for about 6months i've been using a ghetto rigged light switch to manually lock up the tranny when crusing on the street or going down the interstate. Kinda worries me leaving it locked up while upshifting to overdrive (light footed of course) because i can feel a little "flexing" especially at lower speeds.

Anyway since it only locks up with a switch in the ALDL and unlocks when holding the brake or turning off the switch what could be the cause of it not doing it automatically?

Factors that may cause this that were recently repaired:

Coolant temp sensor (ECM)
Coolant temp sensor (Gauge)
Mass Air Flow sensor
Throttle Position sensor (Set to .54 volts)
Idle Air Control Valve

No codes except for Code 54 may be related to not having the float arm in the tank.

As far as engine temp goes mine warms to 200-225 F pretty easily
Another thing is that the systems cycles in and out of closed loop at idle only when warm and idles at 500 in drive if that has anything to do with it...

 

Some stock 200-4R and 700-R4 transmissions use a pressure switch (threaded into a pressure circuit for 3rd or 4th gear) to prevent lockup in lower gears. There were variations on the TCC wiring and switches used, depending on year / model.

There are a few different pressure switches. Some are normally open; closed when pressurized. Some are normally closed; open when pressurized.

Since you're reasonably electronics competent, you could pull the pan and test the circuit to see if it works "on the bench" and if it does, replace the solenoid.. While you're in there. The pressure switch is reasonable in cost and "This all doesn't take much time on top of a fluid/filter change.

 

If it was the solenoid, it wouldn't lock up manually.

Put a test light between terminals A and F of the ALDL and go for a drive. When does the light go on and off?

 

took it for a ride and i realized that it locks up only at or above 85 mph by watching the rpms and not grounding the ALDL...

 

If the test light (see Apeiron's post) comes on at a lower speed but lockup doesn't work until later, then it could be a pressure switch (see my post)