<VV> Odd Gas Gauge problem

[Craig Nicol]

Bob asked:
Now that I have replaced the hoses in my gas tank in my LM I can finally
fill it up.

Here's the problem. After the fill up, the gas gauge read fine , just below
the "F" for several minutes of driving and then slowly drifted down to a
somewhere between a quarter and a half full where it stayed. I'm assuming of
course that I didn't burn 5 gallons of gas in 5 minutes.....  J 

The sender unit is a relatively new unit from Clarks. The ground and gas
gauge connections are all good and I heard the float moving around when I
had the tank out. I didn't remove the sender at the time as there didn't
seem to be any need.
Any ideas for what might be the cause of the odd gas gauge reading?
Thanks,Bob 66 180 convertible

Craig replies:
If the sending unit resistance checks out OK per Harry's comment, it's the
gauge itself.  

Here's why: 
There's a battle in the gauge between two electro magnets connected in
SERIES. One pulls the gauge up to full and the other pulls it to empty.

One side of the "E" magnet is connected to power. 
One side of the "F" magnet is connected to ground.

The other wire from each coil is tied together with the sending unit (which
provides 90 ohms when full and 0 ohms when empty)

The F magnet is more powerful than the E magnet.

When the tank is full and sender resistance is high (90 ohms) it has little
effect so equal current flows in each magnet and, the stronger "full" coil
wins (Reads F)

When the tank is empty, and sender resistance is low (0 ohms), the F-coil is
virtually shorted out (ground on both sides) and the "E" coil wins. (reads
E)

Now, let's say you have a weak gauge ground.  When the tank is full, it
reads full like it should; even with a weak gauge ground current between the
two coils is equal so the more powerful "F" coil wins. As soon as the
sending unit provides any kind of current path, the normal balance is upset
and the poorly grounded "F" coil loses to the "E" coil much earlier than
normal. 

It's most likely that the grounding failure is internal to the gauge.  If
you take it apart you may be able to clean-up the contacts. 

At least that's my theory!
Craig Nicol

No virus found in this outgoing message.
Checked by AVG. 
Version: 7.5.524 / Virus Database: 269.23.16/1432 - Release Date: 5/14/2008
7:49 AM