Hello, again
I am making a bit of an assumption here since I don't have a Morgan
but, from my experience, Jaeger/Smiths have built two styles of fuel
gauge.
In the older style, the needle swings relatively free. It's position
is influenced by two wound-bobbin electromagnets- one on each side of
the gauge. One magnet is hooked hot to ground while the other is hot to
sender to ground. When you turn your car on the reference magnet tries
to pull the needle to full. The variable magnet pulls the needle
towards empty in varying degrees depending on how much fuel you have in
the tank. You can identify this gauge in two ways. The gauge should
read empty with the sender wire grounded and full with the sender wire
open. Also, if you look at the back of the gauge and see two posts
sticking through slots and nutted, you have this type of gauge. My 61
MGA and 58 XK use this type. A sender for this type should read 0
resistance when the tank is empty. If anyone is interested, I built a
simple calibration device for these using 2 bucks of Radio Shack parts
and can share the plan.
The newer style gauge uses a resistance heating element wrapped around
a bimetallic spring. This kind has a lot of natural damping so you
don't get the nervous needle like the magnetic type. I think that these
were introduced in the mid-sixties. Identifying characteristics include
the more stable needle and the lack of adjustment slots on the back.
There may be an adjustment screw but, since I don't have a car new
enough to have this gauge, I am not sure. On these gauges, it will show
empty with the sender circuit open and full with it shorted. 0
resistance at the sender mean more current heating the wire which bends
the bimetallic spring more. I don't have one but I am sure that a
calibration device could be easily constructed for these as well.
Personally, I can't see how either would work with a capacitive sender
as Chip describes but, since my usual cure for an electrical problem is
to give the offending unit a good whack, I may be missing some of the
more subtle electrical phenomena.
Regards,
Bill Eastman
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