Fuel Gauge Needle is Swinging Wildly!

[72MetallicMalaga]

My fuel needle just started moving crazy during idle. I tried searching for this exact scenario and couldn't find anything on this forum.

 

Has anyone had this issue before? All my grounds are fine, and the connections on the sender unit are clean and tight...

 

Video attached...

 

 

 

[Son of Marty]

My first thought is the settling well (metal tube) that surrounds the float has come undone.

[Mike Self]

I'm guessing there's either a bad ground somewhere, or as noted above, something is adrift in the sender unit.  The sender is a float that moves up and down on two hair-fine resistance wires and completes the electrical path.  A full tank is nearly zero resistance, and the gauge shows full.  As the fuel level drops more resistance wire comes into the circuit, and the gauge drops.   The whole thing is housed inside an aluminum tube.  It dampens the motion of gas in the tank, which sloshes around as you drive.  Without the tube, the slightest movement of gas would send the gauge all over the place.  

 

Since you know your grounds and connections are clean and tight, you should probably remove the sender unit, make sure the tube is properly fastened in place and the filter screen at the bottom of the tube is in place.  Also, if the car's been sitting for some time, crud can build up on those thin wires, causing a malfunctioning gauge.  You can clean the wires CAREFULLY with a carb cleaner-soaked cotton ball.  Wrap the cotton ball around the wire to support it from all sides as it's quite fragile.

 

Let us know whatcha find so we can add to our corporate knowledge.

 

mike

 

PS--if the temp gauge was moving in synch with the gas gauge, I'd say bad ground at the instrument end.  

 

 

[Hans]

Remove sender SLOWLY.

[JohnS]

I added an extra ground wire to the sender on my 74tii and that solved the jumpy needle for me.  John76 suggested it a while back here on the FAQ.  It was especially jumpy when the right turn signal was being used. 

[billy g]

I have this issue as well, both at idle and while driving.  Sometimes the needle even dies completely for moment.  It always reverts to normal so I haven't addressed it yet.

[conkitchen]

Maybe it has something to do with gas prices as they swing wildly lately. 

Edited December 21, 2022 by conkitchen

[Dick R]

The jumping around gas needle is the exact same symptoms I had when acquired my 71.  Pulled out the sending unit to find the aluminum tube missing.  As Mike said above, that tube dampens the float motion from the sloshing around that gas normally does in the tank.  When I discovered that the tube was missing, I tried unsuccessfully find another.  Then I realized where the original likely was - at the bottom of the gas tank. Indeed there it was easily found with a flashlight and fished out I think with a coat hanger bent appropriately.  Carefully and easily reassembled it to the sending unit and it's been operating normally for the past 7 years. 

[John76]

On 12/21/2022 at 9:36 AM, JohnS said:

It was especially jumpy when the right turn signal was being used.

The right rear light pod shares the same ground as the fuel sender.

Put your right turn signal and brake light "on" to check for a weak ground.