How do I know that the overdrive is working as it should? I know the overdrive can be either on or off, but when it is on how much work should it be doing? How can I tell that it is working at 100% and not 25%, 50% and so on? Do I look at the change in REVS between OD on and OD off? If so, what kind of REV difference should I see?

 

You should be able to notice at about 70 MPH if it is working or not.
If you hit the button and nothing happens then it is not working.

 

this seems to happen occasionally with my 740T too. I notice at about 60MPH, I am rev'ing at 3500RPM. When I hit the overdrive button (to turn it off), nothing happens. is this caused by the speed sensor, or is there a specific overdrive sensor?

 

Either the overdrive relay or soleniod.

 

A little update on my overdrive relay situation... I've noticed the overdrive works when the car is cold. If let's say, I start it up, drive 50 miles, pop into a store real quick, and then restart the car I find that it doesn't work. The switch with the current relay works fine. It switches on and off. The light comes on and off. I hear the solenoid clicking when the car is off and the switch engages and disengages.

I purchased a relay on eBay, and when I installed it, the overdrive light stays on and the switch does not work. I'm thinking the relay he sent was completely bad. But the other symptoms definitely sound like the relay, right?

 

Thats is signs of a bad relay.

The Transmission might have a problem if the light goes on and off with the switch when hot but the overdrive doesn't work.

 

I might be crazy remembering stuff wrong, but my tailgate wiring was frayed and would blow the tailgate wiring fuse, which includes rear defrost on wagon, windshield wiper, etc. These things happened to share the 4th gear fuse, causing the overdrive to quit.

(This was when I had a 240 automatic)

I cant remember if replacing the tailgate wiring or the overdrive relay fixed my problem, but I wanna say it was the former. So if your rear defrost or tailgate power lock (wagon), doesnt work, check that fuse, it might have to do with that of all things.

Again, I might be combining memories, so dont bet a meal on it.

at least check the fuse though...I'm sure my overdrive fuse was blowing till I fixed the problem.