The regulator supplies both gauges with ten volts +/- half a volt, so your
regulator is just in spec on the high side (this will raise the reading
half a needle width or less compared to ten volts).

If grounding the sender wire *at the sender* makes the needle shoot for the
sky (and the gauge start smelling hot if you leave it grounded for a minute
or two because it's getting twice the current it expects to see) then your
only candidates are bad contact at the sender, the sender itself, or bad
sender ground.  You seem to have eliminated the sender itself.

Open circuits like this are easily located with a voltmeter, by discovering
at what point in the circuit the voltage goes from [in this case] ten volts
relative to ground, to zero volts.  So if you have zero volts on the sender
"hot" terminal it's the connector; and if you have ten volts on the sender
ground terminal, the ground is bad.

Yrs,
d

On Sun, Aug 12, 2018 at 12:05 PM, Jim. Felder <jim.felder@gmail.com> wrote:

> I have one in my hand. I have a working temp gauge in my other hand (don't
> ask how I am typing). I am trying to figure out whey one gauge would be
> affected by a bad regulator and the other would not be. You might want to
> try another gauge. The 700Y just clips its two outer terminals into
> connectors on the foil. No soldering. Which raises the question, on a 33
> year old car, maybe you should remove and clean the terminals first?
>
> Let me know what you end up needing.
>
> Best,
>
> Jim
>
> On Sun, Aug 12, 2018 at 10:18 AM Dan N <dn92610@gmail.com> wrote:
>
> > hi all,
> >
> > I suspect that the voltage regulator is defective, but not sure because
> the
> > gas gauge is working good but I am willing to give it a try...
> >
> > so does anyone know where to get one? please... searching the net but
> found
> > none
> >
> > this is the one I am talking about, sitting on the blue foil...
> >
> > http://www.datasheetcafe.com/tca700y-datasheet-voltage-regulator/
> >
> > thanks in advance
> >
>