[T3] fuel gauge and gauge light fluctuating
Jim Adney
jadney at vwtype3.org
Sun Sep 21 10:25:22 PDT 2014
On 21 Sep 2014 at 2:36, David Yaghoubian wrote:
> Driving home tonight in my 68 square I noticed the fuel gauge was
> fluctuating quite a bit (it is usually steady) and the light to that gauge
> was dim. None of the other gauges or gauge lights were acting up, so I
> thought maybe the harness at the back was loose somehow, and/or bad ground
> somewhere. A slap on the dash above the fuel gauge seemed to confirm this.
> The light brightness of that gauge fluctuated, as did the fuel gauge
> reading, now sometimes tapping out at full, or reading near empty.
This is a fairly common problem. I THINK the problem is due to the
fact that up thru '68 the gauge gets it's ground via the springs that
retain it in the dash, but that seems to be at odds with some of your
observations. I've never seen a problem with the 3 contact posts;
those are always pretty tight, but I can see now that this is a
possibility.
I think this problem arises after many years as the vinyl dashboard
pad shrinks and loosens the gauge units. It can be compounded by
people pushing the retainer springs out PAST the humps in the metal
dash, which actually makes them looser. Starting in '69, the speedo
and clock bodies got their own dedicated ground wires, but the left
gauge body never did. Looking at the lamp bases I have here, they all
have a #31 (ground) faston tab on the back just below the hole where
the black wire exits. On my later cars, that tab gets a ground wire.
Does that #31 tab exist on your '68 lamp base? If not, you could
either solder in a brown ground wire next to the black power wire, or
swap in a later lamp base (I have several) that has that ground tab,
and make a brown ground wire to ground that tab. If you want to
solder in a new ground wire, just slip it in next to the black wire
and solder it with the other wires that are joined to the socket just
below the hole where the black wire comes in.
All the lamp bases I've found in my pile of gauge parts have this #31
ground tab, but it's quite possible that I don't have any early ones.
Looking at the parts list, I see that only one lamp base is listed,
and that is a B version which is listed for all years. That means
there were 2 previous versions, so it seems likely that a
non-grounded version existed. I don't know what other version there
might have been, but people who know the really early cars may know.
I suspect that your attempts to run a separate ground wire were
stymied by corrosion on the housing, making good contact iffy. But
I'm always puzzled when this question comes up. I've seen this
problem in later cars, where the ground wire should be there. I just
checked my '71 and it seems to have a wire of some kind, presumably a
ground, attached to that tab. Any yet I've had other '71s display the
same problem that you have. Maybe you're right that the problem is
with the center ground pin between the metal housing and the lamp
base.
One of the common symptoms is that if you turn on the 4-way flashers
with the key off, the gas gauge needle bangs from limit to limit. Do
you see this? This all seems to get cured by tapping/moving the
instrument. Maybe the rivet under that ground tab loses contact. That
seems unlikely, however, since I don't see similar problems with the
other rivets. Or maybe there's a problem at the other end of my
ground wires.
Let me know if your '68 has that #31 tab. This comes up often enough
that we really should try to get to the root cause of it.
--
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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