Topcase repair reassembly issues: A2179

[phospholipid]

I’m working on an A2179 2020 MBA. My old friend spilled something in the topcase and right-side keys weren’t working. Everything else was working fine when I took possession.

I did the repair once with a case/battery combination unit. When I powered on, the fan went full speed and the CPU was throttled to a near halt. It booted and worked, but barely. Also, the battery wouldn’t receive charge.

I read elsewhere that behavior can result if there is a thermal sensor somewhere not properly sending information, or if the battery is completely dead. I reseated all of the cables to no avail, so I returned the component figuring there was an error with the battery or its board.
I ordered a better grade battery (from here!) and case. I repeated the repair and I’m getting the same result. I’ve gone through and reseated the cables. Same thing.

So I’m stumped. Here are the basic conditions:

1. Previous to disassembly, the CPU worked fine. The only issue was problematic keying. Work arounds included external keyboard.
2. Two repairs with different components have both resulted in the computer protecting itself.
3. I’m reasonably certain the second battery and its controller are good quality.
4. I've come to learn this is a not uncommon issue with MBA topcase and screen repairs, and I even read a conversation about it with an iMac.

Reseating cables hasn’t fixed it. Having said that, I am wondering about some of the cabling around the trackpad. I’m also wondering about the tiny daughterboard just north of the trackpad on the A2179. I do wonder if something is goofy with the power/fingerprint button. I may order a new one of those, but that shouldn’t be sufficient for the unit to go into temperature protection.

Any thoughts or advice is super appreciated. Cheers!

 

[phospholipid]

I DIDN'T CONNECT THE BATTERY CABLE FLUSH F**K ME FGNKJOAI#YU HI#RH(*FQIOHJK B$ *EIUJFA

Thank you to a kind user in iFixit forums.