[T3] Fuel gauge issues (jumpy needle)

[Dave Hall]

The tiny hole in the sender casing keeps the level inside relatively
constant, which dampens rapid changes of the float position quite well. Most
likely the wire is a bit dirty in that area, but it is a VERY delicate
operation to clean off the dirt, and very easy to break the wire trying it.
If you can live with it, you're probably best leaving it alone.  

You may find there is an intermittent contact in the circuit rather than in
the sender.

Dave
UK VW Type 3&4 Club
===================

-----Original Message-----
From: type3-vwtype3.org [mailto:type3-vwtype3.org-bounces at lists.vwtype3.org]
On Behalf Of Jacob Adam Schroeder
Sent: 12 June 2015 05:46
To: type3 at vwtype3.org
Subject: Re: [T3] Fuel gauge issues (jumpy needle)

And I thought I was on to something....  Thanks, Jim.  I have noticed my
fuel needle is much "jumpier" the more empty my tank gets--not sure what
that suggests.  I'll just continue to search for the source of my elusive
ground problem.

Jacob

On Thursday, June 11, 2015, Jim Adney <jadney at vwtype3.org> wrote:

> On 11 Jun 2015 at 20:23, Jacob Adam Schroeder wrote:
>
> > 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?
>
> First of all, try touching your VOM probes to each other and see what 
> resistance you get. That gives you a baseline of what the probe and 
> VOM circuit "zero" is. The real resistance of the circuit you're 
> measuring is actually the measured number minus the baseline number.
> It's likely that your baseline is close to 0.6 Ohms.
>
> When you turn the key on, you have energized the gauge circuit, which 
> means that there is now come current thru that ground wire. Even if 
> its resistance is small, there will be a bit of voltage drop due to 
> that current. The VOM works by putting a bit of current thru the wire 
> and measuring the voltage that results. When there is an additonal 
> voltage, this screws up the measurement. In your case the 2 voltages 
> added so you got a higher resistance reading, but if you swap the 
> probes around, you'll probably get a lower reading, probably less than 
> zero (negative resistance.) Negative resistance is impossible, so that 
> tells you you're doing something wrong.
>
> In short, you can't measure resistance in an energized circuit.
>
> --
> *******************************
> Jim Adney, jadney at vwtype3.org <javascript:;> Madison, Wisconsin, USA
> *******************************
>
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