PSU 24-pin to 20-pin

I recently bought an old motherboard for 5 bucks, and I'm working on building my first computer.  The motherboard comes from an old Dell Dimension 4400, and I'm not sure on the power supply, besides that it is a Dell power supply.  The power supply is 24-pin, and the motherboard is 20-pin.  Unfortunately, the power supply connector is not one of the new breakaway 20+4-pins.  I can still make it fit however by just letting the extra four pins hang off the end.  I get a little green LED on the mobo, but no response when I push the power button.

I know the motherboard works because I tested it with the psu from my PC (20-pin), and it worked fine.  I researched the wiring for ATX power supplies, and the colors all seem to match up with what I would expect.  Is there a good way to test if the power supply is functional or not?  I was wondering if it was possible that the psu could somehow sense that the extra four pins were not connected, and would not power up for that reason.  Could that be the case?

I can't guarantee that this would solve the problem but I'd give it a try: 24 Pin to 20 Pin ATX Converter - Adapter

1.  I can still make it fit however by just letting the extra four pins hang off the end.  I get a little green LED on the mobo, but no response when I push the power button.
2.  I was wondering if it was possible that the psu could somehow sense that the extra four pins were not connected, and would not power up for that reason.  Could that be the case?

1.  It's not plugged in all the way.  Get the adapter.
1.  No.  4-pins are for additional +24V power which you don't need.

1.  It's not plugged in all the way.  Get the adapter.

So, theoretically, if I were to cut off those four pins so that the plug could fit deeper, it would work?

I wouldn't ruin a good power supply doing that.  BTW, did you remember to plug in the separate 4-pin power connector?  Not the 4 extra for the 20, the other one that plugs into a separate connector on the mobo.

I wouldn't ruin a good power supply doing that.

Do you say that just because it will no longer be usable with a 24-pin motherboard, or are you unsure whether it will work att all? Because if it works, I don't forsee taking it out of this machine.  I got it for free anyway.

BTW, did you remember to plug in the separate 4-pin power connector?  Not the 4 extra for the 20, the other one that plugs into a separate connector on the mobo.

Yes, I plugged that one in too.

Well, I pushed the plug in to the point that I think it should be getting a good connection, and still nothing.  Is it possible that it is simply a problem with the "power on" signal coming from the power button on the front?  (I know that the button is good because it worked when I put my other PSU into the machine..) 

Is there any way to manually turn the PSU on besides the button on the front of my case?

And what does the green light mean?  Could it be possible that it lights up the green light, but the PSU is no good?

Is there a good way to test if the power supply is functional or not?

Disconnect it from motherboard and test it.
http://wiki.xtronics.com/index.php/ATX_Pinout

1.  Is it possible that it is simply a problem with the "power on" signal coming from the power button on the front?
2.  Is there any way to manually turn the PSU on besides the button on the front of my case?
3.  And what does the green light mean?
4.  Could it be possible that it lights up the green light, but the PSU is no good?

1.  Possibly, but unlikely.  Button provides a momentary ON connection.
2.  Yes.  See link provided.
3.  Power Standby ON.
4.  Yes.

Every single 24-pin power plug I've ever seen is just a 20-ping connector with another, extra 4 pin connector attached to one side. this is designed so that you can remove the 4-pin connection and the remaining connector is a appropriate 20-pin plug.

Every single 24-pin power plug I've ever seen is just a 20-ping connector with another, extra 4 pin connector attached to one side. this is designed so that you can remove the 4-pin connection and the remaining connector is a appropriate 20-pin plug.

Yes but the OP stated it did not have this breakaway piece.

IF you were VERY careful, cutting the plastic with a sharp craft knife to separate the extra 4 pins from the 20 SHOULD work, but this would definitely void the warranty (if it still has one)

I know, but I swore blind that mine didn't have that piece, so I got an adapter, and then when I plugged it into the adapter... the 4 pins slipped off. I was expecting some sort of latch but in my case is connected nearly invisibly.

[Moderator]

Every single 24-pin power plug I've ever seen is just a 20-ping connector with another, extra 4 pin connector attached to one side. this is designed so that you can remove the 4-pin connection and the remaining connector is a appropriate 20-pin plug.

Exactly.

Every single 24-pin power plug I've ever seen is just a 20-ping connector with another, extra 4 pin connector attached to one side. this is designed so that you can remove the 4-pin connection and the remaining connector is a appropriate 20-pin plug.

I went and double checked and I am now quite sure that the four extra pins are very permanently attached. I tugged and pulled and twisted to see if I could pop them off in any way, and they didn't budge.  I guess since Dell made this power supply for the sole purpose of putting it in machines with 24-pin motherboards, they didn't go to the trouble of making a breakaway plug.

Disconnect it from motherboard and test it.
http://wiki.xtronics.com/index.php/ATX_Pinout

I tried what this link suggests and shorted the green to the black, but got no response.  I did however use a multimeter to test the voltage of the various pins and found that only the green and purple wires had a voltage of +5 each.  I guess that is what you would expect before power on. 

My friend who gave it to me told me that it was working fine.  He had built himself a new computer, and in the process gutted his old one.  He had kept the PSU in a box for a few months before he heard that I was looking for one and gave it to me.  But, since I can't get the thing to power on, I don't know what else to do but give up.

Thanks for the help.

Some psu's need a load to turn on.  Plug in an old hard drive, a fan, etc.  The psu fan should turn on if working.  But, it's probably dead.