Terry S

iTrader: (2)

I – Introduction to the Active Center Differential (ACD)

II – Technical specifications of the ACDIII – Miscellaneous ACD informationIV – Frequently Asked Questions

V – Credits

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Current Articles:
S-AWC:
http://media.mitsubishicars.com/chan...a-1ef54b0dd201
Ralliart:
http://media.mitsubishicars.com/rele...4b06fc7e/print

Terry S

iTrader: (2)

First off, let me cover the reason why I believe this guide is needed. On many different Evolution websites, there are many questions asked regarding the ACD. Most are asked over and over with the quality of answer typically degrading for each new question.

This guide is my attempt to answer all current and future questions regarding the ACD. Additionally, all the information found in this guide has been obtained through an extensive search of multiple Internet resources. All information will be cited and given appropriate credit in Section V. I believe that all the information and pictures found in this guide is/are public and non-proprietary in nature since it/they available to anyone on the Internet.

If you would like to contribute to this guide or make corrections, please notify me and we’ll talk.

Please note that this guide is applicable to the USDM Evolution VIII (2005 model year) & IX (2006 model year) only. I live in the US and we don’t have the AYC here so it’s pointless for me to attempt to describe how things work that I don’t get to play with.

*Warning* If you do not like to read pages with technical heavy content, or anything not found in an off-topic forum, your best bet would be to skip directly to the FAQ Section (IV). This section covers the meat and potatoes of this guide without all the technical jargon.

So without further delay, let me begin.

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Terry S

iTrader: (2)

A. Historical overview

Mitsubishi’s Active Center Differential (ACD) was first employed in 2001 on the Evolution (Evo) VII model. The ACD was to be used in support of the Active Yaw Control (AYC) carried over from the previous Evo models which controls the yaw of the car using the difference in driving and breaking forces between the two rear wheels. (1)


When the Lancer Evolution VIII model was released in the US in the spring of 2003, the Evo was outfitted without the ACD or AYC found in other markets. Instead, it was outfitted with the Viscous Coupling Unit (VCU) which distributes the torque evenly (50:50) to the front and rear wheels. This VCU was also equipped on the 2004 model year Evo in both the standard and RS models. (2)

Beginning with the 2005 model year Evo VIII, Mitsubishi decided to outfit the car with the ACD unit instead of the VCU, yet still left the AYC unit out. Bastards. However, the ACD was available to all variants of the model: RS, Standard, and MR. (3)

In October 2005, the 2006 model year Evo IX was released to the US market. Again, as it was with the previous model year, the ACD unit was standard equipment on all variants: RS, IX, and MR. And again, the AYC was not included. Still bastards. (4)

B. What is the ACD?

*NOTE* If you are unfamiliar with how a Limited Slip Differential works, then please look that up first. Howstuffworks.com, answer.com, and even EvolutionM.net have many good descriptions and diagrams available. */NOTE*

The ACD is a bevel-gear type center differential with a front/rear torque distribution of 50:50. The differential is controlled by an electrically controlled hydraulic multi-plate clutch. For greater dependability, the clutches were made from steel. (1)

The differential itself is located inside the transmission housing while the ACD clutch pack is located inside the transfer-case. (https://www.evolutionm.net/forums/attachment.php?attachmentid=70736 Light green and dark blue sections in this PDF by SuperHatch (5))

The ACD ECU is located to the right side of the glove box, behind the passenger side kick-panel. (6)

The ACD hydraulic unit is housed in the engine compartment and regulates the hydraulic pressure of the multi-plate clutch within the range of 0 to 145 psi. The maximum limited-slip torque of the multi-plate clutch is about three times that of a conventional VCU. Translation: 3 times the grip transferred to the wheels that need torque. (3)

There are many different sensors used by the ACD when determining how it will control the center differential. ABS input, steering wheel angle, throttle opening, wheel speeds, and longitudinal and lateral movements of the vehicle are constantly measured by the ACD. Additionally, the driver can select how the ACD will perform by selecting one of three different modes from a switch in the cabin: Tarmac, Gravel, or Snow. (7) (1) (3)


When comparing the USDM version of the ACD to that of other markets Evo’s, the difference lies with the USDM ACD not connecting to an AYC control unit & hydraulic system since the car does not have them. (1)

