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'[EE]: Auto fuel gauge monitor'
2000\07\21@185043 by Mark Willis

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Have something going on weird with the fuel gauge on one car.

Want to see what's up voltage-wise on that line, using a PIC.

Last I looked at this sort of thing, the gas gauge sensor is a pot from
12V to ground, so you get a fairly high impedance signal coming from
that line, & the gas gauge is basically a somewhat low-current
voltmeter, right?

(The gauge sometimes works just fine, sometimes wanders all over the
place, just want an idea of what's going on there, hopefully without
having to drop the gas tank.)

 Mark

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2000\07\24@011127 by Mark Willis

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It's an early '80ies Mazda;  I've never heard of such a thing, must be
an older design?  That does sound like it'd drive me nuts if I hit it
and didn't know what was going on, though - What era cars have those?

(It's bad enough dealing with bad connectors and corrosion in wiring,
when they throw in oddball things like that, it's time to replace with a
proper part IMO!  <G>)

 Mark

Russell McMahon wrote:
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2000\07\24@030527 by Michael Rigby-Jones

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A lot of older cars, in fact most of them, use a wirewound rheostat (not
actually a potentiometer, only one end of the resistance is used) connected
to an arm with a float to to measure the gas level.  One end of the
resistance is connected to ground, the other goes to the gas gauge.  The
gauge will probably be a "hot wire" instrument based on a bi-metalic strip
with a heating element wound around it.  The current flowing through the
heating element (and therefore the amount of deflection of the bi-metalic
strip) depends on the position of the sensor.  This system is used as it has
a slow response, effectively a low pass filter, and only responds to the
average reading of the sensor, but dosen't cause the needle to jump around
as the gas sloshes around in the tank.

The gauge supply is sourced from a voltage regulator of some description so
that the system is not affected by battery voltage.  The most common, and
crudest regulator was yet another bimetalic strip with a heater connected in
series.  This effectively gives a very low frequency PWM output.  The low
frequency is not a problem because of the response of the hotwire gauges.

There are two common faults.  The first is a break in the gas sensor
resistance winding.  This usualy gives the symptom of working fine with a
full tank of gas, but as the gas level drops the gauge will suddenly drop to
zero.  If the gas level is at a critical point the needle will tend to
wander up and down as the fuel moves around in the tank.

The second common fault is the regulator.  They are actualy surprisingly
relaiable for a purely mechanical system, but they do fail, although usualy
the fail open or shut.  If open, then obviously the gas gauge will be
reading zero.  If shut, the gas gauge will be reading somewhat higher than
normal and will be "pegging" the needle when the tank is full.  The needle
will tend to move depending on engine speed, at least between idle and
cruising speed.

Note that the coolant temperature sensor and gauge usualy work on the same
system and share the voltage regulator, so if the temperature gauge is also
behaving oddly, check out the regulator first.

Hope that helps.  If your car has a fully electronic system, forget I wrote
this :o)

Cheers

Mike

> {Original Message removed}

2000\07\24@051634 by Mark Willis

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Aah.  I was forgetting that it's a rheostat type system - that first
failure mode's a pretty darn good description of the problem;  This
particular car's had a number of corroded joints in the system and some
loose "tab" style (1/4") connectors, to boot.  Runs well, though, which
is the important thing.  I'd thought a loose connector, well, probably
one in the fuel tank sensor.  I'll price one...

Coolant temp gauge had it's own loose 1/4" tab connector, the other day,
other than that seems stable-ish.

 Mark

Michael Rigby-Jones wrote:
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2000\07\24@161957 by Barry Gershenfeld

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On my old car (1963) an open circuit to the sender causes the gauge
to read FULL.  Also since the sender is usually installed with a
rubber seal, there is a separate ground wire from the sender going
to the frame.  What happens if this is dirty or missing is left
to the student.

Never found a "regulator" on this one--unless it's built into
the gauge.

Barry

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2000\07\25@030544 by Michael Rigby-Jones

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> -----Original Message-----
> From: Barry Gershenfeld [SMTP:.....barryKILLspamspam.....ZMICRO.COM]
> Sent: Monday, July 24, 2000 9:03 PM
> To:   EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU
> Subject:      Re: [EE]: Auto fuel gauge monitor
>
> On my old car (1963) an open circuit to the sender causes the gauge
> to read FULL.  Also since the sender is usually installed with a
> rubber seal, there is a separate ground wire from the sender going
> to the frame.  What happens if this is dirty or missing is left
> to the student.
>
> Never found a "regulator" on this one--unless it's built into
> the gauge.
>
> Barry
>
It almost certainly is.  Besides the hot wire meter, the other common type
is a moving iron meter with two coils wound at 90 degrees to each other.
This type of meter is inherently self compensating for battery voltage.  Not
that common on older cars though.  Some of these also tend to stay poiting
to the last reading whe the power is removed, quite usefull to be able to
tell how much fuel you have without having to turn the ignition on.

My car dosen't have a ground wire on the tank, but the sender is held in by
a locking ring which makes pretty good contact with the sender and the tank.

Cheers

Mike

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