there's the symbol and the markings for diodes here:
http://www.kpsec.freeuk.com/images/diocon.gifThe Cathode end on the symbol is the one with the bar across the line. This will usually be marked on the diodes you buy with something like a colored band.
The positive current will only flow in the direction Anode to Cathode, so you want the colored (Cathode) end of your diode pointing towards your alternator connection to stop the motor running on.
No harm done if you put the diode in the wrong way round first time, it's just that your alternator probably won't charge; if it does, the motor will still run on.
Stuart.
