Tim Schwartz wrote:

On 4/29/2014 12:33 PM, Apple_Tester wrote:
IBM PS/2 85 server PSU, API manufactured

Diode #ZD1

It's a Zener for sure, since it's labelede ZD1.

Hello all,

I came across this link:

http://john.ccac.rwth-aachen.de:8000/alf/ps2_85/

While it does not give a schematic, it does talk about the power
supply, and from what I see there is no 18V supply. (See the "Details
about main board" section)
Well, it could be a "suicide diode", if the power supply regulation
fails, the diode quickly fails to short and either the PS goes
into overcurrent limit or blows up as it isn't limiting anymore.
Many cheap Chinese PS modules now use these suicide diodes
instead of a more traditional SCR crowbar circuit.

They actually have a special diode made for this purpose.

Jon