C. How the ACD works

In short, the ACD unit hydraulically controls the limited-slip locking state of the center differential which ranges between full locked, full open, and every point in between these two states. It calculates what the optimum locking amount would be using real-time input from the previously mentioned sensors and which ACD mode is selected. The ACD is extremely fast in operation, bettering the performance of the standard VCU and Viscous units. (3) (7)

When the car is accelerating or decelerating rapidly, the ACD begins to engage the limited-slip locking on the differential. The harder you accelerate or decelerate, the more it locks. This is done to provide maximum lateral stability and wheel traction. (3) (8)

The ACD allows the differential to operate in more of a free state when steering movements are made. When turning, the wheels in the front have to travel a further distance than the wheels in the rear. This is because the wheels in the front travel using a larger turning radius than the rear wheels as you can see from this picture. (turning radius.gif) So in order for smooth turning, the center differential needs to become open to allow for the front set to turn at a different speed. If you were to leave the center differential locked, the wheels would skip and skid badly. (1) (3)


If the front wheels are spinning faster than the rear wheels, then the ACD begins to lock the clutches up. If the amount of force exerted by the front wheels is less than the amount the clutches resist, then the slip is stopped and both front and rear spin at the same speed. However, if the force the clutches can resist is less than the force exerted by the slipping set of wheels, then the wheels will slip, but only by how much remaining force the slipping wheels overpowered the clutches with. (1) (9)

D. How the Tarmac/Gravel/Snow switch influences the ACD

Despite popular belief, this switch DOES NOT change the torque split. The differential is geared at 50:50 and cannot be changed by the push of a button. What this switch actually does is quite simple. Each setting determines how long the ACD will delay in freeing the center differential after a steering input is made. In addition, it will determine how much locking force the ACD will exhibit on the clutch pack and center differential. (10) (11)

Tarmac is the setting to be used in dry, paved conditions. In this setting, the ACD will almost immediately allow the center differential to go into a free state upon detecting a steering input. Additionally, this mode provides the strongest limited-slip clamping force of the three modes. (7) (12)

Gravel is the setting designed for wet or loose roads. In this setting, the ACD will delay in freeing the center differential for a moment (possibly 2-3 seconds). Of the three modes, this one has the second strongest clamping force. (7) (12)

Snow is the setting designed for, you guessed it, snow. In this setting, the ACD will delay in freeing the center differential for much longer than the other two settings (possibly 4-5 seconds). Of the three modes, this one also provides the least amount of clamping power. (7) (12)

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Terry S

iTrader: (2)

A. Parking brake

As stated in the owners manual and in several places around the internet, when you engage the parking brake, the ACD is disengaged to prevent damage to your drivetrain and the ACD unit. The reason this would cause damage is because if you were to have an engaged ACD attempting to lock the center differential then you yank on the parking brake stopping the rear set of wheels from spinning, you are forced to attempt to overpower the ACD unit from keeping the rear wheels from spinning. This puts a tremendous amount of strain on the ACD clutches, the ACD hydraulics, center differential and your brake system. (7) (12)

B. ACD interference with aftermarket downpipes

Yes it is true that some aftermarket downpipes will not initially fit. This is typically because the manufacturer created the downpipe using either an 03 or 04 model for fitment. Since those model years did not contain the ACD unit, the transfercase was smaller and more compact. But starting with the 05 model year, the transfercase contained the clutch pack which widened the transfercase housing. Most manufacturers have either solved this problem or offer downpipes exclusive for the 05-06 model years. (13)

C. Upgrading the ACD

While it is possible to upgrade the ACD ECU for the Evo VII/VIII/IX in other markets, at the time of this writing, there are NO ECU UPGRADES available for the USDM version of the VIII or IX. Reason being is that the ACD unit found in other markets also doubles as the AYC control unit. Since we do not have the AYC in our version, the aftermarket units would probably freak-out and not work. I have found no evidence of someone trying to equip a USDM Evo with an aftermarket ECU however. (14) (15)

