I just got my first cutlass and I am starting to make my list of things that need replacing. Just from driving it I noticed the gas gauge was working properly and now that I have filled the tank up the gauge has moved way past the full mark and hasn't moved. I don't know if its electrical but I didn't know if someone else has had the same issue. I attached a picture of what the gauge looks like.

 

Either you have a wiring issue that just happened to occur at the same time you filled the tank, or more likely you have a defective sending unit. There is probably a bad spot in the coil of wire on the sender, when the float is sitting high in the tank (full of fuel) the contacts of the sender are in the bad spot of the coil of wire (poor contact, open circuit=needle well past the full mark). Its easy enough to verify its not a gauge and wiring issue. Find the wire for the fuel gauge in the trunk. It passes through a grommet near the trunk latch, I believe its a tan wire. With the key on, ground that wire and the gas gauge should read empty. If so, you just verified the wiring and gauge work. Wait until the tank is close to empty, and replace the sending unit.

 

A gas gauge that pegs well past the F mark is the classic symptom of an open circuit and one of the most common symptoms non-working gas gauges exhibit. The gauge is just a 90-ohm variable resistor where zero ohms is E and 90 ohms is F. Since an open circuit is equivalent to infinite resistance, which is much larger than 90, the gauge will go past the F mark. Since it happened after you filled the tank, the problem appears to be the sending unit.

But you can check a few things before getting at the sending unit, which often means dropping the gas tank.

First you want to verify that the dash gauge itself is working OK. Find the place at the rear of the car, usually behind the license plate or in the trunk in the area of the latch, where the orange wire coming from the tank sending unit connects to the orange wire coming from the front of the car. Disconnect it at the connector and ground the side coming from the front of the car. Then put the ignition ON. The gauge should move immediately to E. Then remove the wire from ground leaving the ignition on, and the gauge should immediately go back to where it is now, well past F. If it does both of these things, the gauge itself is fine.

The next step is to verify that the other wire coming from the sending unit, the ground wire, is firmly attached and not corroded or dirty. This involves getting under the car as this wire usually comes forward from the sending unit and attaches to the body or frame somewhere forward of the tank. If this wire is not attached or the connector or where it attaches are dirty or corroded, which can happen after so many years, clean things up and reattach. If the gauge starts working, you're good.

If the problem still persists, then it's time to remove the sending unit itself and probably replace it.

 

Thanks for the replies. Looks like I need a sending unit after looking in to it.

 

I know what I will be doing this weekend. I have the same thing going convertible it has been driving me nuts. I am not a great electrician but if it goes together nuts and bolt I am in

 

If the gauge pegs when you fill it, why not drive it and see whether the reading returns to normal after you use some gas.
As stated above, the probable cause is bad windings near the "Full" area of the coil, but they could also just be dirty (from disuse). As the gas sloshes back and forth, it will make the float bob up and down, which will make the wiper slide back and forth against the coil. If it's just cruddy, that may be enough to clean it off in a few tankfulls, and you may have no need for further annoyance. Alternately, you may find that the bad coil is at the very end, and after you use a gallon or two, the gauge returns to normal operation for the rest of its range, making it far less important to mess with it.

Welcome to ClassicOlds.

- Eric