Fan goes nuts everytime I turn it on...

I have two A1342 Macbooks. One has a fan that spins up like crazy everytime I turn it on and it never shuts off. That one is otherwise normal. The other is fully functional with no issues.

The fan thing was really bothering me. I tried swapping the fans between the two macbooks and that didn't help. I even tried swapping logic boards, superdrives, batteries... everything.

No matter what combination of parts I use, the one has a fan that spins like crazy and the other functions normally. How is this possible?

I'm pretty much stumped and any insight would appreciated.

Answered! View the answer I have this problem too

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Try this, download Temperature Gauge Pro it will tell you what the heat sensors and fans are seeing and doing. Paste a screen snapshot here for us to see whats happening.

I suspect the issue is with your heat sink but without any diagnostics its hard to say for sure.

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It's definitely not overheating. I tried two logic boards and each with two separate fans in two separate top cases. All combinations resulted in the same... one top case configuration has the fan spinning fast and the other does not. It doesn't matter which logic board is in which case.

I've been messing with them all day trying to figure it out. I'm starting to think there must be some kind of temp sensor in the top case that is blown in one top case and not in the other...

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TG will help in isolating which sensor. Please give it a try otherwise you're just groping in the dark.

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There is a temp sensor on the track pad that sometimes goes bad try replacing it.

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Yes I agree. When you power on the logic board without the trackpad attached the fan runs full speed. Try to swap the trackpad from the good machine to the faulty one.

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Run AHT - you have a hardware failure, could be a blown sensor, failing fan, or overheating CPU. AHT should help you narrow the field of possibilities.

By default when EFI detects any of the aforementioned hardware problems the fans switch into high speed to keep you from cooking (destroying) your CPU.

If this answer is acceptable please remember to return and mark it.

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Thanks. I tried all the hardware (logic board, fans, etc.) in two separate top cases. The same logic boards are both functioning correctly in one top case and both function incorrectly in the other top case.

I thought about overheating as well but I felt the air coming out of the fan and it was not hot at all. So I don't think it's that.

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AFAIK Temp sensors are on the logic board (should be moving from case to case), the top case contains no temperature sensors. Could one case be shorting a spot on the logic board? (Burr or long screw in a short hole?)

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Mike will be eternally grateful.
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