Fuel gauge always reads empty 64,110

[river_side]

My fuel gauge always reads empty. If I disconnect the lead at the tank the gauge then reads full. I am quite certain i have a good ground at the tank as i used sandpaper on the leads. What now? 1964, 110hp
Thanks

[bbodie52]

If the gauge reads FULL when the key is on, and switches to EMPTY when you connect the fuel tank wire (GROUND), you have proven the proper gauge operation, including the presence of source voltage from the ignition switch circuit, and a proper ground provided via the circuit to the fuel tank sending unit. If the gauge never changes (even when the tank is known to be full), I would guess that the sending unit is either stuck or shorted, as the motion of the tank float is supposed to move a variable resistor to change the amount of current flowing from the gauge to GROUND. Reading empty all the time indicates that the grounding circuit is fully energizing the "E Coil", and the variable resistor in the sending unit is not changing the current flow as the fuel level changes.

The description below in HOW THE CORVAIR FUEL GAUGE WORKS does a pretty-good job of describing fuel gauge operation and troubleshooting. Your description of the symptoms on your Corvair indicates that the fuel gauge does not respond when the key is turned ON. The position of the needle on the gauge reflects a balancing act between two electromagnetic coils within the gauge. The coil on the FULL side of the gauge is a constant. When the key is turned ON that coil is fully energized and generates its maximum magnetic pull in that direction. The fact that your gauge does not respond to the application of power when the key is turned ON would indicate that a proper ground is perhaps not available to that gauge housing, or that voltage is not available when the key is turned on. Without the ground connection, application of voltage via the ignition key would have no effect because the coil would only have voltage applied, but a ground return — if absent — would prevent the coil from generating an electromagnetic field. With your tank sending unit disconnected, there should be no magnetic pull to the EMPTY side, and maximum pull to the FULL side. A short at the tank sending unit to GROUND should pull the needle over from FULL to EMPTY, and the variable coil becomes fully energized and overrides the fixed magnetic pull from the "F coil".

Make sure that 12 V DC power is available at the fuel gauge input. The voltage comes from the ignition key switch. It travels to a power distribution point in the wiring harness, which also powers the GEN/FAN and TEMP/PRES warning lights. If those lights come on when you turn the key on, power is reaching the power distribution point. A 20 B/P wire travels directly from that distribution point to the fuel gauge power input. If the voltage is there, and the gauge housing is grounded, turning the key ON should fully energize the F Coil. If the E Coil is not receiving a ground via the sending unit variable resistor, the needle should swing toward the F Coil when the gauge is energized. If there is power a the gauge input, voltage should also be present at the gauge output to energize the sending unit variable resistor, where it would then be tied to GROUND. In other words, with the key ON and the instrument panel warning lights illuminated, check for voltage in at the gas gauge via the 20 B/P wire, and voltage out at the Tan wire. If voltage is going into the gauge, but not coming back out to go to the tank sending unit, there would have to be a breakdown within the gauge itself. If the gauge case was not grounded, the F Coil would not be energized. Grounding the gauge output to the sending unit would fully energize the E Coil, which would pull the needle to Empty.

Corvair Combined Wiring Schematics
http://www.corvairforum.com/forum/viewt ... 25&t=12968

http://www.corvair.com/user-cgi/catalog ... ow_page=69

Part number C846: 61-64 STAINLESS GAS TANK SENDER UNIT ** *NEED GKT-C155,RETAINER-C156,STRAINER-C261,*

Weight: 1 lbs 0 oz
Catalog Page(s): 69(2)
Price: $ 48.00



See page 9-23 in the attached 1961 Chevrolet Corvair Shop Manual - Section 9 - Fuel & Exhaust Systems, for fuel tank and sending unit information.

[river_side]

Power and grounded, empty.
Power and not grounded, full.
If battery is disconnected when reading full, gauge continues to read full.
How to unstick if stuck?
Time to replace?
Method of emptying the tank?

[bbodie52]

There are two electromagnetic coils inside the gauge. The coil on the FULL side inside the gauge is a constant-pull (like a spring). With the key ON this coil applies a magnetic pull on the gauge meter to pull it toward the Full mark. This coil is grounded to the gauge metal housing to apply the magnetic pull on the FULL side whenever the key is ON. Power continues across the gauge to the E Coil (which when fully powered is slightly stronger than the F Coil. However, the E Coil is not grounded to the gauge housing. Instead, it is grounded outside the gauge via the wire that leads to the gas tank sending unit. The current flows through the variable resistor in the gas tank sending unit and then to the Ground wire from the sending unit to Chassis Ground. When the tank is full, the sending unit float rises and sets the variable resistor to maximum resistance — minimum current flow — which weakens the pull from the E Coil which allows the gauge needle to be pulled by the other fixed-strength magnetic oil to the FULL Mark. As the tank empties, the sending unit float is lowered with the reduced fuel level. This permits more electrical current to flow through the E Coil, which gradually becomes stronger and pulls the needle toward the EMPTY side. When the float is near the bottom of the tank, current flow is at maximum, which pulls the needle all the way to the E mark on the gauge.

When the ignition key is OFF, no electricity is applied to the gauge, so NEITHER internal electromagnetic coil is energized. The gauge needle tends to remain where it was when the power went OFF.

