fuel tank sending unit

[snowmaker]

Is it possible to install the fuel tank sending unit upside down so that is does not work. Awhile ago I put in a new tank and sending unit in my 64 Spyder a fun job, I checked the sending unit before I installed it and it seemed to work all right. After I had completed the job and had filled the tank I noticed that it was not working thought bad ground, it appears that the ground is good but he gauge still stays on empty, and does not go to full as the gas is used up. I have lived with this for a while just always making sure there was plenty of fuel in the tank and not going to the trouble of draining out he fuel but I thought it might be time to put things right. Thanks for any ideas and help.

[terribleted]

Can not be installed upside down the unit is keyed to go in only one way.

[azdave]

Many people report that the floats on the aftermarket sending units fill with fuel and sink not long after the install. Sounds like you might have one of them. Best way to test is to unhook the wire going into the tank and ohm out the sending unit with a VOM. Dead short means the float has sunk. 30 ohms indicates a full tank (on an early model sender) and infinite ohms measured indicates a broken connection inside the tank.

You can test the dash gauge by simply disconnecting the wire at the tank and leave it hanging. Then turn on the key to "run" and see if the fuel gauge goes past the full mark.

[bbodie52]

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
viewtopic.php?f=225&t=12968

[joelsplace]

They are keyed but if you think about it for the fuel line to be pointing the right direction it has to be installed correctly anyway.

[river_side]

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

[azdave]

Sounds like the float on the sender is either full of fuel or stuck at the bottom. Disconnecting the lead at the tank and seeing the needle sweep way past full confirms your gauge in the dash is fine.