Speed sensor?
 

So i noticed a don't have a speed sensor on my 1988 944S, it runs good. What is a speed sensor for anyway?

Hall sensor
 

Did the hall sensor replace the speed sensor?

Actually, the one that you're missing is the TDC sensor. The 944S and 944S2 used a different arrangement than the 8 valve cars. They use a flywheel with notched teeth and one tooth is missing to account for TDC. The hall sensor is used for knock sensing... since there is only one knock sensor, the hall sensor is input to let the DME know which of the two possible cylinders (either 1&4 or 2&3) is the offending cylinder.

so i have one, maybe the ref sensor, on top of the fly wheel housing, and the round hole is for the tdc sensor?

Well, I guess that I should have put "missing" in quotes. You only have a speed sensor (by design) and it counts the teeth on the flywheel (by design) and it is missing one tooth (by design) so that it knows when #1 is at TDC. Since (by design), it already knows where TDC is, it doesn't need a separate TDC sensor.

Clear? Crystal?

Ok thanks. Good to know i'm not missing anything

Can you just spray with those rust converters? I know it sounds cheesy, but I thought they worked in the same way--by reducing rust to black oxide.

Where does the other end of the TDC Sensor plug into?

It's also got a hall sensor for cam position right?