mgb overdrive

["Richard" <ramsmgb@cox.net>]

Hi listers's:
 
I have a 1972 with an original Laycock overdrive and od gearbox. I had
the overdrive fail recently. It was working intermittently. I have
replaced the isolation switch, all the o rings, sump filter, and the
solenoid. It worked intermittently after I replaced the isolation
switch, but now nothing. All the wiring checks hot down to the solenoid
with the ignition key on, the overdrive switch on, and the gearbox in
4th. 
 
Any ideas?
 
Rich in Tucson