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.

vaccum1 vacuum2



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