How to test Voltage Regulator Modules (VRMs)

Procedure for testing Voltage Regulator Modules (VRMs):
(This assumes that any bad capacitor problems have already been fixed.)

1. Turn off power to the computer.

2. Remove CPU, memory, and all peripherals from motherboard. Disconnect power supply.

3. This step verifies that the board is not damaged, and the VRM is not shorted:
a. Measure resistance from power supply pin (+3.3, +5, +12, -5, -12) to input of VRM. Resistance should be zero (too low to measure).
b. Measure resistance from output of VRM to load supply pin. Resistance should be zero (too low to measure).
c. Measure resistance from input of VRM to ground (should be greater than 1 megohm, usually much greater).
d. Measure resistance from output of VRM to ground (should be greater than 1 megohm, usually much greater).

4. Install CPU, memory, and all peripherals to motherboard. Connect power supply.

5. Turn on power to the computer.

6. This step actually tests the VRM (all voltages taken with respect to circuit ground):
a. Measure voltage at the power supply side of the power connector (+3.3, +5, +12, -5, -12).
b. Measure voltage at the motherboard side of the power connector (+3.3, +5, +12, -5, -12). Anything less than the value obtained in step 6.a indicates a high resistance in the power supply connector.
c. Measure voltage at the input to the VRM (+3.3, +5, +12, -5, -12). Anything less than the value obtained in step 6.b indicates a high resistance in the PC board traces to the VRM, and is probably not repairable, even with a schematic.
d. Measure voltage at the output of the VRM. The value obtained depends on the functions being powered by the VRM, so knowledge of the expected value is required. Anything different than expected indicates a failed VRM (note that output may be higher or lower than expected if bad).
e. Measure voltage at the device being powered. This requires knowledge of the device (which pins are power pins). Pinouts are readily available on the Internet for 30-pin, 72/144 pin, and 168 pin memory, as well as ISA, PCI, AGP, USB, etc. Pinouts for CPUs are either on the Internet or may be obtained from the manufacturer. Anything less than the value obtained in step 6.d indicates a high resistance in the PC board traces from the VRM, and is probably not repairable, even with a schematic.