O?D ECT et al!

G'day David.
ECT = Electronic Controlled Transmission.
The ECT setting will only change your shift pattern to give you more "traffic friendly" acceleration.
It won't damage your vehicle at all, but will effect your fuel economy, depending on how hard you drive the vehicle, city cycle etc.
I seldom use mine as I mainly do highway driving as I live in the bush.
Re Overdrive, I recommend you leave it on at all times, unless you need to "kick back" a gear when overtaking. Give your self plenty of room, switch the O?D off, listen to your engine screem and overtake. You won't be able to get any thurther acceleration from the vehicle.
Always reengage the O/D when cruising, otherwise you'll find your self constantly in third grear only as your top gear.
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Best wishes.

Ken.