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Nuts & Bolts: Fuel Gauge Woes

Posted in How To: Tech Qa on February 1, 2019
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I installed some aftermarket gauges in my early Bronco a while back. Everything works great and appears to be accurate except for the fuel gauge. When the tank is full it reads only half, and it shows empty after only a quarter of the tank has been used. Obviously the sender in the tank is working, so what’s wrong?
Casey S.
Via facebook.com/4wheeloffroad

A couple of things could potentially be wrong. Fuel gauges measure resistance to ground in order to indicate the fuel level. The fuel sender in the tank has a float attached to a rheostat, so the position of the fuel level in the tank creates different levels of resistance, which are read by the gauge.

Fuel level gauges work off of different scales of resistance depending on the manufacturer, so if your original gauge was working correctly, most likely the new gauge is simply set for a different scale than the sender in the tank. Your Ford sender should have an ohm range of 73-10, meaning that it should read about 73 ohms when empty and 10 ohms when full. OE manufacturers often use different ranges for their fuel gauges. For example, GM vehicles about the same age as your Bronco could have an ohm range of 0-30 or 0-90 depending on the year. For this reason it’s important to be sure that you order a gauge that matches the sender on your vehicle. If the gauge and the sender are mismatched, there will be erroneous readings or even no readings at all.

Aside from making sure you have the right gauge, there are some other things to check. Because the gauge works off of resistance to ground, it’s very important that the sender has a good ground. There should be a ground wire that goes from the sender directly to a chassis ground. Make sure this wire isn’t broken and has a solid connection, as a bad ground can cause erratic and inaccurate readings. Make sure that the connections between the tank and the gauge are not compromised in some way.

You can also test the sender with an ohm meter by probing the sender wires at the tank. Though ideally you want the sender out of the tank so you can manipulate the float level manually, that’s not always practical. Instead you can check the resistance at times when you know there are different fuel levels in the tank. With a full tank your sender should have about 10 ohms of resistance; half should be about 40; empty should be about 70. If you get readings way outside of that or no readings at all, then the sender is bad. We like to use an analog ohm meter for this, as digital meters often adjust the scale automatically, which can cause a lot of confusion.

If you have a mismatched gauge, there’s a chance that your aftermarket gauge can be adjusted for the scale of the sender, so double-check the instructions that came with your gauge. Otherwise the only practical option is a different gauge with the correct scale for your sender.

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