Fuel System: Avanti gas gauge

[swvalcon]

The gas gauge on the Avanti doesn't work. Have power and ground at sending unit and touched the hot wire to ground and the book says that should peg the gauge if it's working but nothing. So gauge or sending unit?

[Hawklover]

I had similar situation 50 yrs ago, I needed a new gauge.

[gordr]

Be sure the gauge itself is grounded. It is in a fiberglass dash, after all. If you see voltage at the "hot" side of the sending unit unit, it means the gauge is seeing 12 volts from the ignition switch. Also check to make certain the ground wire to the tank unit is in fact grounded. My guess would be right now that it is not. Try grounding out the sending unit wire to a known good ground, like the battery negative post. If current flows in the "hot" wire to the sending unit, the gauge should deflect up-scale, even if the gauge case is not properly grounded.

[swvalcon]

Gordr Think the sending unit ground is working because if I hook my test light ground to the ground wire and the other end to the power wire I get a light. Not all that bright but does light. maybe I will run a jumper wire from the battery ground to the sending ground and see if anything changes.

[bezhawk]

The "low" ohms of a gas sending unit is never "0" ohms. If you ground out the gauge it sends too much current to the heating element that moves the bi-metals strip that moves the needle. Most likely it is now fried.

[gordr]

The sending unit wire is not really "power". A voltmeter may show 12 volts open-circuit, but the resistance of the gauge element is between the gauge power source stud and the wire to the sending unit. It will pass enough juice to dimly light a test light, for sure. I have tested gauges by grounding the sending unit wire many times. As soon as I see the pointer begin to move, I break the connection. Leaving it on long enough to peg out the gauge will do damage, for sure. It's amazing just how robust those stupid things really are. Also, when grounding out the sender wire, did you wait a few seconds for the gauge to react? They are "thermo" gauges. The current flow goes through a tiny heating element wrapped around a bimetal strip, which bends as it heats up, and moves the pointer. That takes a few seconds to take effect. I just drove my Avanti today, and the gas gauge took maybe a minute to stabilize at the "1/2" mark. And note that those gauges contain two bimetal strips, acting in opposition. One is controlled by the sending unit, and acts to move the pointer up-scale. The other (less powerful) is on all the time, and acts to move the pointer down-scale. If the sending unit unit goes open, that one will acts to press the pointer onto the low peg, and it takes some time for the other bimetal to overcome that. The reason for the two bimetals in opposition is to make the gauge reasonably insensitive to system voltage changes. If voltage goes from 12.6 to 14.2, both bimetals push a little harder, and the pointer stays put. Lacking the compensating element, the pointer might go from 1/4 to 1/2 as the engine is revved up and the alternator kicks in. That would be suboptimal.

[Stormy]

I have no idea if this helps, but some years ago the gas gauge was not working in my 1963 Avanti. On a whim , I removed the sending unit and then submerged it in an ultrasonic cleaner. Whatever was wrong was "fixed" by the ultrasonic, as I reinstalled the sending unit and all worked fine.

[swvalcon]

Pulled the sending unit today and after playing with it for a bit got the gauge to move. Slower than molasses in Minnesota in January but works. Think it will get better with use. This car has sat for over 30 years with no gas in the tank. Going to coat the cork float with a sealer that a firend of mine said he used on his Daytona over 20 years ago and it stll works. Cork looks to be in good shape but now that it's out this is the time to make it last as long as possible. Going to get some electric cleaner spray and see if I can get rid of some of the rust and corrosionon the sender and wire clips. will let you all know what happens.