MBP 13" late-2011 (820-2936-B): bad sensors, need to fix VN0R, IB0R, IC0R

[TheBloke]

Hi everyone

I recently bought a used Apple MacBook Pro 13" late 2011 for my mother, but with a known fault. Her existing laptop is old and dying, and I think she'll do better with macOS than Windows, but neither she nor I can afford a fully working Apple laptop. I knew it did turn on and ran the OS, so I hoped it might be fixable.

The symptoms:
1. Only starts with a 10-second press on the Power button. Therefore it can't wake from sleep, only do a full startup each time.
2. In macOS it runs slower than an asthmatic snail slithering through glue with some heavy shopping. So slow the mouse pointer judders. But in Linux it runs OK, and it also ran OK in macOS' Safe Mode.

Some research indicated that the slow running is being caused deliberately by macOS as a protection measure, because of bad sensor readings. And running ASD 3S148 confirmed this theory.

From a cold start, ASD shows all tests pass except the following 7 bad sensor readings:

• Voltage - VC0C - CPU VCore - above the high limit
• Voltage - VD0R - DC in Rail 0 - above the high limit
• Voltage - VN0R - MCP VCore - above the high limit
• Voltage - VP0R - PBus - below the low limit
• Current - IB0R - Battery - above the high limit
• Current - IC0R - Computing Combined Cores - above the high limit
• Current - ID0R - DC In - above the high limit

Put another way, the only Voltage/Current sensor that does work is Current - IO0R - Other (5v, 3.3v).

However, these sensor failures are variable, and this appears heat-related: when the laptop is hotter, more sensors work. (Inversely related I guess - more heat, fewer failures.)

From a cold boot, seven fail as shown above. But if I remain in ASD for 10-15 minutes, some of them start going OK. After a few minutes I will get down to only four failing: VC0C, VD0R, VP0R, ID0R. And once I got down to only two sensors failing - VC0C and VP0R. I have never seen those two go OK.

I believe I have confirmed that this is related to temperature by running ASD with the laptop back off, waiting until only four sensors showed failed, then holding a hand fan pointing at the motherboard. Within 2 minutes, it went back to seven sensors failing. Then I removed the fan, and 5+ minutes later, it was back to four failing.

VP0R (PBus) is the only one 'reading below the low limit', where all the others are 'high'. I therefore wonder if this has a different cause. I'm hoping maybe the others all have a single cause - or at least, many of them do. I think it's definitely notable that three of them all go OK roughly at the same time, once the laptop reaches a certain temperature. A failure in a component common to all of them?

My experimentation shows that sleep works and macOS runs OK when VC0C, VD0R, VP0R and ID0R are all failed. It's only when I have all seven failures that macOS goes into a sort of 'protection mode' which causes the CPU to drop to ~2% of normal performance, and it won't startup normally.

The bottom line is I just want a usable laptop with sleep/wake and running macOS OK. If that's achieved I don't care about bad sensors. Therefore the minimum is to fix VN0R, IB0R and IC0R. If I can fix more or all, that's great, but I believe it should run OK with only those three fixed.

Here's what I've done so far:

• Resetting PRAM and SMC;
• Everything possible related to software - fresh install on blank HDD, updating SMC/EFI firmware, multiple versions of macOS;
• Disconnected the battery;
• Swapped out HDD, RAM, and re-thermal pasted the CPU heatsink
• Unplugged/replugged all internal connectors and cleaned them with 99% IPA;
• Dismantled the laptop to the point of extracting the logic board;
• Cleaned the whole logic board with 99% IPA and a soft brush;
• Examined the board for any obvious signs of damage - none found;
• Tested without the webcam connector attached (I read that it shares a data path with some sensors.)

Nothing so far has helped, so it looks like I'm down to investigating individual components. I do own a Hakko soldering station, a decent multimeter and an OK oscilloscope (Rigol DS1054Z). Also a bench PSU. But I've never done anything like this before. I do have a little electronics knowledge and have watched a lot of YouTube videos, but not much personal experience, and absolutely no experience of surface mount stuff.

I have the schematics and BoardView for my logic board (thanks to a post on this forum.) Well, I have 820-2936-A, I can't find -B anywhere. I hope that's close enough; they seem to match on the bits I've looked at so far. From these I have identified an area that contains many components related to ISENSE and VSENSE, so I suppose my next step is to check all the resistors and capacitors with my DMM and verify their values.

Before I start, I'd like to ask whether anyone recognises this failure pattern in an Apple MacBook? Do they point toward a particular component or area that I should focus my efforts on? Or do the symptoms (some sensors go OK when the laptop is hotter) point to a particular type of failure? Maybe a bad solder joint, or a specific failing resistor/cap/transistor? I'm afraid my electronics knowledge is basic enough that I don't know stuff like this. I was quite surprised that things got better at higher temps - I'd have assumed the opposite.

Second question: should this be possible for me to repair? I must admit I am quite daunted by the prospect of maybe replacing tiny SMD components on the logic board. It this at least possible with a normal soldering iron? I don't have anything fancy like a reflow oven, and I don't know if that's a problem.

Thanks very much in advance for any help. I'm a newbie when it comes to electronics and below a newbie with SMD components. But I love a challenge and am willing to try anything necessary, so I hope maybe something can be achieved. PS. Hope the post isn't too long - I thought I should include everything I know and everything I've tried.

[TheBloke]

Re: MBP 13" late-2011 (820-2936-B): bad sensors, need to fix VN0R, IB0R, IC0R

I guess no-one here is familiar with these symptoms. For the record and in case it helps any future newbies, I will update this thread as I learn and try more.

To answer my own first question: yes it's possible for me to make these repairs - or at least, it's possible in general. Since I last posted I've watched a lot of Louis Rossmann's component repair YouTube videos and he has a couple where he shows exactly the sort of repairs I need to make: replacement of resistors and caps using only a soldering iron.

So while it's likely a lot easier using hot air and/or hot tweezers, it definitely is possible to replace these tiny components using just a normal iron. Whether I can do it is a different matter of course.

I did need to get a couple more tools, in particular a basic microscope. It's only a basic USB thing, cost less than £20, and it's cheap and tacky and the stand is terrible. But I don't think I could even attempt this without it - 0402 components are just too ridiculously small.

With the help of the microscope I've now done some measuring, starting with the PBUS sensor VP0R. This is the one sensor that's always "below the low point", and initially I thought I might not need to fix it as it was one of the four that seemingly could be bad while the laptop would run normally. But I'm starting with this one because I've seen a couple of videos where Louis reports it as a cause of ultra-slow-running, and because he fixes it in a couple of videos. So I figured it'd be a good one to try and fix first, as I can refer to the videos for help. He also mentioned in one video that 'below the low point' sensor readings were always bad, where 'above the high point' might not be a problem. So I'm wondering if I'll get lucky and find fixing PBUS might fix all the symptoms. Unlikely, I know.

Looking under the microscope in the PBUS VSENSE area showed the board looked quite dirty/nasty in this area; cause unknown. One of the two resistors used in the voltage divider (R5304) is reading half the resistance the schematics report. And the capacitor used in the same divider (C5304) is reading 0.0 on my multimeter's capacitance measurement. Either of these could explain the failure of this sensor, and clearly both need to be replaced.

My next step is to find some piece of sacrificial electronics that I can practice 0402 soldering on. I don't want my first attempt at replacing 0402 components to be my one and only MBP motherboard. I have an old Android phone that I hope might have some components this size. I might even find some suitable donor components, saving me buying them from Mouser or wherever.