Fuel Gauge or Sending Unit?

[Dan S]

1978 Avanti II question:  My fuel gauge flutters and doesn't go past 3/4 when full. How do I know if it's the gauge or the sending unit?

[WayneC]

Here is some information I copied back in 2000, regrettably I didn't document who the author was... I apologize for that.

A short version of it appears in the Avanti Workshop Manual, gasoline section, page 4 

Most fuel gauges work pretty much the same way: A sending unit in the tank has a float on an arm connected to a variable resistor. The Avanti gauge reads the resistance in a (red with white trailer) wire connecting the gauge to the sender. The gauge has 3 wires connected to it: 12v power from the ignition switch, a white ground wire, and the red w/ white trailer lead (sensing) wire to the tank sender. The sender, located atop the gas tank behind the rear seat and removable body panel, has attachment for a ground wire in addition to the sensing lead.

 

1) Check all the connections at the gauge and at the sender for loose or corroded connections.

 2) Use an ohmmeter to check the white grounding wires at the gauge and at the tank for continuity to frame ground.

 3) Disconnect red w/white trailer wire at both the sending unit and the gauge and ensure the loose ends aren`t touching anything, then use your ohmmeter (in conjunction with a long test wire) to check the continuity from end to end; then check its continuity to frame ground: it should NOT have continuity to ground.

4) Disconnect the gauge to sender wire at the sending unit end; with ignition switch on, the gauge should now read `empty`; if not reading `empty` you will need to replace the gauge or have it repaired. If it reads `empty`, continue on.

5) Disconnect the gauge-to-sender wire at the sender; turn on the ignition, then connect the loose wire to the sending unit`s other (grounding) post: the gauge should now read `Full`; if it doesn`t read `full`, then the gauge is likely bad. If it does read full, then the gauge and the wiring appear to be OK and therefore the sending unit is likely bad.

6) You could verify that you have a bad sender by removing the sending unit from the tank and testing it with an ohmmeter to see if moving the arm smoothly through its travel alters the  resistance smoothly between the two terminals. It may be possible to accomplish this arm movement in the tank using a long stiff insulated wire (to preclude any sparks) with a hook on one end to grab the arm... I haven`t tried it and frankly I doubt you can properly manipulate the arm this way .

[kboyd]

I have been experiencing the same problem with the fuel gauge in my 83 as Dan S and though the information provided by WayneC is helpful it does not get to the meat of the question which is where is the best place to start looking.  Removing either the fuel gauge or the gas tank sending unit in an Avanti is a bit involved and it would be good to know if anyone else has experience dealing with this problem before diving in so one would know where is the best place to start looking.

Also does anyone know if Avanti's have a problem with the fuel float in the gas tank getting waterlogged or chemically breaking down overtime.  That would cause similar erroneous fuel gauge readings independent from the electrical connections. 

[grobb284]

I am assuming you have Stewart Warner gages. The Gage can be tested using two resistors, 33 ohms and 270 ohms. One should read empty, the other full. Hook up the sensing lead to one side of the resistor, the other side to ground on the car.  This duplicates the function of the variable resistor in the tank. Do not use the ground on the sender, for this could be the problem. By moving these resistors directly to the Gage, then to the sensing lead at the tank, and utilizing different grounds you should ascertain your problem. You will need a meter to see that you have 12-14 volts into the Gage.

 

We have a board with the resistors necessary for Stewart Warner, Ford, GM, Porsche, and VDO fuel gages made up for just this reason. 

Edited May 28, 2018 by grobb284

[grobb284]

By the way, VDO makes an adjustable sender that has resistance to match Stewart Warner. Make sure you order the correct one 33-270 ohms.

 

I have used this in the Avanti tank several times.

Edited May 28, 2018 by grobb284

[AD]

It could be that the cork float has deteriorated and become soaked with fuel so that it's not "floating" all the way up when the tank is full. I'd check the wiring between the sender and the gauge first, using the resistors as Grob284 suggested. If that checks out ok, remove the sending unit for testing and inspection.

By the way, there should be a small panel in the trunk ,above the tool pouch, that can be removed to gain access to the fuel gauge sending unit. You should be able to inspect the wiring connections and see if anything is corroded or loose. You might decide it's easier to remove the back seat and gain access from the other side to remove the sender though.

[Dan S]

Thanks to all for the advice.  I'm sure I'll be back asking more questions as I discover more about my Avanti.  Much appreciated.