A2779 (820-02841-05) Keyboard and Trackpad Not Recognized After Replacement

Got a 2023 14" MacBook Pro (A2779, board 820-02841-05) that came in with liquid damage affecting the keyboard and trackpad. It still boots fine, screen is good, external keyboard and mouse work no problem. I replaced the keyboard, trackpad, and trackpad cable with new parts. Trackpad clicks (Taptic feedback is working), but neither the keyboard nor trackpad are recognized not in boot, not in diagnostics, not in recovery. Totally dead input-wise.

Here’s what I’ve checked:

-Keyboard power rail (PP3V3_AON_KBD_CONN) is present — getting 3.3V directly on JT200
-Diode mode on all KBD_SENSE, KBD_DRIVE, and KBD_ID lines looks normal (~0.400V range)
-Connectors are clean, no corrosion, parts are seated properly

I’ve read conflicting info about whether Apple firmware blocks keyboard/trackpad replacements on these models. Some say only Touch ID, logic board, lid sensors, etc. are paired. Others suggest even keyboards could be blocked from full function without AST2.

Maybe something on the board shorted or failed that Im missing?

Appreciate any insight, thanks.

You probably have a problem with TP only.
KB can't work alone.

So check first all JT400's power rails.

all PP3V3_S2_IPD = 3.3V
but PP5V_S2_TPAD_CONN = 3.3V instead of closer to 5v

If 5V_S2_MAIN is correct, then replace LT400; a simple wire bypass is good enough.

I should clarify, at pin 17 on JT400 PP5V_S2_TPAD_CONN I get 3.3v, but at LT400 that same rail I get 5v. How is that possible.

I just double checked again for sanity reasons....with the board in the palmrest, i measured 3.3v previously, but with the board out it measures 5v. Maybe i wasnt making full contact with my meter previously? idk....but now im getting 5v. So with that in mind, Ill put the board back in and retest. If it measures 3.3 again, maybe something is dragging it down thats connected that isnt connected when the board is out? Ill retest and post back.

You should check power and signals on JT400 with trackpad connected.
Don't you have replacement TP board/flex for test?

If mentioned voltage drops, replace LT400.

So after reinstalling the board, Im getting a consistant 5v on PP5V_S2_TPAD_CONN and PP3V3_S2_IPD is getting 3V. Could this be firmware rejection? If not, can you suggest the next step? Just for the heck of it, I removed LT400 and bypass with jumper wire. Still no keyboard, no trackpad, but I do get taptic feedback. Have tried 3 different new trackpads and 3 different new flex cables.

Keep the KB disconnected, until fix the TP problem.

Check all circuitries from page 127.
Start with SPI_TPAD series resistors, RT37x.

Quick update — I’ve now gone through 3 trackpad flex cables and maybe 3 trackpads, and I’m seeing the exact same pin burn up every time. It’s always the same spot on the flex cable, but just to clarify — it’s the trackpad-side connector that’s burning, not the JT400 (board-side) connector.

This is a 50-pin flex, and the burned pin is the 10th from the left when holding the cable with the trackpad connector on the bottom and the board connector on top. That may be pin 10, or it may be pin 41 — I don’t know the official direction of the pin numbering on this model.

Here’s what’s happening:

With the flex cable connected, the MacBook either doesn’t charge or won’t power on at all

If I disconnect the cable, the machine boots and charges normally. After a few connection attempts, that same pin melts again

All other related voltages (PP5V_S2_TPAD_CONN, PP3V3_S2_IPD) test fine when the board is out of the case

I "think" Ive narrowed that pin down to pin 37 on JT400 which is a ground ACT_GND.

ACT_GND checks out – trying to confirm if the trackpad was the real culprit

Just to clarify and correct my earlier posts:

• I’ve confirmed pin 37 on JT400 is ACT_GND, and it’s properly grounded
• There’s continuity between that pin and other chassis ground points
• With the trackpad and flex cable disconnected, there’s no short to ground on the 5V or 3.3V lines, and no floating or negative voltage on pin 37
• So the board-side looks clean from everything I can tell

The burned pin on multiple flex cables appears to line up with pin 37 (ACT_GND), though I’m still not 100% on that mapping. My working theory now is that my initial replacement trackpad may have been bad. That may have shorted the ground return or another line and burned the cable. Because I didn’t notice the damage on that first cable, I unknowingly plugged it into another trackpad, which possibly shorted that one, and so on.

I’m trying to avoid repeating that mistake before I order and connect another new trackpad and cable. So my question is:
Assuming I’ve pinpointed the correct pin based on the burn location, and all rails and grounds test clean, is it fair to say the original trackpad was likely the problem? Or is there anything else I should check on the board side before plugging in another trackpad to avoid frying it?

I may have found the real cause. If Im right, this could save alot of people headache (and money by purchasing the wrong trackpad replacement)

After a lot of digging, I think I finally figured out what’s been happening.

Turns out, the A2779 model with the M2 Pro chip uses a slightly different trackpad than the other A2779 variants (M1, M3, etc). Even though parts suppliers list many of these trackpads as interchangeable under the A2779 umbrella, they’re not actually the same internally.

I compared the original trackpad from this machine side-by-side with the replacement I bought, both labeled as A2779, and the circuits on the trackpad board are clearly different. There are changes in layout and what appears to be additional or missing ICs/components. I believe plugging the wrong variant into this M2 Pro model caused a fault condition that burned the same pin on multiple flex cables, even though the board and power rails all checked out perfectly.

That also explains why the machine wouldn’t charge or boot properly with the replacement trackpad plugged in

I’ve now ordered a trackpad that matches my original, not just based on the model number but by board design. Once that comes in, I’ll report back, but I’m 99% confident that was the root cause - wrong trackpad for the right model.