The first important matter to determine is if you have a D type or J type overdrive fitted.

The two types are different and suffer different faults, originally D type overdrives were fitted to all Vitesse and GT6, the Spitfire was changed for 1974 and was fitted up to chassis number FH60000, if in doubt have a look.

The D type has the solenoid on the right hand side, the J type on the left.

The D-Type Overdrive

The J-Type Overdrive