D. ACD unit fault – 3 solid lights

When your ACD unit has all three lights on at the same time (other than at startup), this means your car is reporting back to Mitsubishi headquarters that your warranty should be voided… Just kidding. As stated in your owners manual, this denotes some kind of error in the unit and it needs to be serviced. To service the ACD you need a very special, and very expensive tool from Mitsubishi. This tool is used to engage the ACD pump to bleed the lines. You may run into this issue if you are changing out your transfercase. (6)

E. Upper Intercooler Pipes

There are reports on the internet that some have had issues with installing intercoolers & upper intercooler piping. The issue lies with one of the ACD oil lines getting in the way. Most people report no issue or only a slight fitment issue solvable by slightly moving or bending the line out of the way. (16) (13)

F. Changing the front/rear torque split

There are rumors of a 35/65 center differential made by Cusco, but it does not appear to be available for the USDM market Evo’s. (15)

G. Strength of the ACD unit

The center differential itself appears to be similar to the version in the 03-04 model years, but it is connected to the ACD clutch pack instead of the VCU. Since there are hydraulics & electronics involved with the system, some have expressed concern that something is bound to show signs of weakness. At the time of this writing however, there have been no obvious weak points in the ACD system. (17)

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Terry S

iTrader: (2)

1) All of this is over my head. What can I do to not be so stupid?

- SEARCH! All of the answers I provide in this guide were found searching on EvolutionM.net, SoCalEvo.net, NorCalEvo.net, & LancerRegister.com in a few hours. Digesting it took a bit longer though. If the issue is that you don’t understand the basics or basic terminology, then I would suggest starting at howstuffworks.com or picking up a copy of Car Hacks & Mods for Dummies by David V.

2) What do the Tarmac/Gravel/Snow settings do?

- They determine how long and how strong the ACD will engage the limited-slip of the center differential before allowing to go into a “free state”. See section II – D

3) What is the torque split in each mode?

- 50:50. The center differential is geared at 50:50. The modes do not determine the torque split. See Section II - D

4) Can I change the torque split with a new Center Diff?

- Yes and no. As of the time that this guide was written, there are no center differentials that change the torque distribution for the USDM Evo’s. However, there is supposed to be one made by Cusco that works on other market Evo’s. See Section III - F

5) Is the ACD weaker than the non-ACD Evo’s?

- The center differential itself appears to be similar to the version in the 03-04 model years, but it is connected to the ACD clutch pack instead of the VCU. Since there are hydraulics & electronics involved with the system, some have expressed concern that something is bound to show signs of weakness. At the time of this writing however, there have been no obvious weak points in the ACD system. See Section III - G

6) Where is the ACD?

- The ACD clutch pack is located inside the transfercase, the center differential itself is located inside the transmission and the ECU is located to the right of the glove box. See Section II – B

7) What controls the ACD?

- The ACD ECU reads several sensors that control the limited slip action of the center differential. The Tarmac/Gravel/Snow button further determines is actions. See Section II – B

8) What models have the ACD?

- All 2005 Evo VIII’s and all 2006 Evo IX’s. See Section II – A

9) Is the ACD just a LSD (Limited Slip Differential)?

- Basically, yes. It’s a computer controlled LSD. See Section II – B

10) Can I put any downpipe I want on my ACD equipped Evo?

- No. Before you purchase one, ask the manufacturer or vendor if it clears an ACD equipped Evo. See Section III - B

11) Which downpipes CAN I put on my ACD equipped Evo?

- Many different kinds. Most manufacturers have fixed the downpipe fitment issues with ACD equipped Evo’s, but not all. Eventually this guide will have a list, but in the mean time, ask the vendor you buy from. See Section III - B

12) Do All Upper Intercooler Pipes work with ACD equipped Evo’s?

- Possibly. There have been reports of some UICP’s having minor fitment issues. Again, ask your vendor if it will fit. See Section III - E

13) Which UICP work with ACD equipped Evo’s?

- Just like the downpipes, this guide will eventually have a list, but in the mean time, just ask your vendor. See Section III - E

14) All of my ACD lights are on. What does this mean?

- It means your ACD is experiencing a fault and it needs to be serviced. Take it to a dealership. See Section III - D

15) Can I just service the ACD myself?

- The ACD requires a special tool from Mitsubishi to engage the ACD pump. This is a very expensive piece to purchase. For most people, it would be wiser to just take it to the dealership to service. See Section III - D

16) Why doesn’t my model come with the ACD?

