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MG MGB Technical - Ignition Switch swap/replacement


I'm putting together a car for track day use. I decided to have use a collapsible steering column for safety.

I picked up a steering lock/ignition switch to fit it. Year unknown ... it has one white wire two red and black wires and a single green and black wire. With a key that does not return back from position III.

The harness I'm using is from a 1973 car.

Can anyone advise regarding:

a) Which wires go where, (from the 73 harness onto the later switch) for me to solder onto the steering lock switch. ... or is this one with a relay connection??

b) How to go about putting in a button start system (i.e. sorting the wiring and doing away with the steering lock ignition) onto the 1973 harness.

Thanks in advance.

IA Jones


Found a LUCAS 31912 SPB501 replacement ignition switch.

However, it has connections numbered 1-4. How do the 1973 wires connect as they attach on to 1,2,3 and 5?

In other words (sorry to be a electrical dumbo)... where should the white, red and white, brown and green and white wires go.


IA Jones

You need to confirm the actions of the switch with an ohmmeter as wire colours and terminal numbers vary.

With four terminals I'd expect them to be 12v supply, accessories, ignition, and crank.

In the position where the key goes in and out all the terminals should be isolated from one another.

In the 1st operated position - accessories - the 12v supply (brown) and accessories (white/green) terminals will be connected together, with the other two terminals isolated from anything else.

In the second position - run - the 12v supply, accessories and ignition (white) terminals will be connected together, with the remaining terminal isolated.

In the final position - crank - the 12v, ignition and crank (white/red) terminals will be connected together, with the accessories terminal isolated.

It's easy to step through the accessories position when you make the first turn of the key from off, straight to the run position, a bit easier to find it when switching the ignition off but not much, you need to go slowly and carefully.

Paul is spot on.

If you're comfortable reading schematics, there are color and B/W versions available for all year ranges. Find your year and the year of your switch and you can make the adjustments.

Hi, Thanks for the info., it's not a problem with the schematics for the MG.

This is the diagram for the switch, its not a normal MG switch, I think it's one type that was used for mini's at one time.

Does this help?


IA Jones

Brown - 1
White - 2
White/red - 3
White/green - 4
Dave O'Neill 2

Hi, Thanks Dave. I knew brown would be 1!!
IA Jones

This thread was discussed between 27/07/2017 and 28/07/2017

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