[T3] Fuel gauge issues (jumpy needle)

Jacob Adam Schroeder jacob.schroeder at gmail.com
Sun May 31 07:51:33 PDT 2015


Thank you, Sean.  This makes sense.

On Fri, May 29, 2015 at 10:19 AM, Sean Bartnik <sjbartnik at mac.com> wrote:

> You basically have the concept right, the fuel gauge gets power through
> the black plastic bulb holder and the ground goes through the fuel sender.
> The fuel sender is basically just a variable resistor that changes
> resistance based on the position of a float. So the fuel gauge is basically
> just a voltmeter in that sense.
>
> When you pull the wire off the sender and ground it to the body, that's
> providing a resistance-free ground which is what the sender does when the
> tank is full.
>
> There can be issues though with the grounds of other components (like
> light bulbs or the gauge pod itself) that cause current to take the wrong
> path to ground, such as back through the gauge or through other light bulbs.
>
> So make sure that fuel gauge pod is grounded properly. It is supposed to
> ground by contact with the dashboard via the spring clips when inserted all
> the way into the dash. If you want to improve on that, you could add a
> dedicated ground wire from the metal body of the gauge pod to a ground
> point under the dash.
>
> -Sean
>
> ___________________
> Sent from my iPhone
>
> > On May 29, 2015, at 1:10 PM, Jacob Adam Schroeder <
> jacob.schroeder at gmail.com> wrote:
> >
> > I suspect it is a ground issue, but I am less certain of how to confirm
> > that is the case.  I guess I am having trouble understanding where the
> 1972
> > fuel gauge circuit is supposed to ground.  It appears as though a signal
> is
> > sent from the gauge through the sending unit and that the sending unit
> > grounds to the body through the tank.  Is that correct?  Are there any
> > other grounds?
> >
> > On Thu, May 28, 2015 at 6:23 PM, Craigs List <
> craigslistcsprings at gmail.com>
> > wrote:
> >
> >> Have you checked that is is properly grounded? A bad ground can cause a
> >> jumpy needle. I have one myself.
> >>
> >> Adam
> >>
> >> On Thu, May 28, 2015 at 6:00 PM, Jacob Adam Schroeder <
> >> jacob.schroeder at gmail.com> wrote:
> >>
> >>> Also, I saw the fix that somebody shared previously about drilling a
> hole
> >>> in the center of the sender.  That was interesting but I'd prefer not
> to
> >>> make that mod if I can avoid it.
> >>>
> >>> On Thu, May 28, 2015 at 3:58 PM, Jacob Adam Schroeder <
> >>> jacob.schroeder at gmail.com> wrote:
> >>>
> >>>> I have a 1972 Squareback and have been having a recent issue with my
> >> fuel
> >>>> gauge.  I recall the the gauge used to work, so I am not sure what has
> >>>> changed over time.
> >>>>
> >>>> The gauge appears to be accurate, when it is steady.  But the problem
> I
> >>>> have is that the needle jumps around like crazy (especially when I am
> >>>> driving).  From looking at the gauge, it appears that there are two
> >>> ground
> >>>> points: (1) through the housing to the spring clips to the dash, and
> >> (2)
> >>> a
> >>>> ground wire to the back of the gauge through the black plastic piece
> >> that
> >>>> clips on to the rear of the gauge.
> >>>>
> >>>> Are these grounds redundant in that only one has to be good?  Do both
> >>>> grounds service the fuel gauge or is the wire for the lights and the
> >> fuel
> >>>> gauge grounds through the clips?
> >>>>
> >>>> The bouncing needle to me indicates a grounding problem, but I cannot
> >>> seem
> >>>> to track it down.
> >>>>
> >>>> Also, I followed the test in Bentley--disconnecting the sender and
> >>>> grounding the wire to the sender housing, and the gauge reads full.
> >> So,
> >>>> according to Bentley, the sender is good.  I tested a spare sender and
> >>> get
> >>>> the same jumpy needle.  So I am fairly confident my sender is OK.
> But,
> >>> do
> >>>> the late (1972) sending units need a tab of exposed metal on the tank
> >> for
> >>>> grounding?  I saw something to that effect in Bentley and I did
> repaint
> >>> my
> >>>> tank within the past few years, but did not leave a portion of exposed
> >>>> metal along the circumference of where the sending unit mounts to the
> >>> tank.
> >>>>
> >>>> Also, could be unrelated, but I also get an intermittent fast turn
> >> signal
> >>>> blink that may share a similar cause.
> >>>>
> >>>> Any help would be appreciated.  Thanks,
> >>>>
> >>>> Jacob
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