8100 Magnetek PSU - does the fan always run?

[lobust]

I bought a used PSU (Magnetek 3390-20-001) from, I believe, an 8100 to replace the dead and unsalvageable PSU from a project Quadra 800 I have.

It all works fine but the fan didn't run at all during my limited testing.

Is the fan load/thermal operated on this PSU, or is mine faulty?

 

[lobust]

I bought another identical psu and the fan runs all the time, so I guess the first one is faulty somehow. I will open it up at some point and check it out.