<VV> Gas gauge accuracy

[Tony Underwood]

At 02:55 AM 7/28/2009, J R Read_HML wrote:
>If I recall correctly, the resistance element (LM) in the tank has 2 
>(maybe 3?) coils to it.


Nope, it's a single wire-wound rheostat, one coil wrapped around a 
nonconductive form.


>Should one of those coils fail, the reading will be too high on the 
>gauge with a full tank.


Only one resistor element, so if it fails anywhere other than at the 
point where the resistance wire is crimped onto the lug exiting the 
sending unit, the gauge will read normally until the sending unit 
wiper reaches the break, then the gauge reads over-full.


>BUT, it will work (the dash gauge) close to normal below at about 
>1/3 of a tank.  Correct?


It will work normally until the wiper reaches the break in the 
resistance wire.



>It will provide the impression of good gas mileage for about 1/2 a 
>tank - and then... What the H happened...
>
>Maybe EM is the same?  Just at a different OHM range?


The earlies are a bit more than half the resistance of the lates, 
although exact figures are kinda moot; I've yet to find two of the 
same vintage that read alike with an ohmmeter.   I recently went 
through several late sending units (looking for an early) and the 
three measured between 85 and 100 ohms, give or take an ohm or 
two.    The early I ended up with came out to about 50 ohms.




>Also... I think that you meant to say ONE of two things.


Yep...   ;)   My bad...






>>Incidentally, if the gauge reads above full it's likely being caused
>>by two things:





tony..    considering cutting/welding a section out of a later fuel 
tank onto my '60 tank so I can use a '61-64 sending unit that will 
read correctly