HARD SENARIO: fuel tank sending unit works dry not wet float ok

[Robert K.]

I don't know if anyone would have had this problem before, and I'm hoping there is another answer than replace the sending unit. OK here it is....

When the tank is low on gas and I raise the sending unit manually the fuel gauge registers as it should.

When I fill the tank up with fuel the gauge doesn't register....has anyone had this experience?

dumbfounded........

[Clem Z.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

I don't know if anyone would have had this problem before, and I'm hoping there is another answer than replace the sending unit. OK here it is....

When the tank is low on gas and I raise the sending unit manually the fuel gauge registers as it should.

When I fill the tank up with fuel the gauge doesn't register....has anyone had this experience?

dumbfounded........

sounds like the float ain't floating

[Robert K.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

sounds like the float ain't floating

That's whats odd about this it is floating?

[Roger O.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

I've seen later model year tank units that when the pot or rheostat goes under the fuel level they lose contact but the dash gauge goes to full.
What do you mean when you say " the gauge doesn't register" ?
I'll look at a diagram , but don't the C2s have 3 wires at the tank unit ?

[John H.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

don't the C2s have 3 wires at the tank unit ?

Yes, they do; the midyear powered voltage-divider fuel gauge system was an "experiment" confined to the Corvette, and was abandoned at the end of 1967.

The article on how to diagnose the system and two diagrams that go with it are below.

[Roger O.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

Thanks John for the info.

Thats spooky. It also appears that 0 resistance produces a full reading,thats just opposite of C3s.
John , Robert hasn't said yet what the gauge does when it stops working,if he says the gauge freezes,would that mean the 12v "I" term wire to the sender has been interrupted or the S term wire is open ?

If you remove the S term wire with the key on does the gauge go to 1/2 tank or does it go to empty ?

Spooky....12v + rheostat + ground.... scares me !

[Ronald L.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

Issue is not that scary actually, its just 12 volts!

Follow the steps in the PDF John posted. Pretty simple, and probably the sender. These were a coil and very subject to failure as compared to today's resistor cards.

What ever you get new check the resistance before you use it, many of the replacement parts out there are wrong and that will mess you up on the flip side of this one.

[Robert K.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

Issue is not that scary actually, its just 12 volts!

Follow the steps in the PDF John posted. Pretty simple, and probably the sender. These were a coil and very subject to failure as compared to today's resistor cards.

What ever you get new check the resistance before you use it, many of the replacement parts out there are wrong and that will mess you up on the flip side of this one.

Is there a procedure for checking or just hook up to an ohm meter and check for 87 + ohms of resistance?

[Robert K.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

I've seen later model year tank units that when the pot or rheostat goes under the fuel level they lose contact but the dash gauge goes to full.
What do you mean when you say " the gauge doesn't register" ?
I'll look at a diagram , but don't the C2s have 3 wires at the tank unit ?

when I fill the tank up the gauge doesn't go past a 1/4 tank.....

[Robert K.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

Yes, they do; the midyear powered voltage-divider fuel gauge system was an "experiment" confined to the Corvette, and was abandoned at the end of 1967.

The article on how to diagnose the system and two diagrams that go with it are below.

Thanks John I'll try this..... but will this solve the mystery?

[Ken A.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

Filling and draining the tank is a lot of work for a $65 part.

[Jack H.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

Per the trouble shooting by symptom table in the '63 Corvette shop manual, a gauge whose needle won't move above 1/2 full indicates poor grounding of the sending unit at the tank...

As John Hinckley pointed out the mid-year fuel gauge circuit was unique, based on a Whetstone bridge circuit topology. Why? That rendered gauge readings TOTALLY independent of actual B+ supply voltage that would vary based on alternator/generator charging output and battery condition.

The gauge pointer needle represents the RELATIVE voltage divider action between the FIXED resistance of the two inductors in the gauge (one pulls the needle toward empty, the other toward full) and the position of the center tap wiper arm on the potentiometer in the tank's sending unit.

The diagram John published from the '63 Corvette Shop Manual is MISLEADING! It shows a tank sender resistance of 87-93 ohms, but you have to know how to interpret that in terms of a Whetstone bridge voltage divider!!!

Look at the corresponding position of the gauge pointer needle in that same diagram and recognize that it's at roughly the half full/empty position. That's what the 87-93 ohm tank sender resistance legend refers to. It's NOT a spec for the total resistance of the tank senders potentiometer!!!

See how the individual inductors in the gauge are depicted as being 160 ohms? The tank sender potentiometer corresponds to a dynamic PAIR of these inductors.

Bottom line, all (*& breaks loose with this system IF you don't have TWO, good/solid, grounds (one at the gauge, the other at the tank) to complete the Whetstone bridge circuit as designed... That's why GM/Chevy actually published the circuit schematic that John supplied WITH a troubleshooting by symptom table.

It was very unique and different from fuel monitoring circuits that came before AND after the mid-year Corvette era...

[Robert K.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

Filling and draining the tank is a lot of work for a $65 part.

LOL..... yes it is.... if that was what was happening....Just so you understand what's going on here..... I drive the vette and check the tank when it's low.... and the sending unit is working when I manually lift it...... but not when I fuel up....and were trying to see if sending unit is still good..... this is called a fluke......now do you understand?.....

By the way where can you get the sending unit for $65.00?

[Jack H.]

Re: HARD SENARIO: fuel tank sending unit works dry not wet float ok

LOL..... yes it is.... if that was what was happening....Just so you understand what's going on here..... I drive the vette and check the tank when it's low.... and the sending unit is working when I manually lift it...... but not when I fuel up....and were trying to see if sending unit is still good..... this is called a fluke......now do you understand?.....

By the way where can you get the sending unit for $65.00?

Most of the vendors have repros (non-GM) for this or less. I've seen them as cheap as $39.99 (plus shipping) on eBay as I was shopping one of these myself not long ago. Several choices out there depending on how much you want to pay for quality.