- Because Mitsubishi didn’t want it to. Sucks for you. See Section II - A

17) Why didn't the USDM Evo's get equiped with the ACD & AYC like everyone else?

- It is believed that the reason our Evos don't come with the full AYC & ACD combo is because the AYC equipment goes into the same place that the US emissions regulations equipment goes. That's why the US VIII and IX will not see the full AYC/ACD package. The emissions equipment is all the stuff hanging next to the fuel tank under the driver's side of the car. See "Evaporative emission canister and fuel tank pressure relief valve" as shown on page 17-21 in the service manual. (18)


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Terry S

iTrader: (2)

V. Credits

(1) http://www.mitsubishi-motors.com/cor...001/13E_09.pdf - Mitsubishi Technical Article – 2001
(2) http://media.mitsubishicars.com/deta...56621&mime=ASC – Mitsubishi press release – Nov 4, 2003
(3) http://media.mitsubishicars.com/deta...37442&mime=ASC – Mitsubishi press release – Oct 18, 2004
(4) http://media.mitsubishicars.com/deta...28388&mime=ASC – Mitsubishi press release – Mar 24, 2005
(5) https://www.evolutionm.net/forums/at...chmentid=70736 – Post on EvolutionM.net by member SuperHatch – Jul 21, 2005
(6) https://www.evolutionm.net/forums/sh...&highlight=ACD – Post on EvolutionM.net by member Crazy4Cars – Apr 25, 2005 – Section II – B, 3rd paragraph
(7) http://www.btrprep.com/e7_qa.htm - FAQ for Evo 7 & 8 – date ?
(8) http://www.mitsubishi-motors.com/cor...nce/e/awc.html - Technology Section of the website – date ?
(9) http://auto.howstuffworks.com/differential.htm - How differentials work – date ?
(10) https://www.evolutionm.net/forums/sh...&highlight=acd – Member raywong - Nov 22, 2004
(11) http://www.geocities.com/alias_hery/MMC7.htm - Info on the ACD – date ?
(12) http://www.lancerregister.com/showth...&highlight=acd – Various Posts – May 8, 2003
(13) https://www.evolutionm.net/forums/ – Various posts, no specific posts cited – date ?
(14) http://www.takakaira.com/asp/templat...LlHiIidfEgcjFe – Aftermarket ACD ECU’s – date ?
(15) https://www.evolutionm.net/forums/sh...&highlight=ACD – Post by Adam @ Z1 – May 28, 2005
(16) https://www.evolutionm.net/forums/sh...&highlight=ACD – Various posts – Aug 29, 2005
(17) https://www.evolutionm.net/forums/sh...&highlight=ACD – Various posts – Dec 6, 2005
(18) https://www.evolutionm.net/forums/sh...84#post2644484 - Posts by Ludikraut - December 20, 2005


Also, I would like to thank Superhatch for giving this info the once over and making sure i'm not messing up in any serious way

At this time, I am looking for some serious technical information regarding the ACD, pictures from the service manual for the 05's and 06's specifically regarding the ACD, and even part numbers (and prices!) for all the pieces. If you are interested in helping out, PLEASE let me know

And on a final note, if you find anything in here that is wrong, and you can prove it, just shoot me a message and i'll correct the mistakes.

Thanks and enjoy.

Terry S

Blue Evo 8

iTrader: (40)

I'll have to read this a bit later tonigt

Killboy

iTrader: (2)

Wow, killer info. Thanks!

*EDIT* In section IV (FAQs) point 8, shouldn't it include MR editions from previous years also?

evo 8 ya

iTrader: (31)

holy sh*t this should be a sticky

Ultimate CC

iTrader: (122)

This is def sticky material, great right up

BrianGT

iTrader: (1)

Great article, thanks for the explanations.

--
Brian

keevo54

iTrader: (2)

Very good aticle. It answered a lot of questions

Randy

iTrader: (14)

Killer info this should be a sticky!

SuperHatch

iTrader: (23)

Mods, we should sticky this.... or incorporate it into the drivetrain for dummies thread if Terry desires.

- Steve

voidhawk

iTrader: (20)

very nice .