There are two knock sensors. They are bolted inside the V and getting to them requires removing the intake. If anyone has done this without then it is probably not all that important, because when the knock sensors are deteriorated enough to fail there are many other problems as well.
The knock sensor is really an accelerometer. An accelerometer works on the principle of measuring movement and generates an electrical signal. The signal generated by the senor is in proportion to movement. So when the car moves or the engine vibrates the Knock Sensor or Accelerometer will start generating voltage. (no it is not enough to run your car on - it is a very low level signal). When the engine pings or knocks with bad gas then the acceleration measured is greater than the normal vibration. This creates a spike in voltage that the computer then reads. The knock is taken as a signal for the computer to detune the engine to accommodate for bad gas. Since the 32 valve engine adjusts timing electronically through the EZ unit it can simply use this signal to
set the timing back. Reduced timing will give you less low end acceleration.
The Knock Sensor has three connections. The first is the signal, the second a ground and the third is a shield wire. The shield is needed to keep out other random signals in the air like from the spark plug and other electromagnetic signals that may be running around the area. The connection to the wiring harness is also a three wire system so the shielding runs all the way back to the cars computers.
Yes there are two knock sensors and their associated connectors. The front on is on the left side of the engine (your right if you are standing in front of the car looking at the engine with one hand holding up the hood so it will not hit you in the head) The base of the connector is anchored with a clip to a metal bracket with the wiring harness plugged in from the top.
The second on is located on the right fuel rail towards the rear of the motor. It is mounted horizontally with the harness plug facing forward. The fuel rail should have a plastic cover and the bump in the back of the cover is used to protect the connection. For this reason alone the rear connector is probably better than the front.
In removing the plastic cover (if you have one) there will be some remains of some ugly brown foam rubber. This is there to insulate your fuel rails.
New harness ends can be purchased from Eagle Day
For 10 times the cost of the plugs in bulk - but they have a nice kit and you can buy them one at a time rather than 100 or so. Eagle Day also has color coded wire the match the Porsche wiring.
Dan the Pod Guy Portia Parts Restoration http://www.kondratyev.com/porsche/
Model 1986--1990 (black connector) Model 1991-1995 (blue connector, gold plated)
911 606 141 00 -> 1987-1990, tin contacts 928 606 144 00 -> 1991 onwards, gold plated
This is what a bad knock sensor looks like: