Living with Sally
The MGB Roadster
One of the funny things about driving an MGB with overdrive is that you can change up from overdrive in 3rd gear into overdrive in 4th if you forget to switch out the overdrive off before changing gear

It seems (to me) to be more sensible to change from overdrive in 3rd to a normal 4th gear
THEN switch overdrive on in 4th for easier cruising

It's ok if you remember to switch off the overdrive as you change up a gear (but who remembers?)
So... what I would like is a setup that will automatically switch off the overdrive as I change gear
I also want to be able to still manually switch overdrive off if coming to a hill etc

I thought about this for a while and have decided that the easiest way for me to achieve this is to make the overdrive switch itself off when I depress the clutch
This can be done with a new 'flick' type 'momentary' overdrive switch, some kind of switch attached to the clutch pedal, and a couple of relays
The basic idea is to use one relay in a 'latching' mode so that a flick on the momentary switch will switch on the overdrive and keep it on.
A second relay will be used to break the connection of the first relay to drop the latch and disengage the overdrive
The second relay will be controlled by both the clutch switch and the other side of the flick switch

Depending upon what kind of switch I find for the clutch (normally open or normally closed) I will need to either break the connection on the first relay or activate the second relay to break the connection

The cost of doing this mod will probably only be a few pounds
(probably less if I have a couple of SPDT relays in my scrap box)

Ok.... shopping list:

  • 2x SPDT relays
  • A small square of circuit board
  • A couple of small diodes (1n4001?)
  • Connectors & cable
  • dashboard flick switch
  • Clutch switch
  • Mounting box

    I can probably forget the circuit board and solder the cables directly to the relays - we'll see what I have lying around.

    The plan is to replace the existing overdrive switch with this little circuit and wire in a switch from the clutch pedal

    My original thought was to conceal the relays etc by actually mounting them to the back of the dashboard somewhere as I have the dashboard out completely for refurb. On second thoughts I have decided to mount them in a box under the dashboard, so that I can get to them easily, should I have a relay or diode fail.
    I also want to be able to quickly switch back to the old system if I find this to be unreliable or decide to return the car to standard (although that seems to be an increasingly difficult proposition)