Early MGB Overdrive Reference

Photos and description of the overdrive system on early MGBs.

Written by Various – 2020-05-06
How-To Library > Early MGB Reference Section

MGB Overdrive Vacuum Switch (1963-1967)

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: https://sites.google.com/site/cunhab52/mgbvacuumswitchforod

I (M. Feig) am showing front and back pics from one of my OD Vacuum Switches for illustrative purposes only:

Early MGB D Type Overdrive Under Hood Components and Wiring

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 tracer.

— Early MGB Overdrive Reference —

Written by Various – Created 2020-05-06 Published 2020-05-06
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2022-01-04 02:39:29 # 78986

Comment by Don Scott

Besides a rebuilt overdrive gearbox and shorter driveshaft, I have accumulated a correct dash switch and firewall pieces to put overdrive on my '65 MGB properly. I bought a wiring harness from British Wiring as well. Can anyone describe the right way to run the wiring and what P-clips I need to make wiring the overdrive installation as close to correct as possible?

2022-12-09 14:52:30 # 84166

Comment by Clay Collins

I have the same question as Don. I'm installing an overdrive in a '63 MGB and have a harness that connects to the solenoid and the overdrive interlock switch and runs up to the firewall connecting to the relay. John at Quantum Mechanics does not recommend using the vacuum solenoid, but provides a relay in his kit. From pictures on the internet, routing the harness from the fuse box to the relay looks like it should run in front of the heater and up to the relay which is mounted to the right of the heater (looking towards the rear of the car). I'm wondering if the gearbox part of the harness should be routed on top of or underneath the gearbox and how/where it should be secured with the P-clips as mentioned by Don. Any photos would be a huge help. The Bentley manual has a good wiring diagram and also one was provided with my gearbox by John in case anyone needs a copy - it's pretty straight forward.

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