Fuel Gauge Reading full

[reddie73]

My tank has about 3 gallons of gas in it, but the gauge is reading full. I have new sending unit, black wire attached to pin on sending unit marked G, yellow wire attached to other pin. Tried reversing wires and no reading at all. Any suggestions ?

[Duffman]

Is your sending unit bent too much, thus reading full all the time?

[Duffman]

I mean your float.

[skunkbud280Z]

but I didn't replace the sending unit or change the wires in mine. I disconnected the keyed pins at the tank and removed the float apparatus when the tank was off and being cleaned out. My car hadn't run in about 10 years. It's running now but the voltmeter still reads zero and the fuel gauge still reads full. Because the battery is brand new, the fuel tank has 3 gallons in it, and the gauge is useless reading like this, I just unplugged it. I'm not sure if I placed the float inside the tank incorrectly. I placed it horizontally to the surface of the fuel in its lowest position, assuming the fluid in the tank would cause it to rise accordingly. The arms of my float are perfectly straight so their being physically bent isn't my problem (and might not be yours); likewise, the sending unit might not be my problem either.

Edited October 1, 2008 by skunkbud280Z

[obxtrainman]

Did you place it horizontally rightside up, or rightside down?

[skunkbud280Z]

That maybe I put it in upside down? If I put the float opposite the way I put it in there, it would be in a more elevated position as it sits in the tank. That didn't make sense to me. The float would sit in a low position with an empty tank thereby reading empty, and rise with the fuel level. I guess it's possible I put the thing in there wrong but common sense still tells me I didn't.

But you asked "rightside" up or down. If I remember correctly and I think I do, the float was connected on the arm that was on the right side (passenger's side) and it was in a down or low position (as it rested when inserted into the fuel tank). If I put it in the opposite (and still horizontal) position, the float would be resting in a higher and left (driver's side) position. If the latter case is the correct way to install the float, then I have to assume that gravity alone (via the weight of the metal arm) is enough to drop the float to the level of fuel in the tank. This would seem like the wrong way to install it, but I'm not ready to conclude I didn't make a mistake here either.

[Bonzi Lon]

According to your description you did install it correctly. I got my old one out and photographed it in the position it needs to be in. (Kneeling at passenger rear tire removed)

When mine was removed after sitting 26 years I knew a new one was going back in, just look at the rust and build up. There are a series of wires in the center that the arm is attached to that measure resistance for the guage to function. Corrosion could be giving mis-readings. Just a thought.

What did yours look like when it was out? (getting tank cleaned) Any cleaning before reinstall?

Got my new one at Courtsey Nissan, under $40, and has worked perfect from day one.

Bonzi Lon

[skunkbud280Z]

It is a resistance reading that moves the gauge. At a certain level of resistance or higher the gauge is going to read full, I have to assume irrespective to what position the float is in.

It was somewhat rusty when I pulled it out. Not as bad as the one you're showing in the pictures. It was more spotty with rust than scaley. I used a rust remover gel (a phosphoric-acid based naval jelly) on the (larger) rusty surfaces and rinsed it off. What I didn't do is try to clean the corrosion around the wires. It looked delicate there, there is a very fine metal wire on this that I wouldn't even touch and I didn't want to be "busting my thumbs" all over it trying to clean it immaculately and then end up breaking it. I figured I took off around 75-80% of the rust on aggregate.

As the fuel gauge worked at the car's last running state about 10 years ago, I applied the rule "if it ain't broke don't fix it". Of course now it's broke so the rules have changed. I'm not gungho with climbing under the car and removing it either. BUT someone please tell me I can do this without taking the tank off.

So here I am now and you've got me thinking that A) It's a corrosion issue and I didn't clean the oxidation where the wires attach (sufficiently, or more likely, at all) or Perhaps I left a little bit of the phosphorous coating that jelly tends to leave, as it seems to be chemically attracted to iron oxide as opposed to non-oxidized metal, it might have just "opened the circuit" so to speak and created an infinite resistance at the round piece the arm is attached to. At this point it would have been nice if I had disconnected the fuel pump, installed the battery, and checked the gauge for operation before draining and removing the tank to see whether I "broke" it or if it was already reading incorrectly.

But since it sounds almost sure that I physically installed it right, I'm guessing something on the surface is creating resistance where it shouldn't be.

Oh, and thanks!

Edited October 5, 2008 by skunkbud280Z
gratitude

[EScanlon]

Can someone confirm that the sender has it's highest or lowest resistance reading at the top or bottom range? That way you could do a simple resistance check at the tank and know approximately whether it's on it's way to high or on it's way to low.

When you had the sender out, you might have inadvertently contacted together some of the wires on the sliding surface which would yield a lower resistance, or worse yet broken that wire and have an infinite resistance.

FWIW

E

[Bonzi Lon]

No idea on the resistance thing, I have a hard time with electrical, but thought these photos could be helpful.

The first photo shows the float arm in the full position, this is where the wire wrapings are at the narrowest, approx 6mm. The other photo is when the float arm is in the empty position, wire wrapings at the widest, approx 14mm. This float travels a full 10 inches, the contact arm in the unit travels about 1 1/4 inches, full to empty, and is from a 73.

Bonzi Lon

[EScanlon]

For the record, and to possibly help those who are budding electromagicians...

The width of the wire wraps around the insulated holder, while important in the overall equation of the electric circuit involved are not as significant as the number of turns. The number of turns are factors in the overall resistance that that coil adds/subtracts to the circuit. The more turns between the contact arm (one of the wire terminals outside) and the other contact (the other terminal) the MORE resistance your circuit has. At full number of turns you will have put the most amount of that contact wire into the circuit, and conversely, at the least number of turns the least amount of resistance.

For those old enough to remember slot cars, this was the basic circuitry in your hand controller that allowed you to go faster.

(A side note, while slot car controllers controlled the amount of CURRENT and therefore got warm even hot, this circuit just increases the amount of resistance, hence no worries about a major current flow through your gas tank sender.)

This circuit uses the increasing/decreasing resistance to allow the needle in the gauge to deflect fully or not. I seem to recall that the gauge also had an adjustment hole. Haven't given that a try though.

FWIW

E

[skunkbud280Z]

In my own layman's vocabulary: There is a sliding surface that moves between two terminals. It moves across a number of "winds" or metal coils, making contact with them. When the tank is near empty the sliding surface touches less of those winds, and when the tank is near full the surface touches more of them. Presuming higher resistance indicates greater fuel level.

So now that (I think) I get the fundamentals better, what could be wrong with this thing...

Of note, I did some shaking of the tank before I installed it because when I was carrying the tank out to be installed, I heard a few pieces of crud rattling around in it. So I was sloshing a gallon of gasoline through the tank several times to wash out the loose particles till I couldn't hear anything. A few times while I was swinging the tank in an arcing motion I heard the float apparatus moving around in there (in its 10 inch arc range of motion as was described). Maybe it's possible I caused something on it to shift out of position when holding (shaking) the tank back and forth but I can't see what that would be. The float got stuck on the arm in the full position perhaps? That would seem plausible at least.

There I be, overly cautious while cleaning the thing, but oblivious to breaking it while sloshing the tank back and forth. :stupid: