What does the computer use park/neutral info for? I don't have anything hooked up to my 7747 and I'm wondering if it really matters.

Mostly idle control. On a stick car the pk/nuet line is to be left open, this is drive. And the ECM is always in a closed loop idle state.

On an auto you can use it if you want. In pk/neut the idle is open loop, no real control other then the lower RPM limit and an upper RPM limit. On my ElCamino I left the line open and adjusted the idle screw so pk/neut idle is a little high (at 0 IAC steps). Then when dropped in gear the load pulls the idle down and the IAC counts are 5-10.

This prevents it from stalling when placed into drive. The ECM will add steps when it sees the pk/neut line change to drive (not pk/neut). So it can have benefits to having it connected.

RBob.

Hmm. Could not having this hooked up cause the idle to surge/search at all? That's the symptom I'm having - plus high BLM values in neutral/idle. I tuned my IAC to be low, but not zero, at idle.

Not really, the open/closed loop idle I am referring to is just idle RPM control, not fueling. Closed loop idle isn't the same as closed loop fueling.

RBob.

rbob could the p/n switch cause the car to die when you put it in gear?

Yes, if the ECM doesn't see the Pk/Neut signal go high then it won't add IAC steps. This can be checked with a scan tool or data logger. Look for the pk/neut status to change when the shifter to moved to reverse or drive.

RBob.

my 88 would always die when shift to a diffent gear/pk/neut, any ideas on why? i didnt get any error codes at all...2.8L auto. i always had to stomp the gas to keep it going, wether shifting into drive, park, or even neutral

A bad park/neut switch won't set an error code. First check that the ECM sees the proper mode. Also check the distributor timing is correct (10 deg BTDC with the BYPASS open). These engine eat timing chains, so if it has over 60 k miles on it, it's shot.

RBob.