FUEL GAUGE, WONT SHOW FULL

[Jerry Whitt]

The gas gauge. on my 65 Corsa convertible 140 stick shift, will not show full. I pulled the sender, attached to a ground wire and then the wire from gauge. By moving the float up and down, the gauge would move and show full. After replacing sender, and filling the tank, still would not show full. Thinking the tank may be dented or damaged, and not allowing the float to have full travel, pulled the tank clear out of the car. Wow, what a project! Looked over the tank for damage. Did not see any damage. Looked into the tank, fairly clean and no obstructions. Went ahead and replaced filler hoses, and then put the tank back into the car.

I obtained a new sender from Clarks. Checked with an ohm meter and found the new one a little different from the old, about 2 ohms different. Aha! But after installing the new
float and sender, and then filling the tank at the local station, the gauge still shows about 3/4 full.

Any ideas would really be appreciated.

Thanks in advance

[myhotrod]

Not sure but double check all grounds .

[bbodie52]

Here is some additional material, including some theory on how the fuel gauge circuit functions. Hopefully this will help with your troubleshooting...

The attached wiring schematic shows the fuel gauge circuit on the 1965 Corvair. This is a very basic circuit, with power coming from the fuse block via a 14 Gauge B/P (Black with Pink Stripe) wire to a connector on the fuel gauge. You should be able to measure 12V DC at this connector with a multimeter (with the key ON) with the other multimeter lead grounded. The other fuel gauge connector is a 20 Gauge T (Tan) wire that is routed to the fuel tank sending unit. The sending unit must be properly grounded to function. When the tank is empty, the sending unit basically is at 0 ohms resistance, and full current flows through the gas gauge, giving an indication of Empty. As the fuel level rises, sending unit resistance increases, which reduces current flow and moves the needle toward Full. You should be able to ground the Tan wire at the fuel tank to obtain an Empty reading, and an open circuit (tank wire disconnected) should show a Full meter reading. Note that the gauge itself must also be properly grounded for the "F Coil" (as it is referred to in the description) to be properly energized to pull the needle to the F mark. If the ground connection on the instrument panel is faulty, the "F Coil" will not be energized properly, and the gauge will not function properly.

You should use a multimeter to verify the presence of 12V DC at the gauge input. The resistance (ohm) setting on the multimeter can be used to ensure that the gauge housing itself is properly grounded to the car chassis. You should also be able to measure 12V DC at the fuel tank wire, with the other test lead grounded. If the voltage is missing there, you may have a broken or loose wire between the gauge and the fuel tank sending unit. (All voltage checks are done with the key ON)

If the voltage is present where it should be (at the gauge input and at the fuel tank sending unit wire connector), and if you have a good chassis ground at the gauge housing and at the fuel tank sending unit, there is a possibility that the gauge itself has failed. If you determine that the gauge is faulty, you may be able to get a used fuel gauge on eBay.

Although the circuit itself is fairly simple, there are several connection points using multi-wire connectors that could become corroded or loose. Corrosion in a connector can add resistance to the circuit and lower voltage, which can affect the gauge reading. The gauge itself gets its ground from a ground strap that connects to all of the instruments attached to the plastic instrument panel housing. As shown in the illustration below, that single multi-point ground strap is grounded to the car chassis at one point by one screw. If that connection is corroded or loose, the instrument panel may be improperly grounded, which can affect the performance of the fuel gauge. At the other end of the circuit, a poor ground at the fuel tank sending unit can also impact the gauge. But an open condition (no connection of the tan wire) at the fuel tank sending unit should result in the fuel gauge needle being pulled fully to the right (F mark). 12V DC at the gauge and with the gauge properly grounded should apply full power to the "F Coil", resulting in a full deflection to the right. Only current flowing in the opposing coil (through the sending unit to Ground will apply a magnetic pull in the opposite direction, to pull the needle away from the "F Mark" and toward the "E Mark" as the current through the sending unit increases as the fuel in the tank is reduced.

The schematic wiring diagram and the written description below explains how the fuel gauge circuit functions.

Left-click each image with your mouse to enlarge the image...

I have attached the appropriate section of the 1965 Corvair Chassis Shop Manual to help with your troubleshooting.

[jwarren]

Let me know what you found out. My new clarks sending unit worked for a week. Tried Brads suggestions. I gave up.Went back to the gas receipt with the mileage on it. I may try running a wire to the instrument panel to bare metal to see if its a grounding issue.

[Jerry Whitt]

Additional note. This afternoon, the wife and I went for a ride. As the sun went down, the headlights were turned on. Instantly, the gas gauge dropped 1/4.

Looks like the problem is in that ground circuit. in the dash area, or other area. Will advise after working on that.

Thanks for the help.

[Jerry Whitt]

After looking at your various notes, some tests were run. I pulled down the steering column, pulled the dash out, leaving the wire harness installed. An old testing trick to locate possible resistance in the circuit is to hook up a volt meter from "the most positive, to the least positive". This then is a parallel circuit that the electricity will try to use, instead of the questioned circuit.

The positive lead was hooked to the small screw on the back of the fuel gauge. The negative lead to the negative battery cable clamp. Another wire was hooked to the ground strip on the back of the dash panel, and to the metal parts of the dash. The engine was started, and noted the volt meter (in the 1.5 volt scale). It showed over 1 volt of resistance. With the engine still running, turned on the head lights and the volt meter pegged!!!! Aha! Normal resistance should be about 1/10th volt, maybe 2/10s.

A hole was drilled into a metal dash panel support, a quarter inch bolt with 16 gauge wire and loop bolted to the dash support. The 16 gauge wire was strung along the existing
wire harness, and taped a couple places. Another small loop was crimped on the other end of the wire and after wire brushing a shinny place on the back of gauges, attached
the new ground strap.

A new test shows about 2/10 volt resistance, the gauge is now working, and a bonus, the dash lights are much brighter!

[bbodie52]

I guess you could say that old ground connections never die... they just FADE AWAY. Glad your testing was successful and pointed to the problem area, and that you were successful at fabricating a solution. Thanks for sharing your testing procedure and corrective action. This info may help others with similar problems.