Now this unit came to me after the owner had his share of interference with it .

The problem basically is no beeps, when the 4 pin P4 connector is plugged in to the motherboard the processor fan does not come on , when i tested the voltage through the molex connector i various entries of Zero (0.06)
When i disconnected the P4 and left the 20 pin P1 connecor in, it powered up the fan spins and everything . the reading on the molex is as it should be 12v ,3.3v respectively .

So what suggestions are there to check next ???

the guy said once he saw smoke ascending from the mobo but the unit was still working then .

So what do you think i can further do or check ??

Power supply possibly. If you put in a standard atx power supply you need to disconnect one of the wires for that model Dimension, it's important you know that. I don't remember which wire, but Dell has the wiring diagram on their site for it somewhere.

i worked on about three (3) Dell Dimensions all about power supply and i never got this problem although there was slight difference within the units build .

The only difference between a standard Dell PSU and a regular ATX PSU is a white wire connected to the 20 pin P! connector , but i never got a problem with it so far