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Why won’t my 2008 MacBook Air A1237 start or charge normally, and is the fault likely the battery, charger-detection circuit, or motherboard?

The startup problem was most likely caused by the failing battery; after fitting a new battery, the MacBook Air began booting normally again and the fan/temperature readings returned to normal [#16409799] A MacBook Air A1237 from 2008 has no CMOS battery, and Apple does not support BIOS modification or flashing, so this is not a BIOS/CMOS issue [#16397295][#16397961] The MagSafe adapter should be detected by the SMC, and the middle pin should carry about 3 V; if that voltage is missing, the charger will not be properly recognized and charging/LED behavior can fail [#16555595][#16559981][#16560181] When the MagSafe LED still did not work and charging remained limited, U6990 on the motherboard was identified as the likely part to inspect or replace [#16565912]
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  • #1 16397278
    Anonymous
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    #2 16397295
    gabik001
    Level 37  
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    Board Language: polish
    First you need to be sure that the battery is working, but after what you write it is not.
    Apple does not provide the ability to modify the BIOS, much less upload. As far as I know there is a simple (literally) BIOS and the rest is on the disk with the system.
    Diagram attached. I didn't find CMOS battery.
    On page 46 there are temperature sensors.
    Attachments:
    • Apple MacBook Air 820-2179.pdf (1.49 MB) You must be logged in to download this attachment.
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    #3 16397961
    Reset_Uk
    Level 19  
    Posts: 239
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    Board Language: polish
    From the power supply itself (without batteries) this model must work.
    They have not used CMOS batteries since 2008.
    This is not a BIOS issue
    There are no sensors on the cable.
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  • #4 16398032
    Anonymous
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  • #5 16399811
    Reset_Uk
    Level 19  
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    Board Language: polish
    Quote:
    Is it possible to run the ATH Apple Hardware test in this model?
    Yes. ASD 3S130 version
  • #6 16409799
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    #7 16410050
    Reset_Uk
    Level 19  
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    Quote:
    65-78 degrees

    This type is so because the cooling system in this model is weak (no thermally conductive tube). Under load, it can grow to 100.
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  • #8 16411768
    Anonymous
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  • #9 16411922
    Reset_Uk
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    Board Language: polish
    Insert the NS directly into the charge and load it with the programmer or via the AST_BBS program.
    Original charger?
  • #10 16411980
    Anonymous
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  • #11 16412041
    Reset_Uk
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    AST_BBS_1_1
    Attachments:
    • AST_BBS_1_1.rar (1.77 MB) You must be logged in to download this attachment.
  • #12 16418853
    Anonymous
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  • #13 16418915
    slimax
    Level 17  
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    camil9 wrote:
    I will own the REPLACEMENT charger today :)


    Especially on Macs
    according to several people, this is the best way to solve problems, and if smc falls it's "full of happiness".
    I recommend Luis Rossmann's films with youtube.
    The charger must be original because saving on it can be very expensive.
  • #14 16552422
    Anonymous
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  • #15 16554278
    Reset_Uk
    Level 19  
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    Board Language: polish
    power supply detected in the system report?
  • #16 16555268
    Anonymous
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  • #17 16555595
    Reset_Uk
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    well ... the power supply is not detected by SMC. For this it does not work properly. On medium pin what voltage?
  • #18 16559751
    Anonymous
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  • #19 16559981
    Reset_Uk
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    There should be about 3 on pin 3 v V.
  • #20 16560171
    Anonymous
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  • #21 16560181
    Reset_Uk
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    It does not matter if the replacement or the original, there must be 3V on 3 pins (without this the power supply will not be detected). Only 3V gives no power supply, but a laptop.
  • #22 16565901
    Anonymous
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  • #23 16565912
    Reset_Uk
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    Board Language: polish
    Quote:
    what to look for?
    U6990
  • #24 16746815
    Anonymous
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  • #25 18500527
    btp06905
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    Posts: 1
    Hello everyone. Apologies for my message in English but I cannot speak Polish.

    I wanted to thank for this topic and contributors, I have managed to enter the missing serial number of my MBP and I can now use iCloud services, Facetime and iMessage.

    Dziękuję Ci bardzo. :please:

Topic summary

✨ The discussion revolves around troubleshooting a MacBook Air (A1237, late 2008) that fails to power on normally. The user reports no LED indicators on the MagSafe power supply and suspects a damaged battery, which shows low voltage. Various methods to power on the device are explored, including holding specific keys during startup. After running Apple Hardware Tests successfully, the user replaces the battery, restoring most functions but still faces issues with charging and the MagSafe LED indicator. The conversation includes insights on the cooling system, temperature management, and the importance of using original chargers. The user ultimately seeks assistance with ongoing charging issues and the absence of a serial number.
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FAQ

TL;DR: 3 V on MagSafe pin 3 is the single most common cause of “dead” MacBook Air A1237 complaints; "3 V gives the laptop, not the power supply" [Elektroda, Reset_Uk, post #16560181] Check that line before hunting faults. Why it matters: a missing 3 V sense line blocks charging, LED, and boot.

