12v cut out - dead??

[steve davidson]

Hi,
 
I'm suspecting that the cutout on my 12v system has died.  I'm getting good volts at the Dyno (around 13.5) but the ignition light remains stubbornly on no matter what the revs.
 
I've cleaned and tightened all the earths and checked the connections to the ignition light. 
 
The current cutout looks to be a solid state (see photo attached). So my questions are;
 
1 – Is there a way I check if this type of cut out is faulty?
 
2- If it is faulty are there any suggestions on where a new unit can be procured?
 
3- As a side issue can someone tell me which is the A and which is the D terminal on my dnyamo?  I have two wires connected to the dnyomo one larger than the other.
 
4 – When people speak of cut outs and regulators are they one and the same?
 
Thanks