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MDD G4 Cooling ModsReturn to News Page

MDD Cooling & Silencing by using active cooling
By: Lukas Haemmerle
Posted: May 29th, 2003

Disclaimer/Warning: Although the modifications that are described here are not very risky, Apple usually voids any warranty if they see mods to the system like this when its taken in for repair and your computer could get damaged either by the modification itself or the longtherm effect of the modification. So if you do this modification, you assume all risks/responsibility yourself.

If you've been using a Powermac G4 MDD ("Mirrored Drive Door") - the ones with an aluminum heatsink - you definitely have noticed that they are quite powerful but noisy beasts. They were the first computers by Apple that suffered from a severe noise problem (not quite right, in fact the MDD-users suffered). That's why Apple "generously" decided to offer a silence kit that would take care of some noise problems of the MDDs and that would correct their poor (noise) ingeneering. The kit does an amazing job and working with an MDD has become acceptable again, but one issue that will not be solved with this kit is that the huge CPU fan cycles and so still can get loud and disturbing. That the fan changes it's speed - and therefore its noise level- in proportion to the heat he has to blow take away from the heatsink is of course not really an engineering mistake, in fact the variable CPU-Fan generally helps reducing the noise of a computer. But I personally like fans that run at constant speed, so that you won't notice them after a while. Moreover the CPU fan tends to run at higher speed after some minutes of use. This higher speeds are still acceptable but in my opinion avoidable.

Of course you now could buy the verax silencing kit for about 250 Euro and have a nearly unhearable MDD. Your MDD will in fact be so silent that you probably get annoyed by the noise of your harddrive. So next you will replace your harddrive and then the optical drive fan and then the cd-drive and so on ;-)

But most people will look for a more affordable, easier solution that is not quite as effective but still makes your MDD a bit more silent while not voiding the warranty.

As I explained above, working with a silenced MDD is quite accebtable but the cycling of the CPU fan still is annoying. Moreover the CPU fan in your MDD will accelerate after some minutes of use and run at a higher speed, causing more noise. Under OS 9 it could cycle even more because the CPU fan isn't regulated as good as under OS X. So Primary goal of this modification therefore is to prevent the CPU fan from cycling up and down and attract your attention. Secondary goals are that the warranty for the modified MDD should not be voided and that the solution is more or less affordable. Moreover - while mostly (see Results) running at constant speed - the CPU fan still should be variable and therefore be able react on.

Requirements and limitations
With this solution you're able to use only three of four available harddisk bays, namely you cannot use the bay that is nearest to the CPU. That means that you can use only one of the two ATA 100 IDE drive and both of the two ATA 66 IDE drives in your MDD.
You can't do this modification with the MDD's that have a copper heatsink since you can't attach additional fans to the cupper heatsink in the way it is described below.
I'm not sure what the effect of this modification may be if you apply it to a MDD that occupies all of the three harddrive bays, all the memory and pci-slots and that is attached to a cinema display. In such a case the heat produced of the power supply and the components will be much larger what could be dangerous to the cooling of the CPUs.

In my MDD I use two harddrives (one in the ATA 100 bay), no PCI card (except the Radeon 8500 AGP graphics card) and twice 512MB DDR Ram. The following modification works in this setup without any problems but also without the noise :-)

What Description Costs
Apple MDD Fan Replacement Kit
High quality Papst fan and a new power supply from the Power Mac G4 Power Supply Exchange Program.
Available till June 2003. You definitely should participate in this program! You get a high quality Papst fan and a whole new power supply unit for 20 bucks plus you can keep your old power supply unit and the huge CPU delta fan!
20 Euro
2 silent 60x60x25mm fans with about 20 cfm running at constant speed.
Or you can save some money if you use your noisy delta fans and run them at lower speeds with 5V or 7V. Read below.
0/20 Euro
2 x 4 x Tiger Elastic Fixation:
Alternatives for metal screws that you use to attach the fans to the aluminum heat sink.
5 Euro
5.25" -> 5.25" + Molex:
There are three versions available. Molex connector with 5V, 7V or 12V. If you choose to use your old delta fans instead of new 20 cfm fans you may buy a Molex 5V or 7V version. But in case you buy some new fans that are enough silent (below 30dB I would suggest), you could go for the Molex 7V or 12V version. You will need only the two male connectors of this adapter, but dont cut off the other two, the may become useful somewhere.
2.50 Euro
Y-Molex (Molex -> 2 x Molex):
Since you will use two fans in this manual, you need something to supply them both with current.
2.50 Euro
This one is optional (got this for free with my order :)
2 Euro

