All is well until I change up from 2nd to 3rd, then all of a sudden the overdrive engages again, usually under conditions where it is inappropriate. It may be my age but from time to time I find that I move from 3rd or 4th with overdrive engaged to 1st or 2nd and forget to switch overdrive off. I am already looking at an alternative method that does not increase the risk in any way, watch this space!īut enough of that. If anyone performs this modification and subsequently has a problem with the overdrive, I shall be very sorry to hear it but cannot be held liable in any way. This modification does add wiring to the solenoid side of the switch. However if any wiring is added to the solenoid side of this switch there is a slightly increased risk. Ordinarily the only thing that could cause this is if the switch failed in the closed position but the switch is very robust and I have never heard of one failing in the closed position (open, yes, but that will not cause damage to the overdrive). Anything that prevents this over-ride switch doing its job can wreck the overdrive if the car is reversed. Right up front I give you this warning: The MGB overdrive system consists of a manual switch that the driver can use when they want overdrive to be engaged, and a gearbox over-ride switch that prevents overdrive being engaged when it shouldn't be, even if the driver has the manual switch on.

These are the details of the MkI version.

Laycock overdrive wiring diagram