"USB Drawing Too Much Power" Error Power Problem Late 2008 Unibody MB470LL/A

[Auricular]

Has "USB drawing to much power" error. Trying to repair my mother board. Can only find service manuals with flow chart diagrams that lead to replace motherboard solution. Far from helpful. Can anyone point me in right direction. Tried resetting SMC and PRAM to no avail. SMC shutting down ports. Neither port working. Having difficulty tracing circuit and IC's difficult to identify. NEED SCHEMATICS. I think this is fairly common failure and will post solution if someone can assist. TIA

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Please consider I am electronic tech but know nothing of macbooks until yesterday so go easy.

 

[shoulin333]

Have you checked all your USB ports to make sure there is no foreign object in any of them shorting them out? A short circuit sounds like it could cause this error.

Also do you get this error even when you have nothing plugged into any usb port?

 

[Mrbobb]

This error is EXACTLY as it says. Your ball.

 

[Auricular]

I am reading a low 5 ohms across the +5 to ground. there is nothing foreign and there is no short I can see. If the short was at the connector I would be reading 0 ohms. Of course nothing plugged in and board on the bench.

Schematics of the usb drivers etc. of similar model would be extremely helpful!

 

[drambuie]

Does the ohmmeter read 5 ohms in both directions? If it's unidirectional, then you're reading the source resistance of the USB 5v supply, otherwise there's a 5 ohm partial short, which would obviously draw 1 amp at no load.

 

[BeeJee]

Does the ohmmeter read 5 ohms in both directions? If it's unidirectional, then you're reading the source resistance of the USB 5v supply, otherwise there's a 5 ohm partial short, which would obviously draw 1 amp at no load.

First thing I've read on MacRumors where I didn't understand a thing lol

 

[Mrbobb]

Assuming USB port is capable of delivering 1 amp, 5 ohm will trigger the shutdown. If 1/2 amp, 10 ohms will do it.

No schematic, try using a loupe, but don't expect miracles, you have to assume PCB is multi-layers meaning there are circuit traces hidden from plain view.

So, laptop still works despite error message? Will it bother u alot to leave it as it wo USB ports? Are USB ports on a daughterboard u can unplug?

 

[Auricular]

Drambue - It reads both ways.

MrBobb - Its actually around 7.7 ohms after remeasuring, but should be real high like 1meg + normally. I thinks its a blown regulator or protection diode but just a guess. I'm sure its easy fix if I could trace it. Unfortunately its not on a daughter board it just one big "little" mother board.

I am gonna try looking it again with a eye loupe but its difficult. I am thinking with the board powered down, back feeding around 200-400 ma and see what gets warm. Maybe hit the area with freeze spray and see what thaws?