What does this constant actually do? Does changing it really add RPMs to a (6E bin) park idle. Didn't think these ECM were smart enough to do that in 1989.

 

There is a switch on the auto shifter assembly. That give the P/N signal to the ecm. That's how the ecm can sense park/neutral. When you put the car in neutral or park, the ecm will raise idle speed a small amount. FYI, there is also the neutal safety switch in the same area that prevents the engine from starting while in gear(not connected to the ecm).