A1466 820-3437-B fan spin and stop once

[MicroSMD Lab]

Hi, I need some troubleshooting a fan spin and stop once on a Macbook Air 820-3437-B. I get green then orange, little fan spin then nothing, USB sticks blinks once. Current sensing resistors on U7100 are good resistance.
The board was initially liquid damage, I restored PP3V42 and the device worked well for a while. Then it would only boot if I press the power button until I see the apple logo and when it did boot, it was really slow. I took it apart and saw some corrosion around J1800 and I cleared that up (flux and boil and applied fresh solder then cleaned up). I also noticed C7094 had some corrosion so I changed that as well the work I did before to restore PP3v42 was in this area so everything else was good.

[MicroSMD Lab]

Re: A1466 820-3437-B fan spin and stop once

I think I may have a shorted SMC. Pin 41 of the SMC reads 32ohms to ground, I removed all of the other components on the line individually and the short did not go away..... hmmm, I did not test the components to confirm that they were not shorted, it could have been more than one shorted component. OK, well I don't have a SMC stencil or solder balls so I'm hoping this is normal resistance or more than one shorted component.

[luckydip]

Re: A1466 820-3437-B fan spin and stop once

Is the PP3V42 line restored to the correct voltage? I've had boards in the past with issues when the voltage was 3-3.2v.

Also check the tracks around U5110 as these get corroded easily as they're on the edge.

Did you replace U5110?

[MicroSMD Lab]

Re: A1466 820-3437-B fan spin and stop once

Yeah, it's 3.4...V. I did not replace U5110, its still out, I did check for short to ground pin and there's no short on the ic. Traces around U5110 look good.

[luckydip]

Re: A1466 820-3437-B fan spin and stop once

Normal resistance should be a few hundred ohms with all component on the board.

If you've removed C5125/5020/5021 then that only leaves the SMC. You could supply 2v with a psu and see what warms up. Depending on how bad the corrosion was its either internal board corrosion or the SMC.

[MicroSMD Lab]

Re: A1466 820-3437-B fan spin and stop once

Thanks, good advice there! I went ahead and removed C5125 and C5021 and check them for short and they were good, since I did not resolder U5110, I could test C5126 without removing and it was also not shorted. By elimination, C5020 should also be good since the short was still there when I removed it earlier. The funny thing is that the short is no longer there. I did not resolder U5110 but I get OL between all the pins and pin2 or center pad.
I'm thinking that the heat around the SMC chip helped remove some corrosion that was causing the short. I proceeded to "flux and boil" the SMC. No short now, getting OL when I test for resistance, U5110 is still not soldered. I have a donor board but I wouldn't want to replace the ic without good reason since I don't have much donor boards currently.

[MicroSMD Lab]

Re: A1466 820-3437-B fan spin and stop once

OK, so I decided to go the safe route and replace U5110. Green-Orange, 3 fan spins and light on a USB! I guess I'll find out if that U5110 is not good on another repair!
Thanks again luckydip for all your help.