Fuel gauge not working - how to test?

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rnd5553
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Fuel gauge not working - how to test?

Post by rnd5553 »

On my 66 Corsa, the fuel gauge was working fine until I had to remove the cluster recently, now the needle goes past full to the 3 'o clock position when the ignition is on. I have a newer sending unit, and the chassis ground at the tank is good and clean. I

I have connected a jumper wire to the ground strap on the cluster and use a test light and go to known good ground and nothing - still the 3 o' clock reading.

How do you check the fuel gauge itself to see if the gauge is bad? Again, it was functional until the cluster R & R. Yes, the temp gauge, lights etc. all work fine still.

Thanks for any suggestions.
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bbodie52
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Re: Fuel gauge not working - how to test?

Post by 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 indicate that the fuel gauge does in-fact 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 responds to the application of power when the key is turned ON would indicate that a proper ground is available to that gauge housing. 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.

So we know that the "F coil" is fully functional, and that the gauge is receiving voltage and the housing is properly grounded. The description below reads "If there is a break in the tank sender unit wiring or a failure of the tank sender that creates an open circuit, the fuel gauge F coil will not be balanced by any magnetic force from the E coil and the needle will be pulled to the right beyond the F mark on the gauge when the ignition switch is on." This description would seem to match your description of the response that you are getting from your gauge. It would indicate that no ground connection is present on the wire that leads to your fuel tank sending unit. If the connection to chassis ground via the fuel tank sending unit was present at all, the power that is being fed to the gauge would not only energize the F coil, but it would also be energizing the E coil, which would be producing at least some magnetic field depending on the position of the variable resistor in the tank sending unit. With no magnetic field being generated by the E coil at all, the F coil is the only one producing a magnetic field, which pulls the needle past the F mark as you described.

Clicking on the link below will provide you with a full 1965 Corvair Corsa wiring schematic. (Left-clicking the image twice should fully enlarge the image to its maximum, so that you can zoom in on the area of interest). Assuming that the fuel tank sending unit is properly grounded and fully functional, voltage from the fuel gauge should be present via the 20 T (20 gauge Tan) wire that is attached to the sending unit (when the ignition key is ON). If the connection is broken somewhere between the sending unit and the fuel gauge, you should check the wiring harness. There is a 12 pin multi connector between the tank sending unit and the main instrument panel wiring harness, and another 12 pin multi connector between the main wiring harness and the instrument panel. A loose, bent, or corroded metal pin within either connector is the likely point where the connection between the gauge and the sending unit is being interrupted. (I would guess that the 12 pin multi connector closest to the instrument panel was the one that was probably disrupted when you recently removed the instrument panel and then reinstalled it. If you unplugged that connector or shifted its position you may have caused an interruption in the electrical connection between the two pins that provide continuity in the 20 T wire that leads to the fuel tank. (You can test the gauge function by simply providing a ground to the terminal on the back of the gauge that is connected to the 20 T wire. Grounding that terminal will simulate the ground that would be provided via the tank sending it if the tank were empty. When performing this test be careful not to ground the wrong terminal on the back of the gauge. The other terminal would have 12 V DC applied from the ignition switch, and you do not want to short that one to ground! I believe the 20 T wire on the fuel gauge is connected to the terminal that is on the side closest to the center of the instrument cluster pod. Grounding the terminal that is connected to the 20 T wire should cause the E coil to be fully energized. Since that coil is larger and has a stronger magnetic pull capacity than the F coil, a fully energized E coil should move the fuel gauge needle all the way to "E". This will confirm proper operation of your gauge, and will also confirm that the fault in your Corvair lies in the harness between the gauge and the fuel tank sending unit). If you unplugged the two halves of the 12 pin multi connector closest to the instrument panel, you should examine the metal pins inside to make sure that none of them are bent or distorted in a way that would prevent electrical continuity when the two halves are reconnected. Hopefully your problem will be resolved by simply unplugging and reconnecting the two halves of the multi connector.

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

Image

Image

:chevy:
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible
dave t
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Re: Fuel gauge not working - how to test?

Post by dave t »

Often the pins in the connector push out on the wire side when the plastic connector is plugged in. Check to see if the tan wire is fully seated in both sides of the plastic connector.

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