Guys and Gals,
You don't need to wire a warning light into your overdrive circuit. The
safeguard is already there. While there isn't a mechanical lockout, all the
gearboxes originally equipped with overdrive have an additional switch that
only allows the overdrive to be engaged when the car is in 3rd or 4th gear.
Both the D type and LH type have this feature. Cars have been commonly
straight wired (bypassing this switch) because either a replacment switch
isn't available, the person did not want to go to the trouble to change it
(not very accessible on the B) or they wanted the ability to shift in O/D on
any forward gear. A lot of auto crossers used to do this for quick shifts.
The problem is that if you mistakedly leave the car in O/D and reverese it,
BAM!, there goes the O/D unit. I just this week worked on a TR6 that was
wired this way. I put the car in reverse and the gearbox started making funny
noises. I immediately stopped and started to move forward and the thing
shifted out of overdrive in first gear at 10 mph. Very dangerous situation.
If you're not sure if the switch is good, use an an ohmeter to check it. If
the switch goes bad, the O/D will not work. No harm in that is there?
Enjoy your overdrive,
Eric
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