[T3] Fuel gauge issues (jumpy needle)

[Jacob Adam Schroeder]

And I thought I was on to something....  Thanks, Jim.  I have noticed my
fuel needle is much "jumpier" the more empty my tank gets--not sure what
that suggests.  I'll just continue to search for the source of my elusive
ground problem.

Jacob

On Thursday, June 11, 2015, Jim Adney <jadney at vwtype3.org> wrote:

> On 11 Jun 2015 at 20:23, Jacob Adam Schroeder wrote:
>
> > I found the time to spend a little while in the garage and can post an
> > update.  Suspecting a ground issue, I got my VOM out.
> >
> > Measuring between the brown wire that goes to my gauge pod and ground, I
> > measure about .6 ohms.  I also measure about .6 ohms between the metal
> > gauge clips (when the gauge is removed from the dash) to ground.  It is
> not
> > surprising to me that these measure the same, since I believe the brown
> > wire ultimately terminates in the gauge clips.  My circuit experience is
> > rusty (don't tell my ECE professor), but doesn't this mean I have a
> > connection to ground?
> >
> > But, when I turn the key to the first click and repeat the measurements,
> I
> > measure about 160 ohms.  Meaning, when the key is on, I am getting
> > resistance between the brown "ground" wire and bare metal on the car.
> > Shouldn't my measurement still result in .6 ohms (assuming no problems)
> or
> > is there something I am missing?
>
> First of all, try touching your VOM probes to each other and see what
> resistance you get. That gives you a baseline of what the probe and
> VOM circuit "zero" is. The real resistance of the circuit you're
> measuring is actually the measured number minus the baseline number.
> It's likely that your baseline is close to 0.6 Ohms.
>
> When you turn the key on, you have energized the gauge circuit, which
> means that there is now come current thru that ground wire. Even if
> its resistance is small, there will be a bit of voltage drop due to
> that current. The VOM works by putting a bit of current thru the wire
> and measuring the voltage that results. When there is an additonal
> voltage, this screws up the measurement. In your case the 2 voltages
> added so you got a higher resistance reading, but if you swap the
> probes around, you'll probably get a lower reading, probably less
> than zero (negative resistance.) Negative resistance is impossible,
> so that tells you you're doing something wrong.
>
> In short, you can't measure resistance in an energized circuit.
>
> --
> *******************************
> Jim Adney, jadney at vwtype3.org <javascript:;>
> Madison, Wisconsin, USA
> *******************************
>
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