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Recently purchased the car with the gearbox inhibitor switch(C23291 Lucas34822) bypassed. Managed to get hold of one and fitted it ok; want to check the relay(C24467 Lucas33188) but cannot find it! The workshop manual says its behind the right hand facia but I've searched behind the speedo & tacho with a mirror and endoscope camera and cannot see it anywhere. If you know exactly where it is located do please let me know.

Hi there,
I have a 1965 S type automatic, which I think has an earlier version of the auto gearbox than your 420, and so the inhibitor switch may well be different. Are you sure a relay is fitted?
By "inhibitor switch", am I correct in assuming you're referring to the device to prevent the engine starting if the selector is not in "Neutral" or "Park"? According to my wiring diagram, there is NO inhibitor relay fitted to the S type, although I appreciate my gearbox is not exactly the same as yours.
My inhibitor switch (also operates the reversing lights) is just above the steering column, and just behind the dashboard. (I had problems with it going out of alignment, and preventing the starter solenoid operating).

Hi, the wiring diagram in the 420 workshop manual shows a relay but I never did find it, however the car will only start in N or P so inhibitor switch must be doing its job OK. Must say its a pretty crap design with quite a heavy clumsy leverage operating a flimsy switch, no wonder it goes out of alignment (luckily mine hasn't --- yet!)