Power is going to the motherboard, and you know this by assuming?
An assumption would be you see the Power Supply fan spin, LED lights light up, and maybe computer case fans spin.
Not an assumption, and you would have tested the 3 main voltage power rails, coming out of the Power Supply.
The Power Supply in your computer is an SMPS.
Switched-Mode Power Supply. (Also is known as the short abbreviation - PSU. Power Supply Unit)http://en.wikipedia.org/wiki/Switched-mode_power_supply
The Power Supply in your computer is a Converter.
It converts HIGH AC voltage into 3 LOW DC voltages.
[Depending on country,
USA = 120 Volts AC. UK = 240 Volts AC. Japan is 100 Volts AC, but may depend on area. Australia = 240 Volts AC. India = 240 Volts AC. HOWEVER, do not write the above in stone. I may have made an error ]
The 3 low main DC Voltages coming out of the Power Supply is;
A) 3.3 Volts DC
B) 5 Volts DC
C) 12 Volts DC
Orange wires carry 3.3 Volts DC
Red wires carry 5 Volts DC
Yellow wires carry 12 Volts DC
ALL Black wires are Ground wires. They can also be called Negative wires.
This is a DC circuit now. There is a Positive, and a Negative.
Orange, Red, and Yellow wires are power wires, and also Positive wires.
The first part of your diagnosis will be to test those 3 main voltage power rails.
Using an example;
There are many Red wires coming out of the Power Supply.
These are 5 Volt wires. They are Connected TO, the 5 Volt power rail in the Power Supply.
ALL 5 Volt wires end in one place, in the Power Supply.
The 5 Volt power rail.
When you test just ONE red wire, you are testing the entire 5 Volt power rail coming from the Power Supply.
This also goes for the Orange wires, and Yellow wires ]
With the Red 5 Volt wires, and Yellow 12 Volt wires, you could just use a 4-pin Peripheral power cable to check them,http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral
Multimeter set to DC Voltage, the red (Positive) probe lead of the multimeter; touches the female metal terminal connector, for the Red wire.http://www.mcmelectronics.com/product/28-9420&utm_medium=Affiliate&ref=cj&utm_campaign=CommissionJunction&utm_source=CommissionJunction?t=2&utm_expid=8634549-14
The black (Negative) probe lead of the multimeter, touches a female metal terminal connector, that goes to a Black wire.
You should be reading 5 Volts DC.
Same thing for the Yellow 12 Volt wire.
With an Orange 3.3 Volt wire, this changes.
A straightened out paperclip is inserted, down into the BACK of the ATX main power cable's connector; AND into a socket hole with an Orange wire in it.http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24
The straightened out paperclip, slides down into the socket hole, with the Orange wire in it.
Slides down into the socket hole, RIGHT NEXT TO the orange insulation of the wire, and MUST go down far enough; to Touch that female metal terminal connector.
EVERY wire going down into the ATX main power cable's connector, ends in a female metal terminal connector.
Same thing is down with a socket hole that has a Black wire in it.
ALL Black wires are Ground wires. (Negative)
You can choose ANY socket hole that has a Black wire in it.
Now touch the two probe leads of the multimeter, to their respective straightened out paperclips.
Red (Positive) probe lead of multimeter, to straightened out paperclip in Orange wire socket hole.
Black (Negative) probe lead of multimeter, to straightened out paperclip in Back wire socket hole.
You should be reading 3.3 Volts DC.
(Or if your multimeter kit has special probe lead, that would take the place of a straightened out paperclip, of course use it instead)
A) If ALL of the LED's ('lights') were on at once, they would use less than 1 Watt of power.
B) EACH fan uses 2 to 3 Watts of power.
C) A typical CPU (Processor) can use 51 to 130 Watts of power.
Just depends on what Processor (CPU) it is.
This is why a Power Supply with a weak voltage power rail, will not have enough power to turn the Processor ON, but will have enough power to light those simpy LED's, and spin fans.
[LED - Light Emitting Diode ]