My guess is that it has the wrong sender hence the light going dimmer and brighter, instead of off and on.
there are 3 types of sender. yours should either have the nut size much larger than 13mm (1/2) spanner size, or be fitted with a "converter" which is a brass sleeve which fits in the head to take the newer 13mm nut size sender.
Airflow heads have the 13mm nut sender as std so it may have this head fitted to yours if engine has been changed during its life. therefore you need the right "early" sender. the club sells all types!