# Unable to power on Dell Dimension 4550



## wordbuilder (Nov 12, 2009)

I'm working on a Dell Dimension 4550 for a client. The system will not turn on. With the power cable attached, there is a green light on the motherboard. The power supply tested okay. I also pulled the memory.

I removed the switch and replaced it with another used one (seller claims it tested fine). However, this made no difference. Can I use a jumper to test the switch? Which pins do I need to contact.

Any other thoughts on what might be causing this problem?

Thanks!


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## Tyree (May 10, 2009)

The Mobo light being on doesn't mean the PSU is good. How did you test the PSU to assure that is is not defective?
Your problems relates to a failing/failed PSU. Do you have another PSU to try?


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## wordbuilder (Nov 12, 2009)

I used a PSU tester but it's very basic (plug it in, flip a switch, and an LED glows), so I don't know that it's accurate in the case of a failing, but not fully failed, power supply.


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## wordbuilder (Nov 12, 2009)

I have a more thorough PSU tester on order. When it arrives, I'll post the results.


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## wordbuilder (Nov 12, 2009)

Tested the PSU again with the new tester. All of the lights lit except the one for -5V.

I've read that newer 24-pin PSUs often do not supply the -5V line. However, the PSU I'm testing is only 20-pin. So, is it likely that the non-functioning -5V is the problem?


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## hitech (Oct 17, 2007)

Tyree ask if you had another PSU to try. I still use a digital voltmeter, even so I still double check with a known workiing PSU. As you know, the Dell PSU is not your standard PSU unit. I use a standard 20+4 pin PSU that does not have the white -5v wire. I have got a later Dell to post and load the OS with this. If it does not post, I check the motherboard diagnostic lights and see if the motherboard light is amber and if the power switch light is amber, that leads to a bad motherboard.


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## Tyree (May 10, 2009)

The best test for a PSU is a substitute.


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## wordbuilder (Nov 12, 2009)

Thanks for the feedback. The motherboard has a single light, which is green. The unit won't POST, and I get nothing on the power switch light.

I'll see what I can come up with in the way of another compatible PSU and let y'all know what happens.


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## hitech (Oct 17, 2007)

These are the diagnostic codes for the led lights on the motherboard on the back of the PC.
http://support.dell.com/support/edocs/systems/dim4550/advanced.htm#1118998
I have seen the small flat ribbon with a black end coming from the power switch that plugs in towards the front of the motherboard come loose from someone else working on it.
Good Luck


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