Point is: The Dell 3 pin fans and this particular Dell motherboard with these 3 pin fan connectors type is middle pin is 12V for supplying fan, ground is black for fan power return, and white wire is for no rotation signal, while running that wire is grounded by the *working* fan itself if it was genuine dell fan.
What I modify like you do, with different fans onto these Dell type, jump bit of wire from ground to no spin pin ground to keep that dell motherboard happy.
Keep in mind on keeping meanings straight in future for other fans. The PWM is a signal that comes from a motherboard to control the fan's RPM. Second thing, PWM fans always have rpm signal output from fan to tell motherboard with RPM support how fast fan is spinning.
RPM signal is also true on PC standard 3 wire fans too. Plain fans is simply a supply voltage and a ground which is two wires.
Another type of fans is no rotation error signal comes from fan and to do so to signal motherboard in event fan is not spinning by fan causes that pin to be ungrounded (voltage comes up due to motherboard's design) on the third pin, this is case for Dell 3 pin fans, not anything else.
Cheers,
Great Northern aka Canada.