, any ideas? Thanks.

[JayChicago]

Originally Posted by LM44S

The gauges read full and according to the manufacturer of the sending unit that indicates a break in the wire to the unit.

You were given incorrect information. Some gauges work that way. My '58 Chevy works that way. But on all four old Ford gauges, if connection to the sensor is broken, needle moves to worse-case position. Fuel gauge needle moves to E.

When you ran a jumper wire to the sender, was the original sender wire disconnected at both ends? One thing that could explain needle always on F is a short to ground somewhere along the sender wire. That could also explain your low voltage reading on the gauge sender terminal, cuz voltage is dumping to ground.

[Mickd]

Jay &19Fordu are right. However guys, If you use a 73 -10ohm sender you CAN get a reasonably accurate reading. You might have to adjust the float position a little though.
After mucking about with my original bimetal KS sender unit I finally bit the bullet and bought a 73-10 ohm unit just to see how accurate it would be. I wasn't expecting it to be as accurate as it is ! Pleasantly surprised actually. It's been in my 52 Customline for about a year or so now.

[19Fordy]

Mickd: That's good news. Please keep a record of the gas tank readings at FULL, 3/4, 1/2
1/4 and 1/8 so that we can get an idea of the accuracy and how much gas is actually in the tank at those readings. Thanks.

[JayChicago]

I was able to get my modern sender to give an accurate gauge reading near empty tank. Gauge remains very inaccurate in the upper range. Fed the sender wire thru a 9-ohm resistor just ahead of the sender. Required trial-and-error to settle on the 9 ohm. (driving till emty with a gas can in the trunk) I suspect every sender, with different float arm adjustments, would be a little different.

[19Fordy]

Thank you Jay for your helpful info.

[mgm60]

LM44S, I'm dealing with my own Fuel Sending Unit problems and sharing what I've read. If the wire at the Sending unit isn't connected to anything, the gauge should read empty or bottom of scale. If you ground that same wire the gauge should read full or top of scale. If those two are true your gauge is good as is the wire to the Sending Unit. That test worked for me so I know I have a good gauge but can't get anywhere near an accurate measurement with the aftermarket unit.

[hueyhoolihan]

there are only two or three ways gasoline levels are registered. as mentioned previously, the sending unit and the gauge must be compatible. old sending units from the 40's vary the current with a rheostat in the sending unit. the gauge for those types heat a coil with a bimetallic strip attached to the arm in the gauge. modern units have nothing in common with those types. if you're using an original type of gauge it's highly unlikely a modern sending unit will work.

addressing compatibility, if indeed those are the circumstances here, would be my focus.

to test your gauge, i would simply send some current through it using a battery (possibly a 1.5v battery would suffice), so as to heat it's coil and so. bend the bimetallic strip inside and the needle. but first so as to not waste time, i would check if for continuity. a working gauge should have continuity.

if the gauge has no continuity, or if the sending unit has no continuity, or if the wire running from the gauge to the sending unit has no continuity or the sending unit is not grounded, the gauge should read empty. if there is a short in the circuit AFTER it leaves the gauge it could create a fire, but most probably the gauge would read full (this may have been mentioned in a previous post).

[JayChicago]

Originally Posted by hueyhoolihan

old sending units from the 40's vary the current with a rheostat in the sending unit.

Wrong again!
Modern units use a rheostat. Our original senders used a timed-pulse technology.

[hueyhoolihan]

Originally Posted by JayChicago

Wrong again!
Modern units use a rheostat. Our original senders used a timed-pulse technology.

in answer to your polite response, i won't argue.

but i will suggest one might want to take another look at when timed-sensor technology was invented and became common usage in automobile gas gauge sending units. and how likely that it would be implemented in the 30's or 40's cars. as it would likely require solid state circuitry capable of measuring electro-magnetic waves involving light-speed over a distance of inches. something that is beyond the capabilities of 30's and 40's era analogue circuitry.

the timed/pulse technology (common in 50's Ford sending units) that was mentioned, was dependent on a rheostat. modern sending units, AFAIK, are not.