Author: B. Cunha
The early MGB MKI equipped with the D type overdrive unit used a firewall mounted, vacuum activated switch as a part of the overdrive system. The switch ran off manifold vacuum to activate the switch at around 7+ inches of mercury.
EDIT – Bruce Cunha has a beautiful webpage that not only provides a pictoral of the vacuum switch but also provides guidance on operation as well as calibration.
The website may be found here:
I (M. Feig) am showing front and back pics from one of my OD Vacuum Switches for illustrative purposes only.
Author: A. Henderson
Overdrive became available on the MGB in January, 1963. The early, early wiring harness contains the necessary connections for the dash switch, and the port for the power feed to the transmission, so overdrive was planned for from the beginning. (the car photographed has had overdrive added, as it is too early for it to have been factory equipped with overdrive.)
A correct installation includes a relay and a vacuum switch mounted next to the heater box behind the carburetors. Many early MGBs with added overdrives do not have these components. The overdrive will function satisfactorily without the relay and vacuum switch. Their purpose was to protect the overdrive from harsh disengagement. The overdrive harness, running from the wiring junctions below the fuse panel to the relay and vacuum switch can be fabricated, and is also available from Moss Motors.
Photos show the vacuum line
running from the intake manifold to the vacuum switch, and the correct location
of the switch and relay. The wires to connect are yellow, and yellow with a red