Diodes

you could use led's to isolate, but you have to know which leg is positve and which is negative (cathode or anode), what you can do is hook up the leds to power source through a resistor and see if it lights up if not then you have it backwards.  MAKE sure and use the resistor or you will burn out the led's.  If you are using these to diode isolate each sensor, i'm not sure that it is necessary, unless you already have something on the hood pin.  It would probably be easier to just use regular diodes.

You should be using a 1 amp 1N4001 diode instead of the LED's. If you still wish to use the LEDS, in place of the diode than you should know that the anode (+) positive side of the LED has a shorter lead, and if your looking into (inside) the LED. The anode is the side that is smaller / thinner, piece of metal, compared to the other side. The cathode is obviously larger when looking inside of the LED. Also, if you have a DMM that has a *diode* check mode, you will notice that the DMM will light the LED when properly biased, and will show a aprox. 1.6 VDC and infinite when reversed. As stated above, a properly 1/4 resistor of aprox. 470 ohms will allow you to use it in a 12 VDC vehicle. Regards EVIL Teken...

you should really not use LEDs to isolate.. they have a high voltage drop and can not handle much current, by placing a resistor in line to limit current you are going to drop the voltage even lower...