View Full Version : Wiring a molex backwards

02-14-07, 09:30 AM
I had a problem yesterday with a system fan not working. Computer unplugged, connected fan, plugged in computer, switched on. Everything began the start up process, then this horrible screeching sound started and the computer switched off again. I hit the power switch at the back as soon as I could.

On inspection of the fan, the wires were incorrect going into the molex.

So it should have been:

power supply yellow to fan red
power supply black to fan black

But instead it was wired:

Power supply yellow to fan black
Power supply black to fan red

Is this likely to cause any permanent damage? Has this happened to anyone before? Or has anyone got enough electrical knowledge to tell me what effect this would have?

02-14-07, 12:47 PM
Your asking if anything is damaged? You haven't tried to boot your machine after this event? (without the fan plugged in obviously)

02-14-07, 03:26 PM
I doubt any of your components are damaged. I've fried a laptop drive by having the wrong power wires connected, but most other components will just cause the system to shut down.

I've had countless IDE hard drives survive this (from hot swapping them and having the cable turned the wrong way... heh, yes IDE drives are hot swappable in XP :p) so at most your fan could be toast, but I highly highly doubt anything else is wrong with your system or PSU.

02-14-07, 03:58 PM
Hi, I've tried the PC without the fan and it appears to be ok. I've tried the fan on an old psu and it also appears to be fine.

It looks like I was really lucky... but my heart stopped when the computer turned off. It was near midnight, and I was too tired so didn't try it that night in case I did something more stupid whilst over tired.

I'm going to put the fan back in the PC with the wires the right way round. From now on, anything I buy I will double check the wiring before using it!

02-17-07, 01:41 AM
What you did was simply reverse the polarity of the fan. You didn't ground out the 12v lead, so you should be fine. What you basically did was make the fan try to go backward. The fan probably even works fine, it just apparenetly won't spin in reverse.

02-18-07, 10:36 AM
Hi, everything does work fine, including the fan. I was expecting melted power cables. I just assumed the 12v would go through the power line back to the psu down a line it wasn't expecting.