Ford Repair: 95 ford taurus idle problems, electrical coils, coolant temp

Question
95 Ford Taurus, 3.0 vin: U
My ford taurus has a fluctuating rpm problem, more prominent when the A/C is on. No check engine light. the MAF sensor, tps, and both O2 sensors have been changed. removed and cleaned the IAC valve and re-installed did not fix it. If I leave the IAC unplugged it fixes the problem, but i get no code, which I find unusual. what I'm really trying to find out is if their is a resistance reading I can check on the IAC solenoid. Or maybe their is something else going on in the system that is telling the computer to open and close the IAC? Any advice would be appreciated.

Answer
Chris, the lack of a check engine light when the IAC is unplugged is unusual, and could indicate a PCM failure.  Make sure the check engine light is working; key on, engine off, it should be on - if not, check your fuses and the bulb.  
The IAC resistance is not a spec that I've seen anywhere, but most electrical coils have between 5 - 50,000 ohms, with less than 5 ohms indicating a shorted coil and over 50,000 ohms indicating an open circuit.
The inputs to the PCM that would cause an erratic idle are many!  You could have a PCM logic that doesn't see that you are at idle - after replacing the TPS, the computer memory needs to be cleared... it uses the lowest TP reading in memory as the idle position, and considers anything above that as part-throttle.
If the A/C compressor is cycling excessively, that can make an erratic idle.  A failure of a power steering pressure switch, coolant temp sensor, or many others can also give incorrect load information to the PCM and make a poor idle.  A lean condition could make it idle poorly, usually at a faster RPM than desired.
Usually the engine should NOT idle with the IAC unplugged.  It should stall, or run very low (about 400 RPM) with it unplugged.  If yours runs good with it unplugged, then I'd suspect the throttle isn't closing or their is a vacuum leak somewhere.
The best way to see what the computer is looking at is with a scan tool... the Ford dealer has the most complete equipment.  Knowing the values for "TP" and "close throttle position", "desired RPM" vs. "actual RPM" and "IAC %" would be helpful in knowing what is really going on!  Most dealers will run a diagnostic routine for about $75.
Hope this helps!
Clay