Testing An Inaccurate Fuel Sender With Tanks Inc.

Have you ever purchased a project car that you could drive and enjoy while you work on it? When you first get to enjoy a leisurely drive, I am certain you immediately began to take stock of a few things that need attention. Take for instance the fuel gauge. During one of your early drives into town, although your fuel gauges read slightly more than a 1/4-tank, you realized its inaccuracy when you suddenly have to make a phone call to a buddy to bring you fuel. An inaccurate fuel gauge happens more often than you might realize. To help remedy some of the causes of an inaccurate fuel gauge, Tanks Inc. is here to help.

For this discussion, we’ll assume the sending unit is the culprit of the offending inaccuracy. If it’s not a bad ground at the sender, it’s typically the sender. Because the sending unit is reading resistance to ground, having the proper ground connected to the sending unit is essential to get an accurate reading at your fuel gauge.

inaccurate fuel gauge

All fuel gauge senders have two terminals: One is for the wire to the terminal and one is Ground. If you look at the bottom of the sender’s mounting flange, you’ll see which one the rheostat is wired to. That is for the terminal wire.

Unfortunately, if you do need to replace the sending unit, they are usually only accessible from the top of the tank. This means the tank needs to be removed to gain access — or at least lowered away from the body. However, once it is removed, testing the sending unit is easy. All you will need is a multimeter set to ohms, and that’s it. You will need to remove the sending unit from the tank for testing. This is because you must be able to move the sending-unit arm for testing. While testing, the sending unit should display the required ohm reading at both the empty and full position of the float.

For example, a classic Chevy has a 0-90-ohm fuel sender and gauge. When the sender is at the empty position, the rheostat is not creating any resistance to ground (zero ohms). When the fuel sender is in the full position, the rheostat is creating 90 ohms of resistance.

A Matter Of Ohms

To start testing, set your ohmmeter to the appropriate range that allows you to read the lowest and highest resistance of the sending unit. touch the ohmmeter leads to the sending unit’s gauge and negative terminals. The center post/terminal is usually the gauge connection.

Next, move the sending unit’s float arm from the lowest to the highest position and make note of the meter readings at each position. The resistance reading on the meter should go down as the float is moved to the Empty position. Move the float up, and the reading should go up. If your readings are close to accurate (within a few ohms), the sender is fine. If not, it is time to replace the sender.

inaccurate fuel gauge

As you can see in this sending unit’s opened rheostat, when the sending unit’s float moves with the fuel level, the rod moves the copper tab inside the rheostat to adjust the ohm reading and make the gauge function.

The traditional muscle-car-era sender is made of several parts: a float, a variable resistor (rheostat), a rod connecting the float to the rheostat, and a mounting stem to suspend the sender in the fuel tank. The float used in early vehicles were a sealed brass cylinder. Later, a nitrophyl (plastic) float was used. As the fuel level increases or decreases, the float moves the attached rod up and down in relation to the mounting bracket.

Empty. Half tank. Full Tank. The length of the arm is adjustable to allow fitment in many tanks.

At the end of the float rod — where it meets the mounting stem — is the rheostat or variable resistor that is wired to your gauge. The rheostat is grounded by one terminal and receives voltage from the gauge on the other. It indicates the ohm resistance on a varying scale, dependent on the location of the float (fuel level). A small metal (copper) tab travels across the resistor card and regulates the number of ohms moving through the sender to ground. Those ohms move the gauge needle up or down.

While these simple senders do work, Tanks Inc. can also send you what we feel is a better option — the tubular float. While most fuel senders use a float arm and a rheostat to measure the amount of fuel in a tank, the tube sender offers some benefits over traditional senders. “The tube sender has an internal float that has a magnet in it. As the float passes different reed sensors, the ohms change,” says Justin Somerville, general manager of Tanks Inc.

A Better Float

The tube float is ideal for applications where a conventional swing arm will not fit due to clearance issues. Even if a swing-arm-style float will fit, the tubular design could be considered better, as it also acts as a damper to prevent the gauge pointer from moving excessively due to fuel slosh. This fuel sender has no side-to-side motion, which means it has no moving parts to wear out.

inaccurate fuel gauge

The universal sending unit also offers an adjustable height. Simply loosen the two screws and extend or shorten as needed.

Just like a traditional sender, it is top-mounted and utilizes the aftermarket-standard five-hole mounting flange. It also comes complete with a neoprene gasket and mounting hardware. This sender is available in five different ohm ranges: 0-30, 0-90, 73-10, 240-33 and 10-180. It is compatible with gasoline, diesel, oil, and many other fluids.

If you’re still having issues with your fuel gauge, here are a couple of troubleshooting tips from Tanks Inc.

Does Your Fuel Gauge Only Read Empty?

If you cannot make your gauge read anything but empty, then depending on your unit’s ohm range, you might have one of the following problems:

If you have a 0-30, 0-90, and 10-180-ohm sender, then most likely, your sender lead wire is grounding out somewhere. Or, possibly the center terminal on the sending unit itself is grounding out against the body of the car.

If you have a 240-33 or 73-10-ohm sender, then the sender may not have a proper ground or there is a break in the sender lead wire.

The tubular sender is ideal for applications where a conventional swing-arm unit will not fit due to clearance issues. The tubular design acts as a damper to prevent the gauge pointer from moving because of fuel slosh. It is top-mounted and utilizes the aftermarket-standard SAE five-hole mounting flange. The sender is available in various lengths to fit different applications.

Does Your Fuel Gauge Only Read Full?

If you cannot make your gauge read anything other than full, then depending on your sender’s ohm range, you may have one of the following problems:

If you have a 0-30, 0-90 or 10-180-ohm sender, then the sender may not have a proper ground or there is a break in the sender lead wire.

If you have a 240-33 or 73-10-ohm sender, then most likely your sender lead wire is grounding out somewhere. Or, it could possibly be the center terminal on the sending unit itself is grounding out against the body of the car.

Why Does My Gauge Read Backwards?

The first step would be to make sure that you have the proper ohm-range fuel sender to match your gauge. For example, a 73-10 ohm fuel sender would read backward on a 0-90 ohm fuel gauge. If you find you have the correct ohm-range fuel sender, make sure the float arm is installed on the correct side. The front of the rheostat shows from which side the float should stick out. If the float arm is installed on the other side, the gauge will read backward. Also, if the rheostat is installed upside down, the gauge will read backward.

Hopefully, this short explanation about testing your fuel sender will give you some insight into making sure your gauge is accurate, and you don’t end up sitting along the side of the road. If you have more questions, you can reach out to the folks at Tanks Inc. and be sure to get your gauge reading as it should.

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About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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