Test Fuel Gage out of Dash

Anything Corvair related
TheCommish
Posts: 84
Joined: Fri Apr 22, 2016 5:49 pm
Contact:

Test Fuel Gage out of Dash

Post by TheCommish »

I have a fuel guage I pulled out of the instrument panel, don't know if its good, how do I bench test it? :helpsos:
User avatar
bbodie52
Corvair of the Month
Corvair of the Month
Posts: 11872
Joined: Mon Aug 06, 2012 12:33 pm
Location: Lake Chatuge Hayesville, NC
Contact:

Re: Test Fuel Gage out of Dash

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 Corvair wiring schematic. I will focus on the 1965 Corvair in this example, but earlier schematics are also shown and all model year fuel gauges are essentially the same functionally, with only slight variations from year to year. (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: viewtopic.php?f=225&t=12968

Image

Image

:chevy:
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible
TheCommish
Posts: 84
Joined: Fri Apr 22, 2016 5:49 pm
Contact:

Re: Test Fuel Gage out of Dash

Post by TheCommish »

Not my question....
TheCommish
Posts: 84
Joined: Fri Apr 22, 2016 5:49 pm
Contact:

Re: Test Fuel Gage out of Dash

Post by TheCommish »

I have in my hand, the instrument panel gas gauge only. Out of the car.

.
Attachments
IMG_2141.JPG
IMG_2140.JPG
User avatar
bbodie52
Corvair of the Month
Corvair of the Month
Posts: 11872
Joined: Mon Aug 06, 2012 12:33 pm
Location: Lake Chatuge Hayesville, NC
Contact:

Re: Test Fuel Gage out of Dash

Post by bbodie52 »

If you wish to create a configuration to bench check the gauge, you would have to use the battery and connect the positive battery terminal to the gauge terminal that is normally provided voltage when the ignition key is on. The external housing of the gauge would have to be connected to the negative terminal on the battery to ground the instrument. This configuration would energize the coil inside the gauge that pulls the needle towards the "F" mark on the gauge. It would also provide battery voltage internally to the other coil at the opposite end of the scale. However, since the "E" coil is not connected to ground (via the gas tank float assembly and variable resistor), the fuel gauge needle would remain pulled all the way to the "F" mark on the scale. Temporarily grounding the other terminal (normally connected to the fuel tank sending unit) would energize the "E" coil, which has a larger magnetic capacity than the "F" coil. The energized "E" coil would then overpower the magnetic field that is constantly generated by the "F" coil when power is applied to the gauge. The stronger magnetic field on the "E" side would pull the needle to the other end of the scale. This would basically prove that the gauge is functional. You would have to add a variable resistor or fuel tank sending unit with a variable resistor to the circuit to vary the magnetic field on the "E" side if you wanted to see the needle slowly move between the F and E markings on the scale. This would occur if you varied the amount of current flowing through the "E" coil using a variable resistor. This is not necessary however, when testing the gauge as you only want to see if the gauge operates in its full range between F and E when voltage is applied to the grounded gauge, and when the other terminal is then grounded to energize the "E" coil.
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible
Post Reply

Return to “Corvair Talk”