Have you tested the resistance of the sender? The following link (if it works) refers to MGs but indicates the way that resistance should move.

http://books.google.co.uk/books?id=QwyK7...der&f=false

Better may be the information on the Gomog site here:

http://www.gomog.com/allmorgan/instrumentindex.html

and

http://www.gomog.com/panel/response26.htm

Malcolm T
1966 +4
1957 TR3
1963 TR4
1983 LR 110
1985 Robin Hood 2.0l