Macbook Air 1466 Thermal Sensors

[Askaleto]

Hello guys,
I'm inspecting the schematics of 820-3437 board and on page "Thermal Sensors"(page 44), there are a few acronyms such as:
TBT,MLB, MLBBOT, TBD, CPU Proximity, Inlet ,DDR and BMON. What are these acronyms such as TBT TBD and MLB mean?
How these sensors works? All I can see on the schematics are BJT transistors. Are these BJTs actual thermal sensors? The lines are mutual for a couple of sensors so it is possible that one sensor causing problem and reporting multiple sensors failure.
Actually, the laptop has a problem, full speed fan and Facetime cannot work with message to wait until macbook cools down. So, temperature sensor near camera is failed. Is it possible to remove a resistor or make a jumper in order to bypass this. Fan is currently controled by Macs Fan control and laptop works OK. Zoom also works and camera is OK. There is no place on the computer that is heating, I checked this multiple times, so information about high temperature is fake.

I need a way to bypass this since I don't want to prevent me from normal using of the laptop. The laptop is not working slow.

 

[2informaticos]

Usually ALS sensor issue.
Test if fan spins normal, when diconnect webcam.
If yes, then cut off the traces from pins 8 & 9 if J4002.

As you can see in page 40, ALS sensor is connected in the same bus with U5800, which monitors above mentioned temps.
Of course, you can change LCD assembly...

 

[piernov]

TBT = Thunderbolt
MLB = Main Logic Board
BOT = Bottom
TBD = To Be Determined (ie. they didn't know where to place that sensor yet when designing schematics)
BMON = Battery monitoring

I might be wrong but I think it goes like this (unlike the hints in the schematics seem to imply, also note that it seems to be different from the 11" 820-3435):
Q5810 is TS2P Camera Proximity
Q5820 is THSP T29 Proximity
Q5830 is Ta0P Inlet
Q5840 is TW0P Wireless Proximity
Q5850 is TM0P Memory Proximity
Q5860 is TCHP Charger Proximity
U5800 is TC0P CPU Proximity
U5810 is Tm0P Logic Board Top Side

ALS can indeed make all sensors connected to U5800 to report errors.

 

[Askaleto]

Thank you 2informaticos and piernov for the explanation. It is very strange to use use words proximity for thermal sensing since usually proximity sensors detects presence of an object in its vicinity.
Anyway, @2informaticos, if you by "change LCD assembly" think this:

that is ofcourse out of the question. There is no way to remove screen because of one faulty sensor inside it.

When you say to cut off traces, you mean to use tweezers or something similar to cut (and break) traces i.e. to break conductive path?

Thank you once again.

 

[piernov]

Proximity means "near something", so here the sensors are near the CPU (rather than inside it, which is another sensor), near the RAM, near the Thunderbolt controller, near the camera controller, etc…

First you have to confirm if the sensors problem is solved after disconnecting webcam. Only then you can proceed onto thinking how to fix the problem…

 

[Askaleto]

Yes, disconnecting cable that goes over I/O board caused fan to stop running at full speed.
I wonder how this really works. Does BJT starts conduction when temperature is high or something else? On the schematics, I can see that base and collector of BJT are shorted, and for the NPN transistor voltage between B and E should be above some trehold value in order transistor to start conducting (under assumption that collector is on positive potential).

In this case I have a clear sensor fail issue, since there is no spot on the board that is getting hot. From other posts I can see that these sensors fail upscale which means that temperature reading is above 100 degC

 

[2informaticos]

The bipolar transistor is used in B-E diode mode (short B/C), as temp sensor.
Any diode is sensitive to temperature changes, but simulated diode (with bipolar transistor) is even more sensitive.

 

[Askaleto]

Thank you 2informaticos, very helpful.

 

[Askaleto]

Hello,
can someone please confirm that all A1466 MBA use the same trackpad flex cable? If I'm not mistaken this model is manufactured from 2013 to 2017.

 

[piernov]

2013-2017 is the same, 2012 is different.

