This Power Mac G4 has Mirrored Drive Doors.

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G4 MDD freezes when CPU gets hot - why?

From a cold start the CPU (Dual 1.25GHz) temperature rises until it reaches over 50°C in about 40 minutes, when the G4 freezes. The HD remp also rises steadily to 35°C. I have to press the start button to shut it down.

I have cleaned all dust and fluff out of the machine, checked that all fans are working and replaced the heat sink grease and still it happens.

If I open the side door and use a small external fan to blow cool air over the heat sink the G4 operates normally for hours with the CPU temperature at a steady 35°C and the HD at a steady 26°C.

The technician (Apple Genius) at the local Apple Shop said they did not have the gear to check everything on such an old model

Mac, but all the sensors checked out OK and he thought the problem may be an ageing CPU. What do you think?

Roy Clark

rclark@bigpond.net.au

Sydney, Australia

Answered! View the answer I have this problem too

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I've got one like this that was getting to warm because of a high end video card and Sonnet Tango PCI card for SATA drives, and I mounted an internal fan that cured the problem. I'll take a couple of photos and upload them tomorrow along with how I wired it.

BoldUPDATE

Here's the solution that worked for me. Although the new fan does not vent to the outside, it increases circulation within the machine and cooled it. It's just held in place with velcro. Click on the photos to enlarge:

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Please replace the thermal paste before spending any more time or money. Here's a guide on applying it: How to Apply Thermal Paste

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I may have solved my overheating problem in my G4 MDD. Taking a clue from Richard Mayer (see above) I added a new fan, almost identical to the old one in the G4. Initially it did not seem to make much difference. It still got too hot and ceased up, even after I tried several positions for the extra fan. Maybe the two fans were competing in some ways because it was difficult to position the 2nd fan so as to not compete partially with the old, original one.

Then one of my sons, Philip, who learnt basic computing on an old G3 of mine I gave him, and then went onto become a bit of a whizz on computers, Windows as well as Macs, suggested I try putting the new fan on the back of the G4 so it would suck instead of blow. This seems to be doing the trick. I positioned the new fan so as to be directly behind the fins of the heat sink. I have now had the G4 running for over 90 minutes with the temperatures of the CPU and the HD to reach a steady state of 48°C and 31°C respectively. The two fans are now working together, one blowing and the other sucking.

Main problem is the new fan is rather too noisy, but I guess I will have to put up with it if i want to continue using the G4.

Thanks to Richard for all the helpful suggestions.

Roy Clark

rclark@bigpond.net.au

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Try taking the heatsink off the 7450 processor, very carefully clean the white heatsink grease off both the processor and the heatsink, and apply a good silver-containing heatsink compound to both surfaces. Be verry careful in applying the compound, as too much can cause an electrical fault.

The idea here is to increase the processor's thermal coupling to its heat sink, allowing it to run cooler.

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The MDD is actually designed to shut down as it approaches the overheat stage. There is a temp sense circuit that controls this. Unfortunately, it doesn't give any on screen warning, nor does it do a graceful shut down. It simply dies, as if you pulled the plug. Obviously that can create problems.

As you and most of the posts here have indicated, the machine is also extremely prone to getting too hot.

The engineers did a VERY poor job of managing heat generation and airflow with the MDD. Too many components, in too small of a space, with too little venting.

They allowed their desire for a "coolness" factor to overtake engineering robustness and the machine obviously did not get enough testing before going into production, and the "fancy cool vents" on the bottom of the front are a joke, and only inhibit air flow instead of helping it.

And this doesn't even mention the loud original Wind Tunnel P.S.

The airflow, what little there is, and the geometry of the items inside the case also results in a massive buildup of dust, in a relatively small period of time, depending on the quality of air cleanliness in your house.

I like the idea of redoing the thermal grease. After this many years, most of these cpu chips probably have very dried out, thus ineffective original paste.

As an aside, I will mention that a couple years ago I got tired of opening the door and finding the whole insides sludged up with dust, and also with the van noise (I never got the replacement fan).

So I ended up totally opening and discarding ALL of the original housing, except for the sheet the logic board is mounted too. I now run the entire system with the main board (with alum backing) laying on my desk, and with all the drives and opticals just laying around on the desk. I have three extra fans I placed on and around the components, especially a large one on top of the main heatsink, and a smaller one for the hard drive.

I use SCR based lamp dimmer rheostats to adjust each of the fan speeds.

I blow the heck out of it every week just to keep all the pieces as clean and "non dust covered" as possible.

The whole machine runs very cool now (I use Temperature Monitor).

Unless I forget to turn all the fans on before I start the machine up ;)

-douglas

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rclark will be eternally grateful.
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