Why doesn't Fuel Gauge rest at Empty when car is turned off?

Fuel gauges aren't spring loaded or "held in place" so much by a magnetic field as with other guages. I suspect they are typically glycerin-damped as well, a guess based on observing gauge operation while grounding or opening the signal lead during testing. IOW, if you remove voltage potential to the gauge, the magnetic field around the gauge "stator" will disappear, but the needle won't move because there is no mechanical force built-in to afect it.

The above addresses the modern gauges that I see currently, and is not to say that all gauges behave in this manner. Bimetal gauges would return to empty when power was removed from the circuit.

Toyota MDT in MO

Hi Comboverfish:

If the voltage potential is removed from the fuel gauge, why doesn't the needle return to it's "rest" setting (i.e., the Empty location) as a result of "gravity?"

I look forward to your additional comments.

Regards, Al Gershen Grants Pass, OR snipped-for-privacy@hotmail.com

Comboverfish wrote at Thurs, Oct 12 2006 9:42 am:

Some are driven by tiny servomotors that just stop wherever they were when the system was shut down. My old Ford F150 had an oil pressure gauge that did that. Others I have seen have opposing electromagnets to pull the needle one way or the other, with one being driven by the tank potentiometer and the other by a reference resistor. Some of those are inclined to stay put after shutdown, especially those without any return spring in them. Really old systems used a thermal gauge, where varying current drove a bimetal strip that warped when warm and pulled the needle. Those would drop to zero after shutdown.

Dan

I'm glad Dan Thomas, above, gave US a descriptive, reasonable-sounding, electrically-justifiable (NO sarcasm intended at all) reason. I would have guessed it was due to a tad of friction that gravity would not overcome, but electrical force from the tank unit would. But then, some of the 73-75 Buick Regals had fuel gauges--big and round with center-pivot needles-- that would actually rise toward full, as soon as the ignition switch was turned off. This meant that depending on actual fuel level, sometimes the needle went against gravitational pull direction, sometimes with it. Had a couple of customers bring their newly-purchased cars back to me, thinking the car's fuel gauge was defective. s

I suspect those guages are "vanity' jobs, that is, they are simply analog display devices tied to the car computer, and not actually tied to the sensor in the fuel tank. When power is removed the guage needle just floats around where it feels like.

Unfortunately, auto dashboards have become the domain of the idiot designers. We have 2 vehicles where the dash is 100% digital output, not a scrap of an analog guage at all on them. That makes perfect sense since the dash is tied to the computer and the computer is what is tied to the sensor. Unfortunately, the automakers decided sometime after

1996 (our vehicled were made in 1994 and 1995) that the general auto public couldn't read a speedometer when it was outputing in a series of digits, and could only deal with an old archaic spinning needle. So they put even more, extra, programming in the computer to simulate an analog guage. Completely stupid.

Ted

I attribute it to the glycerin dampening. The needle moves very slowly even when magnetic field is present. I don't think it can move without quite a bit of force.

Toyota MDT in MO

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