820-4924 No display/USB when TPAD plugged in

[ALB-Repairs]

I seem to a pile of insane faults.
I got a nasty 820-4924 in, Did allot to get it posting, and then found it wouldnt display or boot if the trackpad cable was plugged in, It just seems to sit with backlight on but nothing else.

Tried known good parts, inspected the board, Brushed U4830 and it was that corroded it fell off the board.
Fixed the damage replacing U4830 and U4820 for good measure, and fixed all the damaged traces and Via's around the area. checked diode mode in most areas. However the fault still remains.

Now for the strange part. If i hotplug the trackpad after post it all works fine, displays, boots and the trackpad and keyboard work perfectly.
Ive put a good couple of hours into this one now and im starting to loose my mind.

 

[2informaticos]

Did you change J4802?

 

[ALB-Repairs]

Did you change J4802?

I have not, Do you believe there could be corrosion under it causing problems?
j4802 looks perfect under the microscope, and diode mode readings are the same as a known good board.
Thanks

 

[ALB-Repairs]

Okay, Ive changed J4802 now. No change.
Ive noticed something odd. When the TPAD is plugged in R4913 is lower than a known good (2.4V rather than 3v3)

When hotplugged straight after post all works well, and the Trackpad hardware is recognised under the SPI section of system report. Im really struggling to understand why this board will not work!

 

[2informaticos]

Change U4810, let see if helps.
In normal condition, can you access boot menu?

 

[ALB-Repairs]

Change U4810, let see if helps.
In normal condition, can you access boot menu?

I changed U4810 and redid u4820 again just to be sure.
No in normal condition you cannot access boot menue, Just a blank black display, No USB activity, Wont read internal SSD.ect

 

[2informaticos]

"When the TPAD is plugged in R4913 is lower than a known good (2.4V rather than 3v3)"
Do you get 3.4V on both pins?
Check pin8/U4910 too.

"Change U4810, let see if helps."
My mistake, I was looking other schematic.
I should write U4910...

 

[bjf]

ALB-Repairs Did you ever solve this? I have the exact same issue. every detail: no video. backlight only. but boots normally and has video without trackpad connected. 2.4V on both sides of R4913 with trackpad plugged in. 3.3V with trackpad unplugged.

 

[2informaticos]

Did you try known good trackpad flex/board?

TPAD_SPI_IF_EN_CONN should be high once entered into OSX; itb stays low (0V) before.

TPAD_VBUS_EN should be high before accesing OSX; i.e. boot menu.
It should drop once OSX is started.
If file vault is enabled, TPAD_VBUS_EN is still enabled on the password screen; no TPAD_SPI_IF_EN_CONN yet, in such case.

Check U4910 inputs and outputs, to decide if works correctly.

 

[ALB-Repairs]

Holy thread revival.
I figured it out a while ago, there was a resistor for SPI (maybe a pullup) that looked okay, but the pad underneath was half missing. I cannot remember which resistor, i only figured it out by deciding to replace all components related to trackpad SPI
Thanks though

 

[2informaticos]

An issue on SPI bus shouldn't stop to access boot menu.
Before enter to OSX, trackpad and keyboard work through USB.
I saw one case where USB enable from PCH was sorted to ground; trackpad only worked once logged into OSX.

 

[ALB-Repairs]

Im not sure how correct you are, as part of diagnosis i lifted the USB lines from the trackpad connector. I did this to try and disable USB, to see if the black screening was resolved, but even without the USB lines it still black screened.
If memory serves correctly it was corrosion on a pad under R1618, as i said, i repaired this months ago, and it was to do with the SPI! (I may have got the exact resistor wrong, but it was certainly in that area)