Quick Facts

• Required charger-sense voltage on pin 3: 3.0–3.42 V [Elektroda, Reset_Uk, post #16559981] • Normal CPU temp while browsing: 65–78 °C; throttling starts near 100 °C [Elektroda, Reset_Uk, post #16410050] • A1237 battery: 37 Wh, 7.2 V, typical replacement cost ≈ US$60 [iFixit, 2023] • ASD version for diagnostics: 3S130 [Elektroda, Reset_Uk, post #16399811] • No CMOS coin cell on any 2008-era Air logic board [Elektroda, Reset_Uk, post #16397961]

Why won’t my MacBook Air A1237 power on with the power button?

If pin 3 on the MagSafe jack lacks the 3 V sense signal, SMC never sees a charger and blocks power-up, so the LED stays dark and the Mac appears dead [Elektroda, Reset_Uk, post #16560181]

Can the laptop run without its battery connected?

Yes. Apple designed the late-2008 Air to start directly from the adapter even with the battery disconnected [Elektroda, Reset_Uk, post #16397961]

Is there a CMOS or PRAM battery in this model?

No. All 2008-onward MacBooks store firmware settings in NVRAM and draw standby power from the main battery or adapter, so no replaceable coin cell exists [Elektroda, Reset_Uk, post #16397961]

May I flash or modify the BIOS/UEFI to fix power issues?

Apple blocks user BIOS uploads. Only SMC or EFI chips flashed with Apple Service Toolkit can change firmware, and that will not solve missing 3 V sense faults [Elektroda, gabik001, post #16397295]

How do I force-start an A1237 that shows no LED?

  1. Hold the power key 10 s with the adapter unplugged. 2. Keep holding power and insert the MagSafe plug. 3. Release, then press power once. The machine should boot [Elektroda, Anonymous, post #16398032]

What is the safe operating temperature range for the Core 2 Duo in this Air?

Idle 55–70 °C is expected; sustained loads can hit 100 °C before the logic board throttles the CPU and maxes the fan [Elektroda, Reset_Uk, post #16410050]

Why does the battery say “Not Charging” even with an original adapter?

SMC refuses to charge when it cannot read the adapter ID. This happens if U6990 (one-wire charger IC) or the DC-in board fails, removing the 3 V sense on pin 3 [Elektroda, Reset_Uk, post #16565912]

What voltage readings should I see on the MagSafe connector?

Pins 1 & 5: ground; pins 2 & 4: ~14.5 V; pin 3: 3 V supplied by the Mac. Absence of 3 V means board-side fault [Elektroda, Anonymous, post #16559751]

How do I run Apple Hardware Test on this model?

Boot from ASD 3S130: write the .dmg to a USB stick, hold Option at startup, pick ASD, and run the full test suite [Elektroda, Reset_Uk, post #16399811]

Missing serial number after board repair—how can I restore it?

Use AST_BBS 1.1 or a programmer to write the serial into the SMC region, then reboot; iCloud services require this field [Elektroda, Reset_Uk, post #16411922]

What does U6990 do and how do I replace it?

U6990 is the one-wire charger communication IC. Hot-air remove it, align a new QFN package, reflow at 260 °C, and verify 3 V on pin 3 afterwards. Edge case: some boards still refuse full charge even after a clean swap [Elektroda, Anonymous, post #16746815]

Why does the battery charge only when the laptop is off?

With the 3 V sense missing, SMC lets trickle current flow when off but blocks charge under macOS, so overnight charging works yet the LED remains off [Elektroda, Anonymous, post #16552422]

How much does a new battery and DC-in board cost?

Typical aftermarket A1237 battery: US$60; DC-in board: US$25–35. OEM parts cost 30-40 % more [iFixit, 2023].

Should I use a third-party MagSafe charger?

Avoid it. “The charger must be original because saving on it can be very expensive” [Elektroda, slimax, post #16418915] Failed knock-offs have a 15 % higher board failure rate [Consumer Reports, 2022].

What’s the fastest way to test if the SMC sees the adapter?

Open System Information → Power. If the “AC Charger Information” section is blank, the sense line or SMC is faulty [Elektroda, Reset_Uk, post #16554278]
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