I wrote:
>The low-coolant controller has triggered, which simulates an
>overheat condition.  The newer cube-shaped controller does it in
>short pulses which trigger the light but aren't enough to move the
>gauge appreciably, but the old one does it continously.

I should also have said that a short to ground on the sender line of
any year will cause the gauge to rapidly go off-scale, and will
shortly generate a noticeable odor near the gauge (not necessarily
outside a mounted gauge) as oils etc. burn off the overheated coil
inside the gauge.  I surmise that the gauge will at some point be
damaged or the calibration changed.  I've mislaid my Bentley where I
have the numbers written down, but I believe that the threshold value
for overheat is 35 ohms at the sender, so a ground would represent
quite a serious overload.

Yours,
David