Testing MoBo power connectors for shorted supply rails with an ohmmeter (is it safe?)

door_king

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Sep 6, 2017
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My computer won't turn on. It abruptly shut off the other day and would not power up again. The problem was not preceded by any errors or strange behavior. It was as if somebody had just yanked the plug.

It does not seem to be the PSU. It powers up just fine when it is disconnected from the MB and I "jump" it. However, when it is connected to the MB, I get practically nothing. No beeps, no fans, no video output... When I first push the power button, the CPU fan will twitch and the "CPU LED" will flash for a split second and then nothing. I don't get any voltage from any of the PSU wires, except for the standby power. It appears that something "downstream" from the PSU is causing the PSU to go into "protection mode" preventing power up.

I pulled everything off the MB but the PSU, CPU, and HSF and tried seeing if it would at least try to POST, but it was the same thing.

The PSU doesn't have much to go by to send it into protection mode -- MB and PSU don't "talk" to each other. I think the only thing it goes by is detecting excessive current draw, and maybe excessive ripple/fluctuations.

I began to suspect there was a short on the MB from a supply rail to ground, causing the PSU to cut its output. I decided to pull the 8-pin CPU power connector, but leave the 24-pin main board connector to see what would happen, and BINGO, fans spin up! Not much else happened since there was no RAM, GPU, etc. The CPU LED stayed permanently lit since the CPU wasn't getting any power.

So there's something going on with the CPU or the part of the MB that supplies juice to the CPU. Like I said, I suspect a shorted supply rail. I simply want to verify this by checking the resistance between the 12V pin(s) on the CPU connector and one of the ground pins using a DMM, but I don't know if it's safe. Will the small voltage that the DMM puts out in "ohm" mode cause any problems. I wouldn't think it would be an issue on a circuit that's normally supplied with 12V, but I wanted to check first before I dive in.