For this article I will try and cover the complicated subject of a non-working overdrive.

Many things can stop an overdrive from working, the most common I found was an electrical failure. Always check for broken wires, especially on the late model MGB with the overdrive switch in the gearlever. The constant movement of the gearlever will cause the wires to break inside it. Also you need to make sure the fourth gear inhibitor switch is not bad, this is the switch on the gearbox that will only allow electricity to the overdrive solenoid when in fourth gear. 

If you have electricity to the overdrive solenoid, but the overdrive is still inoperative, you may have a sticking piston. This is very easy to check, it is located at the bottom of the unit above the solenoid. When you take the piston out,   be careful not to lose the tiny ball bearing that sits atop it.

Always make sure the overdrive oil filter is clean and not clogged up. The oil for all MGB gearboxes is straight 30-weight engine oil.

Paul Lewis