The symptoms makes it look like a defective sending unit, which is allowing maximum current to flow through the variable resistor all the time. You will have to inspect the sending unit when it has been removed to see if the float is stuck. The float might also be leaking and full of gas, which could force it to sink to the bottom and give you an EMPTY indication, even when the tank is full. I believe Clark's sells new floats to replace a leaking float, or new sending units if the old one proves to be faulty and not repairable.

The shop manual explains removal of the old sending unit. The tank may be drained by disconnecting the fuel line hose. New sending units and gaskets are also listed on the Clark's Corvair Parts online catalog page I referred to in my last post.

[river_side]

Thanks for the excellent in-depth description.
Now I have a plan of attack, thanks.

[64powerglide]

I bought a new sending unit from Clark's a few years ago & the float leaked. I called Clark's & they sent a new float right away, still working fine.

[chrischew]

Hi Brad

It's been a while since I last visited this forum ... busy with work etc. Been enjoying my Corvair more lately. Recently I noticed that my fuel gauge remained near Full (as it should be since I just topped up). However it stayed in the same position after shutting engine off. I did not notice this before and was wondering if my gauge is bad.

Thanks to your wonderful explanation of how the gauge works on the Corvair, I grounded the sender wire at the tank sender end and this made the gauge go to E, implying the sender wire is good (at least). What I could not be sure is where the needle shud be when power is cut off and both F and E coils are not energized.

Tonight I saw your one sentence "When the ignition key is OFF, no electricity is applied to the gauge, so NEITHER internal electromagnetic coil is energized. The gauge needle tends to remain where it was when the power went OFF.". Is this always true?

Thanks
Chris

[bbodie52]

Tonight I saw your one sentence "When the ignition key is OFF, no electricity is applied to the gauge, so NEITHER internal electromagnetic coil is energized. The gauge needle tends to remain where it was when the power went OFF.". Is this always true?

Thanks
Chris

The tank unit of the gauge is simply a variable resistor with a sliding contact whose position is determined by a float on the surface of
the fuel. It is connected to the dash unit or "heat" unit by a single wire, and the resistor controls the current through the dash unit according to
the fuel level in the tank. A simple instrument for measuring this current with scale calibrated in litres instead of amperes could be used as the dashboard fuel gauge indicator but the battery voltage varies from 5 to 8 volts in a 6 volt system, and 10 to 16 volts in a 12 volt system so the gauge reading would vary without any alteration in fuel level. The balancing-coil meter is independent of the battery voltage.

The dash unit has two similar electromagnets, one a control magnet and the other a deflecting magnet. The magnets influence an iron vane or armature to which the pointer is attached. Because both windings are equally affected by variations of voltage, the pointer always indicates a true reading. This arrangement of the coils gives a variable control.

Coil A, that moves the pointer towards the "empty" position, is the control coil. It is connected directly to the battery through the ignition switch and has two indirect connections to earth, through the other coil and the tank unit respectively. Coil B, that moves the pointer towards the "full" position, is the deflecting coil. It is connected directly to earth and has an indirect connection to the battery through coil A. A resistance shunted across coil A and has the effect of reducing the current in coil A and its magnetic strength. When the wire leading to the tank unit is disconnected the two coils are in series, a being, of course, shunted by the resistor. The current in coil B is equal to the sum of the currents in a and the shunt resistor, so that the magnetic effect of coil B is strong and that of coil A weak. The pointer therefore swings to the "full" position under the strong influence of coil B.

If the gauge reads "full" when the tank wire is disconnected indicates an open circuit fault. When the tank unit wire is earthed, coil B carries no current because both its ends are earthed. The only magnetic action therefore is that resulting from the current through coil A, so that the pointer moves to "empty". If the gauge reads "empty" when the tank unit is earthed indicates short circuit.

When the tank is half full, and half the tank unit resistance is connected between the coil junction and earth, both coils carry an
intermediate current and have equal effect on the pointer, which now reads "half". The state of the circuit for other scale readings changes gradually as the float is raised or lowered.

The needle has little weight and no springs to position it to a NEUTRAL location when no electromagnetic forces are applied to it. It just sits randomly where the magnetic forces that were last applied left it. When the key is ON, the two electromagnets are energized, with one coil at a constant, fixed strength, and the variable coil current flow (permitted by the variable resistor in the gas tank sending unit) causing a counter-pull on the needle that is determined by the position of the float in the tank that moves based on the fuel level. With no current in the variable coil, the fixed coil pulls the needle all the way toward its side. The sending unit float position permits more or less current to flow in the variable coil, which determines just how far the needle is pulled away from the fixed level coil.

When the ignition key is turned off, neither coil is energized, so the gauge needle just sits where it was left when the magnetic fields disappeared.

RE: School Science Lessons — Motor Vehicle Electricity https://www.uq.edu.au/_School_Science_L ... h32.6.html

When the key is turned off, it is as if both sides just dropped the rope simultaneously. The flag just remains where it was when the power was cut.

[joelsplace]

Short story - there is no "right" reading with the key off.
Your test is all you can do short of removing the sender.
D
If you are worried about it watch the odometer and see what the fuel reads after 100 miles or so.
If you are really worried about it keep a gallon of extra fuel in the trunk.