The Dell Dimension 4500 Desktop PC motherboard, has a 20-pin ATX main power cable connector,
http://www.auctiva.com/hostedimages/showimage.aspx?gid=1655902&image=561702676&images=561702587,561702631,561702676&formats=0,0,0&format=0(Top left of motherboard)This is a general example of a 20-pin ATX main power cable, and it's connector on the motherboard, (Note color never matters),
http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20The 20-pin ATX main power cable is left plugged into the motherboard, as shown in the photo to the far right.A jumper wire is used in the connector for the 20-pin ATX main power cable.IF the Power Supply comes on you have a bad Power On switch.IF the Power Supply does Not come on you have a bad Power Supply.You can still use the method to check the Power Supply too.Just because it's new doesn't mean it's any good. DOA power supply's do happen. (Dead On Arrival)Again looking at the photo to the far right note the Green wire.This is the Soft Power On wire. (Abbreviated PS_ON)Note all the Black wires. ALL Black wires are Ground wires.The preferred jumper wire is a paper clip straightened out, then bent into a U-shape.The top of the U is wrapped a few times with black plastic electrical tape. (Did you know that one wrap of black plastic tape, is good enough for 600 Volts DC?)Turning the U upside down there are two 'legs'. The legs are what is used.One leg goes down into a socket hole with the green wire in it.The other leg goes down in a socket hole with ANY Black wire in it.Where the wires go into the connector of the 20-pin ATX main power cable, is the Back of the connector.This is where the jumper wire is used.One leg goes down into the socket hole of the Green wire, RIGHT NEXT TO the insulation of the green wire, and slides down into the socket hole.Has to slide down in far enough, to touch the metal terminal at the end of the wire.Same with the other leg of the U-shaped jumper wire.Goes down into ANY socket hole with a Black wire in it, next to the insulation of the wire, and down in the socket hole far enough to touch the metal terminal.[All wires going into the connector of the ATX main power cable, end in a metal terminal,
http://www.molex.com/molex/products/datasheet.jsp?part=active/0002081202_CRIMP_TERMINALS.xml&channel=Products&Lang=en-USThe left side is the side that get's crimped on the wire. This is what the U-shaped jumper wire's leg touches. The right side is the font of the connector, and in the middle photo of the Playtool link you can see the tips of the metal connectors. (Brass colored ]There may be a small spark when the jumper wire 'legs' make contact.The Soft Power On circuit uses 5 Volts DC.In comparison two D cell flashlight batteries store 3 Volts DC.The contact made is 2 seconds. The Power On switch is a Momentary Contact Switch.
http://www.ebay.com/itm/Dell-Dimension-8400-Front-USB-Audio-I-O-Panel-Cables-M5989-/290780863258?pt=LH_DefaultDomain_0&hash=item43b3e3db1aThe Power On switch is on the small circuit board, that is at the upper right, and next to the word Upgrades.
http://support.dell.com/support/edocs/systems/dim4500/replace.htm#1114002For additional questions please post in a Comment.Regards,joecoolvette
