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Technical Area => Transmission Related Discussion => Topic started by: march2022 on February 08, 2022, 01:11:05 AM

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Title: Micro Switch on Transmission
Post by: march2022 on February 08, 2022, 01:11:05 AM
I am in the middle of having my 1977 MGB restored.  Peter Mittler of the MG Car Club of Toronto gave me your club as a possible resource to assist me in sourcing a part.  Peter could not recognize the part.
 
My mechanic has been unable to find this part.  He called it, “Micro switch on transmission not in manual not sure could be for neutral light guessing”.

I am not mechanically inclined, however I thought I would try to help source this mysterious part.

Thank you for any assistance you can offer.
Scott Crichton 
Title: Re: Micro Switch on Transmission
Post by: JohnTwist on February 15, 2022, 09:10:39 PM
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

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