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

[gassyal]

Hi group:

I notice on many cars that when you turn off the motor, the fuel gauge
setting does not rest on Empty. Also, if you remove the gauge cluster
from the car, the fuel gauge setting behaves the same.

The other gauges (speedometer, tach and temperature) seem to rest at
their low setting when the engine is turned off; but the fuel gauge
behaves much differently.

Why is this?

I look forward to your comments.

Regards,
Al Gershen
Grants Pass, OR
***@hotmail.com

[Comboverfish]

Post by gassyal
I notice on many cars that when you turn off the motor, the fuel gauge
setting does not rest on Empty. Also, if you remove the gauge cluster
from the car, the fuel gauge setting behaves the same.
The other gauges (speedometer, tach and temperature) seem to rest at
their low setting when the engine is turned off; but the fuel gauge
behaves much differently.
Why is this?

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

[gassyal]

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

Post by Comboverfish
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

[D***@yahoo.com]

Post by gassyal
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

Post by Comboverfish
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

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

[sdlomi2]

Post by gassyal
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

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

Post by gassyal

Post by Comboverfish
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

[Comboverfish]

Post by gassyal
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.

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

[Ted Mittelstaedt]

Post by gassyal
I notice on many cars that when you turn off the motor, the fuel gauge
setting does not rest on Empty. Also, if you remove the gauge cluster
from the car, the fuel gauge setting behaves the same.
The other gauges (speedometer, tach and temperature) seem to rest at
their low setting when the engine is turned off; but the fuel gauge
behaves much differently.
Why is this?

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

[phaeton]

Post by gassyal
Regards,
Al Gershen
Grants Pass, OR

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-phaeton