Mac Pro Northbridge Chip Runs Too Hot
How to reduce the Northbridge temperature from over 80°C ( 176°F) to 45°C (113°F)
by MIKX (in Japan)
( I take no responsibility for any damaged caused by performing this modification. )
I have a Sept. 2007 Dual Quad Core 3Ghz Mac Pro with an ATI X1900XT 512mb, 9gig of RAM.
On the first day I powered up my Mac Pro I installed Hardware Monitorand was horrifed to see that the Northbridge chip was reading 80°C (176°F). This struck me as ridiculously hot so I decided to finhd a solution to this.
(NOTE: All temperature readings were done with the Mac pro at idle.)
Step 1. Install smcFanControl
I downloaded and installed a wonderful little free program called smcFanControl (make a donation) which allows you to control fans speeds in Mac Pros. I ramped the fan speeds up by 10%
The result : Within 30 mins I was seeing the Northbridge reading 63°C (145°F) in HardwareMonitor with a room temperature of 20°C (68°F) . This was a vast improvement but I still saw room for more reduction so I had a think about the Apple installed heatsink which sits on the Northbridge chip.
The heatsink is held onto the Northbridge chip by a spring clip, it is a minimalist effort on Apple's part compared to some of the glorious heatsinks available on the market at the moment. This heatsink can be easily removed using (non-conductive) fingers with a slight twist to the right.
Apple Installed Northbridge Heatsink from the Mac Pro.
Step 2. Replace Apple Installed Thermal grease.
One day later I decided to replace the Apple installed thermal grease on the Northbridge heat sink with Arctic Thermal 5 Grease .
Note: If you intend to go ahead with this mod then please consider buying the following :
Use lint free cloth to clean the chip and heatsink contact surfaces and then using a separate piece of lint free cloth use the Purifier to prepare the surfaces. Examine the surfaces for any smears, streaks or whatever. The goal is to end up with a mirror surface.
So I went ahead, removed cleaned and prepared the heatsink and Northbridge contact surfaces and then applied a "largish, grain of rice -sized" dab of Arctic thermal grease. NOTE : If you use too much thermal grease you riisk shorting out the Northbridge chip.
The result : After a re-boot with the same room temperature and with the the Mac Pro having been on for 30 minutes Hardware Monitor showed the northbridge at 58°C (136°F) . That is more like it but still not good enough.
Step 3. Replace the apple Heatsink and the X1900XT fan array.
One more day later and I took the plunge and splurged (¥1,400) on a beautifully made copper Northbridge heatsink.
I also bought a ThermalTake AT1 alternate fan/heatsink arrayfor the ATI X1900XTfor around for around ¥4,000.
Note: Make sure that the heatsink you buy has the same sized spring clip. 
I faced two problems with this step.
(Problem A) The copper heatsink fits like a glove and it's spring clip slots right in but it stands too high with the Thermaltake AT1 installed.
(Solution A) I ground away about 25% of the Northbridge copper heatsink's vane area. Don't let this worry you because this type of heatsink is almost fractal in nature compared to the feeble bumps on the Apple installed heatsink. OK, now both the new copper heatsink with Arctic Thermal grease installed and the ThermalTake AT1 are installed happily together.
(Problem B) The ThermalTake AT1 uses a Molex male & female (two way) connector. The nearest molex connection for 12V is way up in the Mac Pro's drive bays.
(Solution B) There are two solutions to this.
1. Remove the hard drive from Bay 1 and remove the two screw that hold the SATA II socket. You can now run a long Molex cable with a "Y" connector at the drive bay end down to the ThermalTake AT1 without having to cut or solder any wires.
2. Next to the socket that suplies power to the X1900XT is an identical, spare 6 pin PCIe power socket! This will happily supply power to the Thermaltake AT1 fan and it is very close to the graphics card.
I have yet to find a 6 pin PCIe to MOLEX (male or female) cable but I haven't really tried that hard yet. Either a cable or even better, if I find the correct socket; it is quite small and there is a slightly larger version of this socket that will fool you if you are not vigilent. I will take a photo of the one socket I have and use my finger as a size reference in the pic for when I go hunting.
The result . . .
With the same room temperature as in the previous two steps, the Mac Pro has been turned on for 30 minutes, and with the new Copper Northbridge heatsink (with Arctic Thermal Grease) and with the Thermaltake AT1 installed Hardware monitor shows 45°C (113°F).
This is a reduction of 35°C (95°F) at idle.
The ThermalTake AT1 is an inspired device. Its intake fan just happens to sit right above the new Copper Northbridge heatsink - it is sucking in some of the hot air and throwing it straight out the back of the Mac Pro. The Thermaltake AT1 is so well designed that this little bit of extra hot air doesn't affect the operation of the ATI X1900XT at all.
Here is how my Mac Pro looks three days later . . .
As you can see, the new Northbridge copper heatsink fit perfectly. You can see where I ground away the area of the vane array that was blocking the ThermakTake AT1.
Here both the new heatsink and the Thermaltake AT1 together.
Incidentally, while I was fooling around inside the Mac Pro and listening to Bryan Ferry (I am old) I decided to try to strip my Mac Pro down to the motherboard for a good de-dusting.
I live in a particularly dusty part of Japan and have long been in the habit of pulling my old G5 apart for this rite but I was shocked at how much dust had accumulated in this new Mac Pro, especially around the front CPU heat exchanger vanes; they were almost completely clogged with thick clumps of greasy dust. I am glad I took the plunge in stripping out the ram cage, rear fan, front fans assembly and CPU heatsink. With the new Northbridge heatsink, the ThermalTake and the de-dusting my Mac pro is silent as a grave and the Northbridge still reads 45°C as I type.
Stripping A Mac Pro
Stripping a Mac Pro to this level is not hard (the second time) but a little challenging. Here is how to do it.
Have a long hard look at THIS VIDEO on YouTube. It is in German and as far as I can tell they are working on one of the first Mac Pros.
I noticed some differences in my Mac Pro stripdown.
1. After you remove the four screws in the RAM bays area, reach down and detach the rear fan power cable (gently)
2. Now pull the rear fan into the RAM bay area to make room to allow the RAM cage out.
3. Now grab the whole RAM bay, fan and all and twist it upwards towards where the graphics card usually sits.
4. CPU Heatsink : Reach down behind it, grab it and pull up and back.
5. Front fans assembly : They still, like the G5, slot into a rail. On my Sept 2007 Mac Pro there are TWO screws to be removed; One is down near where the BlueTooth module socket is. The other (not shown in the video) is next to the CPU heatsink and cunningly hidden under it. The German video doesn't show the second screw. Remove these two screws, pull the top end of the fan assembly away from the latch on the front case edge.
6. Now, wiggle the fan assembly up and out.
Thats all there is to it. The second time took me about 5 minutes tops.
NOTE: When it comes time to re-assemble, install the CPU heatsick cover last, it snaps in easy as pie.
WARNING
DO NOT OVER TIGHTEN THE TWO LITTLE SCREW IN THE EDGE OF THE RAM BAY ASSEMBLY.
If you do the standoffs may lose their grip and you will not be able to tighten them.
I hope my little modification is useful and helpful to Mac Pro owners everywhere.
MIKX (in Japan)
