Got a P0700 and a P0872. One for trans pressure switch and the other for trans control system malfunction. Any possible solution for this? where to start and where is shift solenoid C? thanks

P0700 is just the general trans code that turns on the light and flags that there are other diagnostics to be read.

P0872 is "E" shift valve stuck on. This is the same indication that the 6 speeds get for P0762 you can search on. In general, this means that there is fine debris in your fluid that has gotten between the "E" shift valve and bore and jammed it.

You should drain the fluid and drop the pan and check it out. If the valve is stuck, you'll have to remove the VB split it apart and gently free it.

If you don't find any chunks of metal or other troubling debris, change the fluid and filters. Be sure the magnet in under the spin-on and wipe it clean before reinstalling. Clean the pan and install fresh fluid. DO NOT POWER FLUSH the Allison. You can get most of the fluid exchanged by doing the "Hoot Method" if its really dirty. Otherwise, just drain and refill.