As before stated NONE of the OHV explorer's came equiped with a knock sensor, and yes some cam with a camshaft postition sensor mainly the cailfornia vehicles due to their strict emission policy's. The camshaft postiton sensor or CMP has nothing to do with detonation monitoring it just tells the computer where the camshaft is at and when to fire the injectors according to so.
AS for your guy at the parts store its nothng unusual , they usually spit out something they no nothing about anyway. You'd think since he worked at a parts store he have the sence to look it up on the computer and find out this. Of course there again like most it'd probably say unavailable and he's say it was a dealer only item
hahaaa
For a quick reference to what the sensors are and do on a 91 4.0....
The one located on the upper intake near the vacuum tree is the Intake Air temp sensor or IAT.
The one directly below the throttle body on the lower intake is the coolant temperture sensor for the computer. It has a two wire connector going to it.
The one directly accross from it on the lower intake is the coolant temp sensor for the guage on the dash. It has nothing to do with the computer. It has one wire on it.
The one located below the power steering pump is the oil pressure sending unit for the guage, there again it has nothing that goes to or has to do with the computer.
The crankshaft position sensor is located down below nearthe harmonic balancer.
If you have an EGR valve....
The silver box on the side of the upper intake that has two lines going to it from the EGR tube is the Delta Pressure Feedback Exhaust sensor or DPFE, it tells the computer when the EGR is open and flowing.
The Round top black valve with a vacuum line going to it and one going to the EGR valve is the EGR vacuum regulator valve or EVR solonoid. This controlls the vacuum signal going to the EGR to open and close it.
Then the Camshaft position sensor located in the very back of the intake behind the coil pack directly in the middle of the block.
That's it for sensors mounted ON the engine its self.
Later Doug904.