[T3] Fuel gauge issues (jumpy needle)

Mike Fisher fisherfarms at gmail.com
Sun May 31 10:02:34 PDT 2015

I noticed my gas gauge was flickering a little on the drive to the 30th
Northwest Bug Run at the Woodburn Dragstrip yesterday, but it doesn't
bother me enough to worry about eliminating it! :-)

On Sun, May 31, 2015 at 9:17 AM, Jim Adney <jadney at vwtype3.org> wrote:

> On 31 May 2015 at 8:01, Jacob Adam Schroeder wrote:
> > 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.
> Yes, that black wire goes inside the black lamp base and solders to
> one of the circuits there. This is rather clearly shown in the wiring
> diagrams in the Bentley. Not so clear on those wiring diagrams is the
> brown ground wire: That's shown as if it's connected to the metal
> gauge body, but no such connection exists. It actually connects to
> tab #31, as you describe below.
> > > 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.
> If you had done something to protect the paint from those clamps,
> like put a  plastic sheet in there to prevent paint damage, then you
> would have a  problem. Otherwise, it seems unlikely that at least one
> of the clamps would have cut thru the paint.
> Either way, it's easy to test this with an Ohmmeter. Just check for
> continuity between the sender base and the body. You should get
> something less than an Ohm. For the least confusing results,
> disconnect the wire from the sender while you check this ground.
> > > 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.
> I THINK this indicates that there's a poor ground somewhere.
> As a test, I just went outside and checked 2 cars in my driveway. The
> '69 gauge needle did not flicker at all with the 4-way flashers,
> either with or without the key ON. It's tank was nearly empty,
> meaning <10 Ohms in the sender.
> BUT, on my '73, the gauge flickered wildly with the key OFF, not at
> all with the key ON. However, this may have been due to the tank
> being full, meaning ~100 Ohms in the sender. I haven't noticed
> flickering gauge readings on that car, but it looks like I need to
> look into the grounding.
> > > 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?
> Okay, you've clearly looked into this carefully, so I went down and
> looked at several of those plastic bulb bases. You're right, they
> have a term 31 on them. Yes, that's ground. I had forgotten that
> terminal was there. Yes, from '69-on there should be a brown wire
> that daisy-chains all the dash instruments together, including the
> gas gauge, and then connects to a tab on one of the braces under the
> dash. Yes, that ground makes the spring clip ground redundant, so I
> think you should follow that brown wire and make sure that it's
> getting to chassis ground properly.
> On cars earlier than '69 that brown wire would not exist, so either
> that #31 tab was left bare, or the early black lamp bases had only a
> rivet there and no tab. Or maybe someone with a '68 or earlier can
> fill in the details.
> I think you're on the right track, but I don't know anything special
> to suggest. One thing to keep in mind is that it's possible for there
> to be a poor connection between that #31 tab and its rivet. One more
> data point that I remember from long ago was that my very late '71
> had a bad problem with that ground, and it was fixed by getting a
> better ground at the spring tabs. Maybe this points to a systematic
> problem with that #31 tab and its rivet....
> Another test one could do, would be to do the 4-way flasher test at
> night with the instrument lights on, and see if the instrument lamps
> also flickered with the flashers. That would tell you which side of
> the ground wire the bad connection was on. I'll try to remember to do
> that test on my '73 tonight.
> The actual gas gauge circuit is a bit of a mystery to me. If you take
> it apart,
> you'll find 2 different resistors wired in with the meter movement in
> a way that doesn't lead to any obvious conclusion. Since it has
> always worked for me, I've been willing to give VDO credit for a good
> design and move on to more  pressing issues. If you manage to figure
> it all out, please let us know how it actually works.
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
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