Does anyone know what specifically the -5V does on a power supply.

-5V is just another voltage path for the current to go down to.
It is purly for the the components within the computer that require a negative current, such as Microchips. There are the main supply voltages which are -5V , 0V and 5V for general use for the components inside the computer. The +/- 12~13V lines are for the main positive and negative voltages. This gives a greater potential difference compared from going to 12~13V to 0V, basicly it gives you double the voltage to play with.

I constantly see powersupplys in working order that test bad in the -5V range. (With multiple testers but still work)

Does that mean that they are bad, are going bad,...? Probibly a stupid question.

By the way thanks for the reply!

I do electronics so this is my knowledgeable area. What do you mean by "test bad in the -5V range."? That makes no sense seeing as it is or it isn't -5V! The only possible thing that it could mean is that, there isn't enough current going through, due to too much resistance in the wires. But that would very very rarly happen if they use a wrong compund to make the wires in the first place. What was the site that showed you the testing? If you could post it here i will take a look at it, and judge what they are on about.

That would explain your knowledge lol, thanks for helping me!

I operate a small business, we have guys from network engineers, to master web designers, to programers. I run the repair and building side of the business. When i have the need to test a power supply I use one or all of the 4 PS testers that I have. Basically I test a working power supply that does not indicate in the -5V region. All others good (PG, +12V, +5VSB, +5V, +3.3V, -12V) excluding the Four I
don't really have anything to show.they are physical testers. like this.

The testers just do not indicate that the -5V is working. The indicator light does not light.

This question spawns from an argument over what the -5V truly is and how it will affect a machine. A customer wants to replace a fan in a power supply that is no longer made, and we replace it. And as part of a routine we test it. the -5V doesn't light up, but the power supply still works.

Thanks for helping, if I can elaborate in any area let me know, thanks!

Oh those kind of testers, plug in the respective cable and read of the results. Well charge will always flow to the more positive, so 0V to +5V or -5V up to 0V for example, i think they include the -5V just to make the microchips happy, because from my experience with them, via college, there has to always be a + and a - terminal to connect power rails. Some microchips can't cope with +/- 12V, so they use +/-5V instead. But if the -5V power supply doesn't work, or is not there, there are probably microchips in the system or even small transformers that scale down the voltage from - 12V down to -5V.

So in your opinion would the scaling down of the 12V or just the lack of a -5 damage the mobo?

Well as long as there is the correct electrical components on the motherboard or even in the PSU, there would be no difference at all! As long as there is a transformer (a very tiny version) it will work perfectly fine. If it was in full working order without the -5V rail, the computer will have a system to correct that by scaling down one of the other voltage rails.