klcdenver

Try pressing the mode button for 10 seconds. This is suppose to reset the fault codes. After this it should go to N.

Owners manual also attached. The manual has a diagram for wiring and it is all one harness.

Allison 3000 MH Owners Manual.pdf

Here is a summary but it omitted to make sure tranny is up to temp. You can get tranny up to temp by starting and put in gear while holding brake. Be sure and watch temp and do not overheat it.

Transmission checks.docx

Yes unhook battery to help reset.

Try removing power for a few minutes. Then start and bring tranny temp up to a temp above 150 degrees then try checking fluid level again by pressing up & down arrows on pad at same time.