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.

The Hayes Shop manual lists the following as the function of the switch:

“When the actuating switch is moved to the normal position with the accelerator held down to open the vacuum switch, the circuit is broken and the direct drive is re-engaged.”

“If the accelerator is not depressed when the switch is moved from the overdrive to normal, the vacuum switch overrides the electrical switch until the speeds in the overdrive are the same to give a snatch-free change into direct drive.?

It has also been noted that the switch is not necessary for the operation of the OD unit.  Many owners have not installed the vacuum switch and apparently have no problems.

DESCRIPTION AND PICTURES OF THE SWITCH.

The OD vacuum switch is mounted just the left of the heater box on the engine side of the firewall.  The unit runs off manifold vacuum pressure with the hose coming off of the intake manifold.

Once the screw is removed, the cover can be removed to show the plate where the actual switch is located.

The picture shows the electrical switch system.  It consists of two brass bars (A and B) with a contact point on each bar (C).  When not under vacuum, the top bar is slightly raised by the pressure of a spring in the main body.  When vacuum (7+ inches of mercury) is present the diaphragm is pulled down and the switch closes.

The back of the unit has a brass pressure adjusting screw.  A soft white material, apparently used to “fix” the screw in place normally covers the screw.  I used a white bathroom caulk to fill the screw area after adjusting the unit.

On the bottom of the black disk there is a round plate that pushes up on the disk. The plate rests on a spring in a hole in the body of the switch.

OPERATION

With no vacuum to the unit; the spring in the base puts pressure up on the metal plate under the diaphram and this pushes the nipple up through the switch body to push up on the upper brass contact.  When vacuum is applied (adjustable through the screw on the bottom of the unit, but normally 7+ inches of mercury) the diaphram is pulled down and the switch plates make contact engaging the switch.

Given the thickness of the brass switch contacts, unless the diaphram itself deteriorates, the unit should be functional.  Given that the pressure that the unit activates at can be regulated, any diaphram of similar diameter and thickness should be suitable for a replacement.

CALIBRATING THE UNIT

I calibrated my switch by attaching a hand-operated vacuum pump used for bleeding brakes to the unit.  I hooked up a multimeter set on conductivity to the electrical poles.  Vacuum was applied until the multimeter conductivity buzzer sounded.

For my unit, I found that one full turn of the adjusting screw raised or lowered the activating pressure by about 1.5 inches of mercury.  With a few tries, it was relatively easy to set the switch to engage at 7 inches of mercury.

Once calibrated, I used white bathroom caulk to seal the adjusting screw.