The P8 plug you removed supplies the +12 and -12 volts to the 5150/5160 motherboard.

CONCLUSION:  Because the fan is now turning, and before it wasn't, we now know that your motherboard has a short in either the +12 or -12 volt lines.


What Now?

It is very highly likely that either a tantalum capacitor on the +12 volt line or a tantalum capacitor on the -12 volt line of your motherboard is short circuit.
Faulty tantalum capacitors is a very common problem with the 5150 and 5160 motherboards.

Visually inspecting the tantalum capacitors may (repeat: may) reveal the faulty capacitor.
In some cases, the faulty tantalum capacitor may only have a small black eye/hole, as pictured below.



But the faulty tantalum capacitor may have absolutely no visual indication of failure.


5150 motherboard (PC) - type 16KB-64KB

This is the early version of the 5150 motherboard. It is marked "16KB-64KB CPU"


5150 motherboard (PC) - type 64KB-256KB

This is the later version of the 5150 motherboard. It is marked "64KB-256KB CPU"
On this motherboard, there is one component on the +12 volt and one component on the -12 volt line. Both components are tantalum capacitors.
Click here to see a photo that shows the two capacitors.
The two capacitors are not critical to the operation of this motherboard, and this motherboard can operate without them.
Remove the capacitors either by desoldering, or by using a pair of sidecutters to cut the capacitors off the board.
Your motherboard should work after the capacitors have been removed.

Should you later wish to solder in replacement capacitors (the ideal situation), then click here for replacement details.


5160 motherboard (XT)

On a 5160 motherboard (of both types), there is one component on the +12 volt and one component on the -12 volt line. Both components are tantalum capacitors.
C56 is the tantalum capacitor on the +12 volt line, and C58 is the tantalum capacitor on the -12 volt line.
Remove the capacitors either by desoldering, or by using a pair of sidecutters to cut the capacitors off the board.
Your motherboard should work after the capacitors have been removed.

Should you later wish to solder in replacement capacitors (the ideal situation), then click here for replacement details.