Gas gauge barely moves off full when empty

[John555]

I have a 2006 Crownline LS I just bought and when the gas tank is full it is past "F" on the right. When it gets just past the "F" to the left the tank is empty. Any ideas why it doesn't register fuel level properly?

[PAR]

You likely have a short in the sender wire, that's making contact with a ground source. It's not a real good contact or the gauge would read as full as the gauge can read, but it's enough to give you a darn near full reading, meaning it's a hard short.

To test the gauge, pull the center terminal off the fuel tank sending unit and attach a jumper to it and ground this jumper. The gauge should read full nuts full. This means the gauge is working and the wire is okay, so the fuel sender or wire is defective. If the gauge doesn't do anything, the wire and/or the gauge is defective. To test the wire, just preform a continuity test on it. If it has continuity, then it's the gauge, if it hasn't continuity, then replace the wire or find where it chaffed through and fix it (splice in a repair).

[CDK]

There's a high probability the voltage regulator is defective or has no good ground contact.
Some gauges have an external regulator with 2 terminals (12V in, 8V out) and a ground lug, others have the regulator internally. In that case the gauge has 2 ground wires for the regulator and the illumination.

[John555]

Thanks guys. So I did what PAR suggested and grounded the jumper and the gauge showed full nut full. So I replaced the sending unit and with an empty fuel tank the gauged showed almost full. What is remaining for me to fix this?

[gonzo]

Make sure you connected the wires correctly. In some installations both wires have a spade connector. The pink goes to the gauge sender.

[PAR]

Well, the logical thing is to check the gauge as I described above. Ground, full, remove the ground, it should drop to empty with the key on. If it did this you have continuity along the wire, though it still could have a short, which will give you a false reading if the short is grounded.

This is a super simple circuit to access, but it does require a systematic approach.



For example CDK could very well be right and the instrument regulator may be bad or needs a good ground or better connections. The one pictured is fairly common. It's usually mounted on the back side of the instrument panel or very near it.

In fact, most of the time these seemingly weird issues are simply bad connections, often the grounds (or lack of good ones).

The very first thing I do is check connections, particularly the grounds. Next would be to reset the connections, which often exposes enough fresh metal to make them work, even if momentarily. If this happens you know you need new or cleaned connections. Next on the list is continuity of the related wires. Finally, once I've established the wires have good connections and aren't broken, it's time to test the actual components. You know the gauge works and the sender is new, so it has to be the regulator or an intermittent short to ground.

[pistnbroke]

none of you have established if its a crossed coil or bi metal gauge ..if it has a voltage stabilizer then it will be bi metal if no voltage stabilizer then its crossed coil. The tank units are not the same ..A bi metal is low resistance tank full and a crossed coil high resistance tank full .The voltage stabiliser does not produce a steady stabilised voltage are there are 5v and 10v types.....Crossed coil gauges may be damped and not move to e when power is disconnected.
The WRONG idea of joining the tank wire to earth persists .....do this only if you want to risk a smoked glass gauge ..you should use a 20 ohm resistor .

[CDK]

Originally Posted by pistnbroke

none of you have established if its a crossed coil or bi metal gauge ..

That is always the problem with remote diagnosis.

My bet is on a thermal instrument with PAR's tin can dangling from a wire because it is the cheapest way to do the job.
The sender is a wire wound pot, approx 270 ohms with an empty tank and near zero with a full one. Without ground to the regulator the gauge shows 3/4 full for an empty tank.

[pistnbroke]

assuming its a bi metal ...then a voltmeter from the I terminal on the voltage stabiliser will show 12v if its contacts are stuck or it has no earth. If its working OK a digital meter will jump around as its a pulsed output ( analogue meter will go down and up not up and down ) the stabiliser is usually marked B for battery and I for instrument ..the case is earth usually through a mounting tag ....Arrh the old technology ..bi metal heating coils magnets and contacts ....lovely

[PAR]

This is a common difficulty here as both Piston and CDK point out. We can't see what's really going on. With one look, we'd know which way to pursue a diagnoses. A picture or two would quickly solve the issue.

[hoytedow]

See if its hot. One of mine shorted out and melted the face of the gauge.