[T3] Fuel gauge issues (jumpy needle)

[Jim Adney]

On 28 May 2015 at 15:58, Jacob Adam Schroeder 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.

I believe the gauge circuits are the same for all years.

> 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.

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.  

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.  

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.

> 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?

No, they both need to be good. They are not redundant.

> The bouncing needle to me indicates a grounding problem, but I cannot seem
> to track it down.

It's also possible for there to be corrosion or residue on one part 
of the resistance wire in the sender. This will show up as jumping 
around when the gas gets to that particular level in the gas tank. 
This can be fixed by gently rubbing the wire in the sender between 
your fingers until it's cleaned off. BUT, you have to be careful, 
because that wire is soft and it's easy to stretch it and make it 
unusable.  

Or you can send the sender to me for rebuilding. There are other 
bad/intermittent connection problems that commonly occur in the 
senders that I will fix.

> 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.

[If you really wanted a better test you could connect the tank wire 
securely to ground thru a 40-60 Ohm resistor (that's ~half scale) and 
then drive around a bit to see if things are still jumpy. But you 
should be able to fix this without going to all that trouble.]

> 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? 

No, as long as at least one bolt is snug and makes a good connection 
to the threads.  

> Also, could be unrelated, but I also get an intermittent fast turn signal
> blink that may share a similar cause.

I think that's probably unrelated. More likely a bulb that's making 
poor contact.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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