transmission ?

[pjohnston]

How does tge tranny control the shift to O D.

[RhinoZ24]

O/D is just what allows you to shift into higher gears, now-a-days its all computer controlled with the click of the O/D button. With it off, it keeps your vehicle in lower gears and higher RPM's, limiting your speed, you usually use it going downhill to save your brakes or extreme uphill and altitude if your engine starts bogging down. Before the button that your vehicle has now, it used to be a gear setting, like today you have P-R-N-D-1-2, you used to have something like P-R-N-D-O/D-1-2 and so on. Its not really a gear, just a setting in your transmission that keeps your vehicle in lower gears, it won't allow a shift into the higher gears, it basically saves you gas with it on.