820-3437: No boot. Fan ramps to max, then shuts off. Repeat

Hell gents,

Have a MacBook air logic board that first came in for no power. Found a couple issues:

1. 10k resistor install on R5189 (schematic has this as omitted, so pulled it off)
2, C5008, C5007, C7714, R5177, R5100 all corroded. Replaced.
3. R5177 missing solder pad on pad 2, Jumpered to R5178 pad 2.

Other than those components above, no other corrosion found. It appears the board was once cleaned, so i'm assuming it there was more corrosion originally.

Now the board powers up (doesn't boot/chimes). The fan spins, the gradually ramps up to max speed, then the board shuts off. Repeats the same thing over and over.

Any suggestions?
 

2informaticos

Administrator
Staff member
This board should make two attempts to start and remains on in the third one.
Disconnect LIO flex and see if this time starts.
Also try to start in SMC bypass mode.
Check which voltages appear (big coils) in the time fan spins.
 
Same repeat power with LIO flex removed.
SMC bypass mode, the board boots and chime (don't have display connected to confirm full boot). But after about a minute it shuts off. Does this mean anything?

L7130 - 8.6v at start, then mid way before fan shuts off, drops to 8.593
L7520 - 5.116
L7560 - 3.328
L7430 - 1.2060 at start, then slowly drops to 1.2053 before fan shuts off
 
Last edited:

2informaticos

Administrator
Staff member
I'm curious if you even have an idea what % difference between 8.6 and 8.593 is; even less for 1.2060-1.2053
I hope you will not buy an "atomic" multimeter, with 6 decimal...
Do you really think that is relevant???

If starts in SMC bypass and stays on much more time, should have some SMC sensor issue.
Check first AVREF_SMC, then check all ISENSE/VSENSE sensors output.
Also try new BIOS, just in case.
 
Yeah, was just stating what I noticed during the fan ramp as i thought that was what you were asking for. Yes the % diff were minimal.

Only ISENSE with a greater difference in value is the one below. The rest were pretty close to each other.
SMC_1V2S3_ISENSE- 0.0447 on bad board and 0.202 on good board (fad at about the same speed as good board)

VSENSE values were comparable.

All measurements were compared on a good board, once its fan was at its highest speed.
 

2informaticos

Administrator
Staff member
That means lower DDR current and shouldn't stop the machine, I suppose.
However, check 1V2_S3 power supply area for corrosion.
If you test the board alone, only with USB mouse, no LCD, nor LIO board or other device, still turns off?
 
I just realized the test board i was using is a 00165 board. Currently searching for a 3437 board, but below are the finds and readings i collected

No corrosion found.
Yes it still turns off with just USB mouse (no LCD, nor LIO cable).

Chasing the SMC_1V2S3_ISENSE line, led me to U7400, Pin 11 (DDRREG_VDDQSNS) starts at 0.841 once power on, then after a few seconds drops to 0.477. On the 00165 board, it remains at 1.996v.
Also, DDRREG_DRVH goes from 1.336 to 1.276 while on a 00165 board it stays at 1.53. I replaced U7400 twice with the same result.
 
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