In my 96 i6 i have on my thing to change from PNDR.. etc... on end of that stick a button that says overdrive with off light... so when the light is on that means its on or is it off.. its weird... but i dunno what its used for or anything when to use how to why... how do i know when its on or off.. if off is lighted up or not.. there is no on light.. :P

 

I bet your D has a cirle around it, and the shift lever says Overdrive Off. This means when the light is lit up over drive is locked out.

When over drive is locked out you should notice the transmission downshift (if you going fast enough) and the RPM's will be higher. You are now in a 1:1 drive ratio with the transmission input shaft turning the same speed as the output shaft. This would also been known as direct drive.

Overdrive just makes the output shaft turn faster with a slower input shaft speed. This makes the engine turn slower for the same vehicle speed in direct drive with a higher engine speed.

 

Light On = Overdrive Off
Light Off = Overdrive On

It's set up this way because nobody wants another Light on in the cab.

Normal driving conditions you want the Overdrive on, meaning the Light is OFF.

If you determine that the Overdrive is hurting you, such as towing a trailer up a big hill, you turn the Overdrive off and the Light comes on.

The light stays on while the Overdrive is locked out.

The 'Light On' is Ford's way of saying to you, "Hey, you still towing that trailer? Because the Overdrive is still locked out you big dummy!'

 

You lock out overdrive (light shining OFF) when you only want the tranny to run in gears 1, 2, and 3. This is handy for highway speed slowdown with a load, or if the tranny is hunting between 3rd and 4th gears. This usually happens when in traffic at 50ish or in hilly country.

Adrianspeeder

 

Read your owner's manual for info on when to turn the OD off. If you don't have one, download it free from Ford.