Title says it all, can someone explain the OD system, how it works and how to trouble shoot it? I understand the TPS, but what else can go wrong with it and how do you check it?

 

I'll give it a try, hopefully I don't forget something.
You already know its electronic and the TPS is a major input to the PCM, but there are several other inputs as well.

OD switch, momentary switch with an indicator light, tells the PCM to cycle the OD light on/off. Light off, OD will engage when the other inputs are met. Light on, OD is locked out and will not engage.

Water Temp, 66°F IIRC.
TPS as you already know.
Speed from the VSS on the trans or transfer case.
The crank sensor is a what tells the PCM the engine is running, not used for OD control, but if its not working it won't shift into OD. Won't do a few other things ether, like charge.
There is a high temperature cutout switch on the hot line coming out of the trans, should open around 260°F and disable OD till the temp comes down and it closes the switch to complete the circuit again.

There's also a solenoid on the valvebody in the transmission that could possibly be not working.
The connector is behind the shifter linkage and is a 2 wire plug. 1 wire is key on 12volts and the other wire is ground by the PCM to activate OD.

First thing I would do to trouble shoot it is install a switch so you can manually control it and eliminate all the other things and make sure nothing in the trans or valvebody is wrong, then you can work on isolating the rest of the mess.
I don't remember which wire, but from another post Dave mentioned the blue wire is +12v, so don't ground that one. I would double check with a meter or a test light before you splice into anything.

 

Well, that one seems to have been the problem!... had cracked insulation on teh one wire adn after cleaning and taping it, everything seems to be working as it should..

THANKS!!!!!!