I agree with Donnie, start with a new OVP relay.
But if you want to figure it out here's where to start. Get a cheap radio shack multimeter with a 2amp scale (volt-ohm-amp meters).
Rig some wiring so that the idle control valve (sometimes called air-slide, idle motor) current will run through your meter. Then observe the readings. You should find readings around 6-700ma at idle. If you turn on the A/C you should get about a 50ma increase in current.
Now for the fun story on OVP problems. You have found that disconnected the idle raises. You might also know that the OVP relay powers the controller. You might deduce from this that if the OVP didn't power the controller that the idle valve would react like one that had been disconnected. In some cases this might happen, BUT!! , that ain't the story we want to hear. The one we want to hear is the one where the engine dies.
Well here is that story and I have witnessed it repeatedly (was very confusing for me for a long time - how loss of OVP could cause the motor to run slower than when disconnected altogether).
You will notice that the current flow is always above 600ma. If you use long wires as I do, then you can watch the activity as you drive (don't do this on the street - pay attention). The current often goes to 8-900ma when the system is off idle. This is so that if the throttle slams shut the idle is high and can be slowly brought down. Now comes the problem. When the OVP has a problem it doesn't just disconnect, it acts as a high resistance in the circuit, which limits the current that the controller can apply. During these times the current will drop to 4-500ma for slow running or even lower which causes the engine to die.
This bothered me for a long time; if you follow the logic, lower current equals lower speed; zero current should be very low. Thats not the case at some low current the idle valve returns to its disconnected state; which is slightly more open than standard warm idle.
Fun story, no?
[This message has been edited by stevebfl (edited 07-29-2000).]