OK I've managed to test a 1.5v battery using the +-Omega-A port on the multimeter. The probes are small enough for the 4-pin HDD power connectors - which pins should I be connecting the probes to?
The fan not running could mean a power supply problem or a motherboard problem since the motherboard switches power to the fan, depending on internal temperature sensing.
That meter should have some sort of battery inside to provide current for the resistance measuring function. It's probably very old by now, and you should take it out before it leaks, and put in a new one.
You can operate the other functions of the meter without that battery.
There is a possibility that the meter has 2 batteries, one for the resistance measuring, and another to power a Field Effect Transistor "front end" that let's you get more accurate measurements that don't divert as much current from what you're measuring. You will need that other battery to operate the Voltage and current measurement functions if you have that kind of meter.
If you have to plug the meter into a wall socket to get it to work you have an old vacuum tube meter which will probably still have a separate battery to operate the resistance measuring funtion.
If you've got two batteries, you should probably replace both.
Without seeing it, I'm guessing that ordinarily you put the black lead into the com (common) jack and the red into the
(current, measured in Amperes, or fractions thereof)
In addition to those jacks do you have some sort of range selector switch?
If so, describe it.
And while you're at it, give us the brand and model of that meter.
It sounds like it might be a fairly good and originally expensive old one.
The black wires on the 4 pin Molex connector are the ground connectors, the yellow is the +12, and the red is the +5.
The 2 black wires are tied together inside the power supply, so they are at the same potential (O Volts), so you can measure the difference in potential (the voltage) between either the yellow or the red and either one of the black wires.
The meter should have a very high internal resistance (except for the current measuring part, which is beyond your current knowledge and skill), so it puts very little load on the power supply when you measure the power supply's output voltages.
If you don't have the drive connected it may read higher.
If you have the drive connected you may see some fluctuation as the drive draws more or less current from either line, depending on what it's doing and needs electricity for, but a good power supply shouldn't let it drag it down for long by more than about a Volt or less.
I'll try to post back soon with info on what voltage you should be able to read on which wire on the cable assembly that comes off of the power supply and plugs into the socket on the motherboard.