When a healthy computer to measure the output voltage of the unit power supply is possible, using analog-to-digital converters built into the motherboard. Restart the machine and immediately start repeatedly pressing the "Delete" or "F2" (depending on model) until the entrance in the CMOS Setup utility. Select the menu item "PC Health Status" and you will see what voltage is present at the outputs of the unit power really is.
A motherboard does not show in CMOS Setup voltage source power standby mode. Measure it with a multimeter, to measure DC voltage limit of 20 volts, connect it between the black and purple wires. Make sure it is not much too high compared to the nominal as in working and in normal operating mode of the computer.
When malfunction of computer, lack of features CMOS Setup "PC Health Status" or doubts about the correct operation of the ADC on the motherboard to measure all voltages, use a multimeter operating in the same mode and at the same limit. The measurements were performed while the machine is running, while ensuring not to cause the probe short circuit.
To check voltage of the unit power at variable load use a special stand. Load each of the outputs of the parallel connected automotive lamps in such quantity that the total current consumed was somewhat less than nominal for a given output. Note that at voltages of 5 and 3.3 V automotive lights consume low current: measure in advance what it at these voltages on a single lamp.Disabling part of the lamps on each of the outputs, we can simulate the reduction of the load on the power supply. Spend the rest of the measurement in the same way as in other cases.
If a particular voltage is too high, immediately stop operation of the unit power supply and give it in for repair. Most likely, the voltage increased due to drying of electrolytic capacitors, but to change them, even off a high-voltage device, which is switching unit power supplymay only be a qualified technician.