Tim,
Simple things first. Is your coolant fresh or looks muddy? 50/50 ratio? Gage contacts clean?
Those problems should be looked at first. Those gages are finicky. To check condition just unplug and unscrew from expansion tank. Clean 2 metal pins with fine sandpaper or emery cloth.
You will need new o-ring for it when installing back. If expansion tank cap looks old it's time to replace it.
This will eliminate part or even whole problem.
Leon
85 Subwagen Westy

Tim Timothy wrote:

There is a control module (labelled 43) up under the dash forward of the
fuse panel. Hard to get to with the dash on but possibly could reach it
through the panel opening.

Thanks David,
Finally found the control module (relay?) with your description of the
location.
With it removed, the warning lamp continues to flash.
(DEFINATELY a faulty gauge?)