The knock sensor reacts to detected knock and retards the timing to prevent engine damage. The sensor is, I believe, a piezoelectric sensor (ceramic?) and should register like 4.2mo resistance.

On older bricks teh sensor can become overly sensitive or not sensitive enough. In the case of the former, normal valve slap can set it off and trigger false knock detection to the ECU (causing the car to run very lean... examing your plugs for signs of this). In the case of the latter your engine might start knocking.

I don't think your problem is the knock sensor, but if as you indicate in your post you are "missing" a knock sensor I HIGHLY recommend you replace it. Driving without one on a car that needs one is unsafe and unwise.

Are you sure your problem isn't the AMM or something? Rough idle (not "smooth but fluctuating" or "stalling") is generally caused by only a few specific items. I would read through the 700/900 FAQ and see if you can't find something there (top of the page, middle pull down menu, bottom option). Pay paritcular attention to the large "performance section".

good luck,
rt