A2681 MacBook Air without keyboard & trackpad functionality (820-02536)

sekidata

Member
This liquid-damaged Air has no trackpad and no keyboard. I need help troubleshooting.
I already tried a known-good trackpad cable and a good keyboard without success. A known-good logic board in the same topcase shows functional keyboard & trackpad, so the topcase and daughter board should be ok.
I did find a short to ground on PP3V3_S2SW_IPD which I traced to a shorted UT360; replacement restored PP3V3_S2SW_IPD but not functionality.
 

sekidata

Member
During testing, this board has decided to shut down. I'm now stuck with 5V on USB-C, PPBUS of 12.3V, and no shorts on the big coils. 3V8_AON rail is stable at 3.8V but the outputs of both PMUs fluctuate wildly. No obvious shorts on any of the PMU outputs.

What's the best way to troubleshoot why the PMUs don't produce stable outputs? Remove the buck converter output coils one by one to find a bad secondary power rail?
 

sekidata

Member
From the LDO OUTPUTS on pages 34 and 39 : some are stable (PP1V5_LDOINT_MPMU, PP1V8_AON_SPMU, PP1V2_AON_SPMU), some are fluctuating (PP1V8_AON, PP3V3_AON), and some are 0V (PP3V3_S2_LDO, PP1V2_S2_CIO, PP0V72_S2_VDD_LOW).
 
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sekidata

Member
I removed all BUCK output inductors from both PMUs and U8100 was still fluctuating. I then diode-mode checked all LDO outputs of both PMUs and found PP1V2_AON_MPMU at 5Ω to ground. Removal of U8100 removed the short. Thanks for helping troubleshoot this one. Can I source a replacement for the MPMU from a similar model, such as A2941 or A2337?
 
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sekidata

Member
Ok, I went for replacing the MPMU with the correct subtype (343S00554) and functionality has been restored.

Now back to getting the trackpad and keyboard to work. LT400 is good and at 12.3V. LT200 is good and at 1.8V. However, IPD_OCP_FLT is 1.8V, which means that a fault is asserted, correct?
 

sekidata

Member
Yup, one of the inputs to UC861 (PBUS_AONSW_IPD_FLT_L) was low, and 20Ω to ground. Replacement of UC861 removed the short and the fault is no longer there (IPD_OCP_FLT is 0V). However, there is still no response to the trackpad or the keyboard...
 

2informaticos

Administrator
Staff member
Check all LT40x coils (page 87); also LT20x (page 85).
Non working trackpad also causes non working keyboard.
I suspect some damaged circuit on page 86.
 

sekidata

Member
All filters are checking out good. I'm going through the page 86 trackpad circuitry looking for failure points.

What's the connection between trackpad and keyboard in this model?
 

sekidata

Member
I found two level shifters that weren't working: UT350 and UT320. Inputs for both were 1.25V but with 0V outputs. Replacing both of them from a donor board restored 3.3V on both outputs. Trackpad now clicks and tracks and the keyboard works as well. Your help is much appreciated!
 
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