A couple of people asked for the details on calibrating the old,
differential magnet-type fuel gauges. Here is what I did when
calibrating a friend's gauge for his MGA. I checked the results on my
three Jaguar gauges so I am pretty sure that it is correct.
Go to Radio Shack or some other electronic's store and pick up a sleeve
of five 15 Ohm resistors and an alligator clip. Go home and solder them
together in series and attach the gator clip to one end. If you can, it
is nice to have a short wire with a connector attached to the two fuel
gauge leads. This will reduce the chance of shorting everything out.
I used a battery charger for a 12 volt supply- polarity doesn't seem to
matter. Hook the 12 volts to the gauge input and the case. Hook the
gator clamp on the resistors to the case as well- my gauges have two
mounting tangs that make this easy.
The gauge will swing hard past full with the sender wire open and empty
with it grounded. On the sender, full gives about 70 ohms resistance to
ground so just work down the resistor array from the end to the gator
clip for 75, 60, 45, 30, 15, and 0 Ohms to ground. 45 and thirty should
bracket the half full mark with the others near the quarter marks.
If your gauge does this, you are done. To adjust, ground the sender
wire then loosen the nut on one of the two magnet cores- doesn't really
matter which one. Move the core back and forth until the needle points
at empty or just below empty- you don't want it above empty or else you
will have a false sense of security. Then move to the other end of the
resistor array and adjust the other magnet until the gauge points at
full or just past full. This is an iterative process. Once you are
satisfied with the endpoints, check the middle points again to see if
they make sense then install and enjoy!
I think the resistors I bought were one-half Watt 1% resistors but I
expect the 1/4 Watt 5% resistors would work as well. The sleeve of five
resistors was 99 cents at the mall. I bought them while my eldest
daughter was doing whatever 14 year old girls do in malls when they know
their parents may be watching.
If this is unclear, I can try again. Once you have the parts in front
of you, it will make a lot more sense.
Happy Holidays,
Bill Eastman
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