[T3] Fuel gauge issues (jumpy needle)

[Jacob Adam Schroeder]

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?

Jacob



On Sun, May 31, 2015 at 8:01 AM, Jacob Adam Schroeder <
jacob.schroeder at gmail.com> wrote:

>
>
> On Fri, May 29, 2015 at 11:47 AM, Jim Adney <jadney at vwtype3.org> wrote:
>
>> On 28 May 2015 at 15:58, Jacob Adam Schroeder wrote:
>>
>> > 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.
>>
>> The black wire that comes out the back of the plastic bulb holder is
>> a power wire, not a ground.
>>
>>
> That black wire, for me, is soldered to the black plastic pod.  Is that
> really how it is supposed to be connected or is it supposed to be a push-on
> style connector?  Either way, that is more a curiosity than related to my
> present issue.
>
>
>
>> There are 2 grounds in the circuit:
>>
>> 1 is at the  gas tank, where the sender is grounded via its mounting
>> bolts. The sender does not need an unpainted place there, as the
>> bolts should make contact in the threads in the tank. You could turn
>> those bolts back and forth a bit to make sure that they have cut thru
>> the paint. The tank also needs a good ground to the body, so make
>> sure the 4 clamps and bolts around the edge of the tank are tight.
>> You could loosen and retighten each of those to make sure they are
>> making a good connection.
>>
>>
> Because I recently repainted the exterior of the tank, I will give this a
> shot.  I can see how the threads in the sending unit bolts could make a
> good ground connection to the tank.  I may turn those back and forth a bit
> to make sure any paint is cleared out.  But what about the paint along the
> edge of the gas tank where the 4 clamps are?  Those clamps (IIRC) thread
> into the body and only press against the exterior of the tank.
>
>
>
>> The second ground is via the spring clips that hold the gauge in the
>> dash. Sometimes these get spread too far out and they loose tension.
>> There's a hump in the dash that those spring clips should NOT go
>> outside of. Or sometimes it's just that the plastic dash has shrunk
>> slightly and no longer hold the spring clips tight against the
>> backside of the dash.
>>
>> If the gauge head ground is bad, the gauge needle will jump around if
>> you turn on the 4-way flashers. I'm not sure if this happens with the
>> key OFF or ON, or both. With a good gauge head ground I think it will
>> not happen at all.
>>
>>
> I do get some (very minor) jumping in the fuel gauge needle when using the
> 4-way flashers.
>
>
>> It should be possible to add a 3rd ground in the form of a brown
>> (ground) wire from an appropriate place in that plastic piece out to
>> a ground terminal under the dash. This will fix a poor ground at the
>> dash.
>>
>
> I think I may have this ground already.  My black plastic piece has a
> terminal (labeled 31) with a brown wire connected to it that appears to be
> ground.  Once it connects to the black plastic piece, it is connected to
> the gauge pod through one of those copper nipples on the back of the gauge
> pod.  Assuming this provides ground to the body of the gauge pod, then why
> would the ground through the spring clips be necessary as opposed to
> redundant?  Are the grounds for the lights separate than ground for the
> fuel gauge?  Either way, I know I have a ground problem but I am just
> trying to understand the system.
>
>
>> > 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.
>>
>> Other way around: What that test tells you is that the gauge head is
>> good. It bypasses the sender, so the sender didn't get tested. It's
>> also not a good test of the head ground, because it was just a
>> momentary test, so you didn't get to see if it was still occasionally
>> jumpy.
>>
>>
> Yes, you are correct that I misspoke.
>
> Jacob
>
>
>
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