Fuel gauge adjustment?

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Austinsguns

Austinsguns

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71 duster... The fuel gauge was working intermittently, so I cleaned all connections and pulled the guage cluster (ralleye). I works now, I filled it up with gas and drove it this week and it never dropped more than to 3/4 of a tank. I know I used over a half a tank as I filled it yesterday and it took 9 gallons.

Anyone ever see a gauge out of adjustment?

What is the small back knob next to the terminals for the fuel gauge on the circuit board of the back of the gauge cluster? Some kind of adjustment? (no im not talking about the dash lights)
 
Its not a knob. Its a cap similar to a wire nut. Its ther for a couple reasons...
it keeps the noise cap' connector from being placed on the wrong stud.
A defrost tube passes very close to that stud. That tube has a wire in it. If that wire should chaff to that gauge stud there could be another dash fire blamed on the ALT' gauge.
These gauges are adjustable to a point. In most cases its a waste of time. You'll adjust a bad gauge again and again. Here's the scoop... Inside the gauge is a bi-metal strip that bows when heated. It started out straight as an arrow and should return to that at room temperature. the heat comes from a ni-chrome wire wound around the bi-metal. That wire is insulated with a spiral wound, fibrous, nearly invisible, very flexible, cover. Over time that insulation becomes crunchy black carbon. Touch it and it will fall off. I've seen some that looked many times its original size. This restricts bi-metal movement. See pic...
 

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How can I test the sender and gauge to determine which is bad. I have yet to buy a service manual. I do not have info on ohm readings for these.
 
80 ohms or greater = no needle movement
73 ohms = empty
23 ohms = 1/2
10 ohms = full
 
Or run a jumper wire from the guage wire at the tank long enough to watch the guage and touch it to a ground.If it goes to full the guage is good.
 
Drop the tank, fill it with different amounts of water and adjust the float rod until you see it reads what you want it to read. I was never able to get mine to read accurately. My advice is to just leave it be, they are what they are. As long as you know where empty is that's all that matters really.
 
wannadrag said:
Or run a jumper wire from the guage wire at the tank long enough to watch the guage and touch it to a ground.If it goes to full the guage is good.
Click to expand...

Shorting the gauge to ground does 2 things...
Supplies / = 0 resistance ground path.
Tells the mechanical limiter it should send up to full battery voltage to all gauges.
All the test really does is show the gauge isn't burned open inside.
It's a good way to kill a gauge too. It could burn open, toss the crusty insulation like so much black pepper in the bottom of the gauge or stick the movement at the extreme end of travel.
If/when that happens... DO NOT TAP ON THE LENS !
 
I crawled underneath to test it... And I noticed that the ground strap was not hooked up on the sending unit. I attached it, tested it (at 3/4 full according to gauge) and it said 13 ohms. Then I drove 8 miles and topped it off and it took 5 gallons. So I think it is close.



I will let it get closer to empty this time and repeat.
 
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