Because the right tank suddenly failed to register at all on the fuel gauge, a new sender was ordered and the bad sender replaced.  An infinite resistance across the sender terminals confirmed that the old sender was bad.

The sender for each tank can be accessed by removing the tail light "bucket", but the sender is affixed to the tank with a ring with 3 prongs.  Those prongs are used to rotate the ring, counterclockwise to remove and clockwise to tighten.  Unfortunately, the prongs are not readily accessible, so I made a tool from 2" ID Schedule 40 ABS with 3 notches for the prongs and a cutout to clear the fender.  Here's a picture of that tool: sender R&R tool

Here is a picture of the bad sender: close up view

And here is a close-up showing the broken wires, which of course is the reason for the infinite ohms reading across the terminals: close up view

The new sender was purchased from Q & E Auto Parts (eBay) for $46.40 on 10 January 2012.  It fits perfectly, includes a new gasket and was shipped promptly.  Note that the new part number for the right tank is DAC5498.

Because the vertical position of the hole for the sender is well below the top of the tank, I drained the tank by attaching a 5/16" ID clear vinyl hose to the CAC2028 Air Bleed Valve (between the pump and the filter), selecting the right tank, and using the fuel pump to transfer the fuel to the left tank.  I expected the dregs to contain both rust and water, but the clear vinyl tubing showed that neither was present.  Perhaps this is due to the ethanol in California gasoline?

I should interject here that I (foolishly, as it turned out) bought a siphon hose from Harbor Freight ($8), expecting to drain the tank with it.  However, a Jaguar tank has a horizontally located flat metal piece under the cap that is far enough down to accomodate a fuel nozzle but is way too high to allow much fuel to be siphoned.  I thought you might like to know that the gas cap locks are essentially superfluous.

With the tank drained, I used my fabricated tool to remove the old sender and temporarily install the new one, using the old gasket.  I then measured 11 quarts/2.75 gallons of fuel into the tank and checked the dash gauge reading, which I wanted to be a bit less than 1/4.  Because the Jaguar tank holds 11 gallons, 2.75 gallons from the "sucked dry" condition (as described above) is 1/4 of the tank capacity.  The gauge showed about 1/8.

A word about my ohmmeteter: it does not zero, so do not take the following ohm values as exact, they are for comparison and a general idea of the correct resistances.  In the completely empty position, the sender provides 275 ohms and in the completely full position it provides 33 ohms.  With 11 quarts of fuel, the reading was 140 ohms, which I adjusted - by bending the float arm slightly - to 115 ohms.  This puts the dash gauge just below 1/4.  Had I not bent it, the new sender would have provided almost exactly the same reading as the original part (1/8).

Finally, I took the sender out, installed it with the new gasket, and snugged the ring down.  The gauge still read 1/4 so I tightened the ring as much as possible, which is not completely against the stops but was as far as a 24" pipe wrench and a plastic tool would allow me to rotate it.

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