LHI968 PIR problems

I recently purchased 6 LHI 968 PIR sensors (alone - discrete components) with a plan to use these between DCC model railway tracks to sense a loco entering various storage roads (tracks).

The theory all seems great, I have written and tested the code for the Arduino using a 5v probem wire to trigger the inputs as required, and it all works just great.

However, connecting the PIR into the circuit as input 1 instead of the 5v probe either does nothing, or it ALWAYS triggers. This seems to depend on the polarity of the Vcc and GND connections, although I would not swear to that.

In desperation, I set up a 2K resistor and LED to the output pin of the PIR, and tried everything to make the LED light, I even tried putting a small soldering iron bit up close to see if it sensed that heat. all to no avail.

I have tried 2 of my 6 PIR's in this circuit now, in case I had blown the first one up somehow when fiddling with the wiring, but still the LED does not light ?????

Viewed from underneath the PIR, with the centre pin on the far side of the PIR, my wiring is VCC, Signal, GND. I have tried adding pulldown resistors to the input pin of the arduino to ensure a zero signal at rest.

Any help would be greatly appreciated, it is driving me nuts right now. :-((