[T3] Fuel gauge issues (jumpy needle)

[Jim Adney]

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.

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