How is the gas gauge *supposed* to work?

So I'm aware that these cars often have wonky fuel gauges. I've heard the stories, from just going bad for no reason, to the idea that leaving the ignition on while fueling causes it to work strangely.

Usually by "work strangely", people mean that the gauge sticks.

On my 2000 M Coupe, however, it is just wildly non-linear, and I'm trying to figure out if this is also a failure, or if it's just the way it's supposed to work. The previous owner apparently had the sender replaced under warranty once..

Anyway, when I fill the car, it's a good 3+mm above full, and the car runs easily 100+ miles before it's even at the 7/8 full mark. But the lower the tank goes, the faster the needle accelerates towards death. I've noticed this on a number of tanks. Last night I started the car up with 204 miles on it, and the gauge was at the 3/8 mark. Within 3 miles, it had passed the 1/4 mark, and by 212 miles total, it was halfway thru the reserve block and the light was on.

The gauge is extremely consistant in its behavior, and I filled it up once near empty and still had 1 gallon left (well okay, I put 12 gallons in, I HOPE that means I had a gallon left).

It's almost as if this sender was designed for one tank, and then used in another -- like a sender for a relatively square tank was screwed into a conical tank that narrows at the bottom.. I suspect I might not be far off, if everyone's car does this, and perhaps it's the E36 part in a non-E36 tank?

- reid

Reply » How is the gas gauge *supposed* to work?

The gas guage is non-linear. It doesn't quite reflect the miles to empty that you would expect. From memory, I get about 150 miles on the 1st 1/2 tank and then about 80-100 miles on the second 1/2 tank. Below 1/4 tank you seam to have about 30-40 miles before the light comes on.

No practical way to fix this unless you have a EE degree and know how to do a signal conditioner to take out the non-linearity.

Reply » How is the gas gauge *supposed* to work?

Kind of like my boat. It has a V shaped tank in the bottom of the hull and the needle will stay between full to 3/4 for ages. But man when it gets down to 1/2, time to start looking for fuel because it is probably really down to less than 1/4 tank. You can almost watch the needle drop!

Reply » How is the gas gauge *supposed* to work?

The senders are know to fail and stick in places. Several of us have had the changed. yours might be going south.

Reply » How is the gas gauge *supposed* to work?

Quote:

Mine is exactly like that. I reset the odometer every time I fill up. If I get close to red, I fill up. I hardly ever drive with the red light on.

On occasion, I stays on empty for a while after I filled up. That doesn't happen very often though...

Reply » How is the gas gauge *supposed* to work?

In any car, the gas gauge sender is basically a variable resistor connected to a float.

If your tank is regular, meaning rectangular or square, or a cylinder flat side down, the gauge would read with some semblance of linearity.

In the vast majority of cars nowdays, the tanks are irregularly shaped to fit within a space in the car that protects it in-case of accident.

In fact, I'm under the impression that our gas tanks are plastic. Ths allows even MORE creativity in making it fit a space.

Anyways, the irregularities prevent a linear device from reading with much accuracy. Some makers compensate some by printing the lines on the meter in places other than the 1/4, 1/2, and 3/4 places.

Then the other item is some senders are affected by deposits built up on the resistor from additives in the gas. These create areas where the sliding contact gets no signal, or incorrect signal, causing the wacky gauge syndrome.

EDIT: Actually, I just realized that the resistance can be adjusted some, by wire winding with different spacings, or changing the properties of the resistance, to compensate for the non-linearity of the gas tank shape. It still leaves some accuracy issues tho...

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