New Sending unit issues?

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lostboy
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New Sending unit issues?

Post by lostboy »

I have a new sending unit that I got special order from AutoZone. It's a spectra brand unit. I'm having problems where sometimes my gauge reads accurately and other times it's either always full or always empty. The wiring seems ok. It does it randomly and doesn't seem to change when I hit a bomb, but I have no way of knowing if the gauge had an issue because the car came with quite a bad tank and sender. Has anyone had issues with fuel readings on a new sender? Thanks.


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-Steve
1961 Corvair 700 Sedan (80hp 3spd Gasoline Heat)
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toytron
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Re: New Sending unit issues?

Post by toytron »

I hope you don't hit bombs too often. Lol

Ed Stevenson
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firstgencamarojunkie
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Re: New Sending unit issues?

Post by firstgencamarojunkie »

I think the sending unit would be the least of your worries when you hit a bomb.... :chevy:
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bbodie52
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Re: New Sending unit issues?

Post by bbodie52 »

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

Study the schematic wiring diagrams in the shop manual and supplement for your vehicle. This is a very basic circuit, with power coming from the fuse block via a 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 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 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.

Image

Image

The schematic wiring diagrams below were for a 1962 Corvair, but they will be very similar to your 1961. I find the diagrams in the 1961 Shop manual to be very poor and confusing. The diagrams in the 1962 and later supplements were much better.

Left-click each diagram to enlarge for better viewing...
1962 Combined Passenger Compartment & Engine Compartment Wiring Diagram.jpg
1962 Wiring (Passenger Compartment).jpg
Image

:link: http://www.corvair.com/user-cgi/catalog ... IN&page=69
Image
Image

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

Weight: 1 lbs 0 oz
Catalog Pages(s): 69(2)
Price: $ 48.00
Brad Bodie
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Re: New Sending unit issues?

Post by 64powerglide »

I had that same problem with a new Clark's sending unit, I took it out and the float was letting fuel leak in through the soldered seam. Clark's sent me a new float. free. Just another thing for you to check if the electrical stuff checks out but make sure you have a good clean ground before pulling the unit. :my02:
Good luck & I hope you find a bad ground, that's the easiest fix.
64Powerglide, Jeff Phillips

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lostboy
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Re: New Sending unit issues?

Post by lostboy »

Thank you all. I will check all of these things. Now that it's cold out I have a bit of other work as well.


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-Steve
1961 Corvair 700 Sedan (80hp 3spd Gasoline Heat)
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