This particular PowerBook G4 laptop had the following issue: Under normal operation conditions, word processing, typical web browsing, the computer didn't suddenly go to sleep. However, when performing tasks requiring high levels of CPU usage such as Adobe Flash videos, the laptop CPU fan starts up on high and eventually goes to sleep.
Some of the solutions on the Internet called for the removal of possibly a faulty heat sensor or paying the flat rate to get the sensor replaced. Personally, I'm not willing to spend the money for that kind of thing especially if the problem is due to poor hardware design.
To diagnose the issue as to why the PowerBook G4 laptop was going to sleep, I used a Temperature Monitor application to show me exactly what was going on with the internal temperature of the system. The Temperature Monitor showed all the system components were below the threshold, but once I got Adobe Flash videos, the CPU topside temperature climbed from 40 C and hovered between 77-79 C, where 79 C was the upper threshold for the CPU topside.
I opened up the PowerBook G4 (a guide on how to open a PowerBook G4 can be found here) and noticed a blue heat distribution pad that pulls heat from a small part of the heat sink to the top cover of the notebook underneath the keyboard. As word of warning, taking apart an Apple PowerBook G4 is the worst experience in notebook computer disassembly. It's poorly designed for serviceability. The screws are all different and you have to keep track of where they go otherwise, you'll end up using the wrong screw in the wrong place. Keep a log of where all your screws came from so that you remember where all those damn screws go when you reassemble. Luckily, you don't have to pull the laptop completely apart, otherwise, you'll spend a whole day disassembling and reassembling the computer as it's layered and you have to take completely apart to get to some components.
Below, I've highlighted the area of the heat sink that the heat distribution pad touched with a red circle below. I figured that perhaps the heat distribution pad wasn't pulling enough heat away from the heat sink. I don't really know where to buy those heat distribution pads, and I only have thermal paste, so I decided to use aluminium foil folded up in the shape of the heat sink as a heat conductor between the heatsink and the top cover. It's important to make sure the aluminum foil is thick enough so that there's enough friction between top cover and the heat sink so that it doesn't move around because the last thing you'd want is for the aluminium foil to move away from the heat sink touch an electrical component and short it. I think I'll go back in and glue it down. Not sure what to use though because I need glue that will conduct heat and also not breakdown because of the heat.
Once I got my poor man's heat distribution pad in place, I started the Apple PowerBook G4 and maxed out the CPU by starting up Adobe Flash, a movie, and mp3's all at once to see what would happen to the CPU temperature. The good news is that the temperature of the CPU topside stayed below 70 C and the narcolepsy has stopped. What you will notice is that the left side of the top of the laptop underneath the keyboard and the palm rest will get warmer, but that's not an issue and is a result of the aluminum foil redistributing the heat from the heat sink to the aluminium top cover.
Update 2011-06-13:
If you don't like the idea of using a slab of aluminium, I've discovered what that blue pad is on the top cover assembly. It's a heatsink thermal pad. Sorry I can't tell you what the size of the pad is because I don't have access to the laptop and I didn't measure it before had. You can find this pad on eBay. Look for heatsink thermal pad. I've not used this before, so I don't know what the thermal conductivity of it is and I don't know how well they work. I'm posting this for those of you that prefer to do a professional job instead of my make makeshift solution. Don't judge me! I didn't know about it when I was trying to initially fix the laptop.
Good Luck.
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