<VV> Gas gauge accuracy

[Tony Underwood]

At 12:36 PM 7/27/2009, aeroned at aol.com wrote:
>Tim,
>
>There's an easy way and a slightly more difficult way.
>
>The gas gauge is an ohmmeter, I can never remember which is the high 
>resistance, F or E. You just need to get the right value resistor to 
>put in series with the sender to pull?both readings down. That's the 
>"easy" fix.
>
>The correct solution (somewhat more difficult) is to pull the sender 
>and adjust the float arm.




Earlies use a tank sending unit with resister elements that are lower 
in resistance than the lates.   Most lates have the resistance of the 
rheostat element stamped on the housing.   Earlies, not so many...

E is what the gauge should read when the sender is at its lowest 
resistance, in other words -zero- ohms which is the same as grounding 
the wire feeding the sending unit.    If the gauge won't read E when 
the tank is dry, the float arm is bent and needs adjustment.    When 
this is the case you can sometimes hear the float itself clunking 
against the tank bottom when fuel is really low in the tank.    A 
correctly adjusted float arm (and yes they are adjustable, via simply 
bending the arm itself) will place the float about 1/4 inch off the 
bottom of the tank.   It shouldn't actually touch the tank bottom, 
ever... because when fuel is really low, sloshing will allow the 
float to bounce off the tank bottom and it's possible to eventually 
wear a hole in the float which will then fill with fuel and sink.


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


That same bent float rod, combined with a bent stop on the variable 
resistor wiper housing (look at one closely, you'll see what I'm 
talking about), which is allowing the wiper inside the sending unit 
to run completely off the resistance unit (which is 
wire-wound).     When this happens, the gauge will read over-full and 
actually hit the peg past full.   Adding resistance in-line with the 
gauge wiring won't help.    In a late model, with a full tank the 
sending unit should read around 90 ohms or thereabouts.    One quick 
way to test the "bent stop tab" on the sending unit is to have the 
tank full up topped off and note where the needle parks.    Then 
simply pull the wire off its plug contact on the sending unit and see 
if the gauge moves farther past full.   If it doesn't, that tab is 
likely bent and allowing the float arm to rise too high and causing 
the wiper to run off the resistance element.

...or something like that.    ;)



tony..


PS:    a dented tank can cause similar issues especially if the 
flange surface where the sending unit mounts is also bent or warped