I bought all these things at Cosh Computersystems in Germany since they are specialized for modding stuff and since I don't know of any other shop with such a large assortment. I don't want to make advertisement for them but I definetely can recommend this shop. Shipping costs for Switzerland are about 8 Euro.

Modification Instructions

  1. Install the Apple Fan Replacement Kit as described in their installing manual.
  2. Realize that the following modification could damage your MDD and that the author won't take any responsibility in such a case .
  3. Shut down your MDD
  4. Touch the metal case of your MDD to discharge yourself.
  5. Unplug your MDD power cable. Don't walk around from now on to prevent electrostatically charging yourself and probably damaging your MDD.
  6. Take your 8 Tiger Elastic Fixations and stick them into the holes of the two fans. Stick them in the holes on the side where the air gets blown out. This usually is the side where the label of the fan is attached to. The output of this operation should look like below.

  7. Now press the fans with the Tiger Elastic Fixation on the aluminum heat sink. You must place the two fans close to each other and align them with the case of the harddrive bay. If you close the case of your MDD the fans should inherit the place of the second harddrive. You should ensure that the two fans don't touch each other because that could cause vibrations what again causes a disturbing sound.

  8. If the fans are in the right place connect the the wires of the fan to the Y-molex cable.

  9. Connect the end of the Y-molex cable to the male connector of the 5.25" -> molex adapter

  10. Connect the 5.25" connector to the second 5.25" connector from the harddrive cabling.

  11. Put the spiral bound arount the cables.
  12. Put the cables in such a way so that the don't get damaged if you close the case.

  13. Close the door of your MDD. If you placed your fans correctly there should be no resistance.
  14. Start your MDD and check if the fans are running properly by opening the door again. You will notice that (even) if the door is open the CPU fan won't accelerate. It definitely would if you hadn't the two fans placed on the heatsink.

You should check that there is no metal wire that touches the metal case of your MDD. Unless you create the wires for this modification yourself this is not really a problem since the bought cables are all properly isolated.


Because of the (now) active cooling of the CPUs the variable CPU fan (Papst) will always run at its lowest speed since it is not really needed anymore (or only in extrem situations where the environment temperature is quite hot). This means your MDD is as silent as it can be in that state since, the fan won't cycle anymore and you don't have voided your hardware warranty. The CPU fan ran only once not at the lowest speed since the time I made this modification. That was some weeks ago when it was really sunny and hot (more then 30 degree Celsius), but I have to say that the 20 cfm fans I use run at 7V and not at 12V altough it hardly would make a difference in noise I suppose. Before I made this modification my fear was that the air coming from the heatsink may not be blown properly out of the case and stay in there because of the additional fans on the heatsink that could blow the hot air back against the CPU fan. You would notice this effect if the variable CPU fan would accelerate and somewhen run at its highest speed since the hot air inside the case could not sufficiently cool the two CPUs. But this is not the case for my MDD and therefore I suppose that the modication does not prevent the air from leaving the MDD case.

Summa sumarum I personally consider this quite an easy, effective and elegant solution, but as mentioned above: I don't know (yet) how this solution will work in other setups.

Alternatives and Extensions
Haven't tried these (yet). But if somebody does, please let me know.

  • Use one 80mm fan instead of two 60mm fans:
    One guy posted a similar solution to the g4-noise mailinglist where he suggested to use an 80mm fan that he attached directly to the harddrive bay with isolation tape. 80mm fans can push more air through and are generally more silent. Problem is that the harddrive bay is about 80mm wide.
  • Turn around the 120mm CPU fun so that it blows the air to the same direction as the two 60mm fans on the heatsink (the blow on two sides, out or the back end and out of the front of your MDD).
  • Replace the 120mm CPU fan with a more silent fan.

Other MDD Cooling/Mods and Systems Related Articles:

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