I've been researching this for a while, and I think I have a very good answer. Remember those disassembly images from Kodawarisan? I decided to look those over and this is what I found:
Here's the motherboard of the iMac G5 (iSight):
Here's the motherboard of the white Intel iMacs:
It's the exact same thing, but with an Intel chip insted of a PowerPC G5. Notice that the GPU (center) is cooled by a simple passive heat sink. Both systems are separate.
Now lets move into the Al iMac motherboard:
This time, Apple decided to give the GPU (the top one) some more cooling by giving it a heat pipe and a fan.
But the problem is that a single fan cools both the CPU and GPU. The fan also blows through the CPU heat sink before the GPU one:
That means that the air that the GPU is being cooled by has already been "used" by the CPU. Small amounts of heat might even be transferring from the CPU and into the GPU.
This leads me to believe that the Al iMacs that are crashing are overheating. That explains why boosting the CPU fan speed with a fan controller fixes freezing problems on some systems. The air has less time to be heated by the CPU and so the GPU isn't "cooked," at least not as much.
But if this is the case, why aren't all of the Al iMacs doing this? Is there a variable that affects this? I blame it on thermal paste. Apple most likely built the the preproduction units with a ton of care, making sure that the right amount of paste was applied. And when the right amount it used, this cooling system works flawlessly. That's why this was never caught during testing.
Remember the problem some early Macbook Pros were having? During the manufacturing process to much paste was being applied. When too much is applied, the paste acts less like a heat transferee and more like a heat vacuum, causing components to get warmer and not helping them stay cool.
Perhaps when too much paste is applied:
It's like a cascade effect. But the real problem is that over time this could damage the video chip, making the problem worse.
(Sorry if this is filled with a bunch of grammar errors and typos. It's late at night and I'll try to fix these later on when I have time.)
Here's the motherboard of the iMac G5 (iSight):
Here's the motherboard of the white Intel iMacs:
It's the exact same thing, but with an Intel chip insted of a PowerPC G5. Notice that the GPU (center) is cooled by a simple passive heat sink. Both systems are separate.
Now lets move into the Al iMac motherboard:
This time, Apple decided to give the GPU (the top one) some more cooling by giving it a heat pipe and a fan.
But the problem is that a single fan cools both the CPU and GPU. The fan also blows through the CPU heat sink before the GPU one:
That means that the air that the GPU is being cooled by has already been "used" by the CPU. Small amounts of heat might even be transferring from the CPU and into the GPU.
This leads me to believe that the Al iMacs that are crashing are overheating. That explains why boosting the CPU fan speed with a fan controller fixes freezing problems on some systems. The air has less time to be heated by the CPU and so the GPU isn't "cooked," at least not as much.
But if this is the case, why aren't all of the Al iMacs doing this? Is there a variable that affects this? I blame it on thermal paste. Apple most likely built the the preproduction units with a ton of care, making sure that the right amount of paste was applied. And when the right amount it used, this cooling system works flawlessly. That's why this was never caught during testing.
Remember the problem some early Macbook Pros were having? During the manufacturing process to much paste was being applied. When too much is applied, the paste acts less like a heat transferee and more like a heat vacuum, causing components to get warmer and not helping them stay cool.
Perhaps when too much paste is applied:
- The CPU gets warmer then it should which...
- Causes it to output more heat which...
- Causes the GPU to collect more heat from the CPU which...
- Causes the extra paste that is also on the GPU to heat it more then ever intended which...
- Causes a crash
It's like a cascade effect. But the real problem is that over time this could damage the video chip, making the problem worse.
(Sorry if this is filled with a bunch of grammar errors and typos. It's late at night and I'll try to fix these later on when I have time.)