Only the 1977 MGBs have this micro switch. It’s for the TCSA circuit.
The Transmission Controlled Spark Advance (TCSA) circuit was designed to allow vacuum to the distributor vacuum advance ONLY in fourth gear. This was to meet emission standards. The B runs better with this plumbing routed directly from the intake manifold to the vacuum advance on the original distributor. The circuit also operates the overdrive – only in fourth gear. AND – the circuit is unfused so when one of the wires at the gearbox chafes and shorts, the entire wiring harness burns.
SO…..if you do not have overdrive this switch and the electrical circuitry is not necessary. If you do have overdrive, then the switch on the remote control will allow the OD to work in third and fourth.
To make that original electrical circuit hot in only fourth, two switches wired in series did the trick. One switch (the one at the top left of the remote control) went continuous in third and fourth.
The micro switch went continuous in reverse, second, and fourth. Together, the circuit only carries current in fourth.
If you do have overdrive, but CERTAIN to fuse the circuit at the junction of the main, rear, and gearbox wiring loom at the rear of the right front fender.
Questions? Just call.
John H Twist