 

[Askaleto]

Thank you piernov. Just one more question. Usually when sensor fails, CPU starts throttling and computer is very slow. I have found some information about the "kernel_task" activity in Activity monitor. I can use the laptop and watch YouTube/Netflix streaming without any glitches.
Maybe this is a stupid question, but I have no experience with MacBooks and especially macOS.
Activity monitor screenshot is attached. Is it possible to see if the laptop is entering "limp" mode occasionally from the Activity monitor?
Thank you!

 

[Askaleto]

piernov, are these sensors labels same on 820-3437 and 820-00165 (A1466, years 2013-2017)?

Q5810 is TS2P Camera Proximity
Q5820 is THSP T29 Proximity
Q5830 is Ta0P Inlet
Q5840 is TW0P Wireless Proximity
Q5850 is TM0P Memory Proximity
Q5860 is TCHP Charger Proximity
U5800 is TC0P CPU Proximity
U5810 is Tm0P Logic Board Top Side

Thank you!

 

[piernov]

kernel_task taking 400% CPU time is forced software throttling. There's also hardware throttling that prevents the CPU from boosting or keeps it at the lowest core clock, can be seen with Intel Power Gadget: https://logi.wiki/index.php/Throttling
820-3437 is 2013-2014, 820-00165 is 2015-2017. I think the sensors should be the same (still not 100% sure). I added the info there: https://logi.wiki/index.php/Mac_sensors_reference_designators

 

[Askaleto]

Ok, thank you. I can conclude that there is no throttling on this MBA, which is "under observation". These sensors are really confusing me. without special control app, fan would go full speed. But the laptop works normal, wherever I found information about failed sensors it was also followed by slow computer response, throttling etc. On this particular laptop, youtubevideos can be watched without any glitches. The laptop unusually slow boots, but once the macOS is on, everything works OK. I'll examine further using Intel Power gadget.

 

[Askaleto]

I have installed Intel Power Gadget and make a test. I have opened youtubevideo, thunderbird, Libre Office Writer, Spreadsheet and Presentation and during that time diagrams is recorded and given in the attachment. Because frequency is not getting above 1 GHz, does this mean that this MBA is actually hardware throttling or that CPU is kept from boosting?


I'd like to see this diagram on MBA 1466 without problem. During this test MBA was only on battery.

 

[piernov]

Did you check the link I posted above? Of course you need to plot the graph while running a CPU stress test software.

 

[Askaleto]

I did try to use Prime 95, but after downloading and extracting archive, I have tried to start Prime95 and got the following message:
"“Prime95” can’t be opened because Apple cannot check it for malicious software."

Because of that I used CPU stress test and this is the diagram. Since CPU is i5 1.6GHz and there is a limit to 1 GHz, I can conclude that HW limitation of the CPU frequency is active on this MBA. Is this conclusion correct?

Thank you!

EDIT: In the sensor table list, ALS is mentioned at the begining but there is no sensor data in the table. Does ALS also belongs to SMC "3" SMBus?

 

[piernov]

Right click → Open like for most apps downloaded outside the App Store.
That CPU is from an 820-00165 board, not an 820-3437 board like you said in the original post. In any case this i5-5250U boosts up to 2.5GHz all cores, so if you don't see it get to that frequency when stress-testing then there is a problem for sure.

 

[2informaticos]

If you get errors about more temp sensors, there is not a problem with a particular sensor.
Various temp sensors are connected to the same bus through an interface.
If one interface fails, it will block correct comunication on that bus.
For this reason, SMC will cannot properly read several sensors connected to the same bus.

 

[Askaleto]

Yes, currently there are two laptops, both A1466 with the same/similar issue, one is 2014 and second one is 2015.
On this from 2014, when disconnecting cable from J4002: Camera Connector, fan is not running full speed, so with help of 2informaticos it is concluded that it is most likely sensor in LCD assembly, and exchange it is not an option for the customer.
Then I tried 2015 laptop once again (here it is suspected that trackpad is causing the problem), but I saw in HW Monitor that all sensors SMBUS_SMC_3 reporting erroneous reading.

I don't have a spare trackpad to test and can only bought one, but I'm not sure if this is a cause of the problem and don't want to wast money.

As 2informaticos wrote, this is not a problem with particular sensor, but with failed interface". By interface, you mean U5810 chip, right?

I was thinking to disassemble and check for voltages from BJTs to try to see if problem is caused by one particular sensor, but it's not an option now.