[T3] Fuel gauge issues (jumpy needle)

Sean Bartnik sjbartnik at mac.com
Fri May 29 10:19:45 PDT 2015


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

___________________
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> 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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