Just purchased a 85 Iroc with 31K miles, 305 TPI set-up. Everything is working fine except the 700R4. It wanted to lockup right after 2nd gear so I replaced the lock-up solenoid inside the pan along with updating the filter and 5 new qts. of fluid but still same problem! Then it was suggested to add the lock-up switch (which is just an oil pressure switch) and wire it up to the the "A" wire 12V side of the factory harness and adjust the pressure cable. No lock-up in 1st, 2nd or 3rd now but when it hits 4th---boom, instant lock-up! Now it won't unlock going up hills etc...Does anyone know a "Factory" fix or repair for this very well known GM 700R4 problem? I hate to take it to a tranny shop and pay big $$$ if I can do it at home.

not sure which problem you want fixed?

GM designed the trans to lock up in 2 - 3 & 4th gears

Sounds like you have 4th gear lockup only now.

The trans will not unlock up in any gear going up a hill or only 4th?

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I want it to work the way it was designed to work. Since this isn't a Rod retro build and all the factory components are there, I'd like to fix the problem without jerry rigging after market fixes. Any clues? Oh yea, I have 4th gear lock-up only. Do you know what the shift points are suppose to be on this model?

Here's a thread that will help understand how it works and wiring diagram, signals at plug etc. May help you trouble shoot your problem.
http://www.turbobuick.com/forums/tr...trans-plug.html

A bad brake switch or vacuum switch in the lockup circuit could cause your initial problem.

Thanks, that's a good thread but addresses a rodders conversion more than a factory condition but still helpful in understanding what wire does what. The diagram is cool too!

There is an aftermarket lock-up switch that makes it work just like the factory ($140). I'm cheap and I used a micro switch taped to the floor shifter. I just ground the tan/black through the switch. My brakes still unlocked the TC because it cuts the power to the solenoid.

Like SSedan64 said "A bad brake switch or vacuum switch in the lockup circuit could cause your initial problem."

Btw my conversion was from fuel injected to carb on my 83 Z28. The computer didn't like the conversion (big surprise) so that is why I went to a full manual and cheap set-up. The fact that your car is all factory and has this problem means you have a different issue. Here are some options if you want to repair it yourself but not a factory fix.

Jet lock-up kit.

B&M makes one that is adjustable and more money.

Also see this. Another way to do it.
http://img45.imageshack.us/my.php?image=yo5la.jpg