Like the title asks what is the thing under the cap and rotor its a plastice thing with a wire going to it and a metal piece on the underside of it? I didn't notice it when i changed the cap and stuff before and have never seen one of them on any other car or truck. Should it be replaced with a tune up or is it fine? I wish if it needed chenged someone would have mentioned it before I went to napa to pick up my other stuff cause now they are closed

 

After looking around on Advance's website I found out its the "Pickup Assembly" and they are all around 60 bucks so untill it starts totally running like **** it can stay old

 

That's the sync sensor. It doesn't wear so generally you don't replace it unless it fails. Not a commonly replaced part.

 

What exactly does it do?

 

I believe that's the cam positionsensor (the round pancake looking thing with the wires on it?) It sets spark sync as the cam rotates around (well actually as distributor shaft rotates around). Twisting it around (by moving the entire distributor on it's shaft) in an old car used to allow you to advance or retard the spark to the A/F mix in the cylinders, but with the newer computer cars it doesn't work quite like that any more.

I think it's just a magnetic pickup. So like rabbler mentioned: it either works, or it doesn't.

 

On these trucks I think the cam sensor is used for fuel sync only, for the sequential part of the injection; the crank sensor is used for ignition timing.

 

That's right, I had that backward in my mind. Damn sensors