Diode in Lock Module

[Jim Reeve <ultra@xxxx>]

I currently have my lock module taken apart to "upgrade" the internal
relays in it. While I was looking over all of the electronics, I
noticed a yellow diode that was looking pretty bad. I checked the
wiring diagram on the DMCNEWS tech page...
http://www.dmcnews.com/Techsection/graphics/lockcirc.jpg
Only to find this diode (D4) unmarked as to it's properties. Do any you
electronic experts out there know what the purpose of this diode is for,
and what type it is? I am somewhat familiar working with electronics,
so I know how to replace it if I had the proper replacement. However,
since I'm no electrical engineer I'm baffled as to the purpose it
serves. It is directly connected to the positive and negative inputs to
the unit, so I'm guessing that is why it has taken such a beating. 
Thanks for any input.
-- 
Jim Reeve <Ultra>
'81 DeLorean-#6960
http://www.dmcultra.com
AIM-Ultra2169