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S-AWC System For discussion of the S-AWC system and it's components including (but not limited to) driveshafts, differentials, programming, etc.

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Old 07-14-2011, 04:10 PM   #1
discogodfather
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AYC / ACD System Explained SERVICE and BLEED Alternative

This is an edited post to reflect the consensus information that was uncovered by this thread. Various contributors from around the world shed light on different parts of the system and how they worked and what to do and when. If you read this thread, it will get deeply into how the system works.

The AYC (active yaw control) and ACD (active center differential) system in the Evolution X is very complicated mechanically and there has been much interest in how to service the system. The USDM market has not seen the combined ACD/AYC system until the Evolution X. the factory service manuals from 2008 and 2010 versions are very poorly written and confusing. They have the correct info, just not very well stated and more than misleading.

There are three main parts of the system:

1) The Transfer Case. This houses the ACD clutch pak and the ACD activation ring piston circuit. It also houses the front differential and parts of the center differential (actual center diff is in Transaxle on both 5 speed and SST).

2) The Rear Differential. This houses the AYC clutch paks (there are two, one for the left wheel and one for the right wheel) and the AYC activation ring piston circuits (there are two as well). It also houses the rear differential.

3) The ACD / AYC Pump. This is in the rear passengers side fender / bumper. This pump produces hydraulic pressure to active the AYC and ACD circuits, which can be as high as 150 PSI. It also has an accumulator to store pressure (around 300 PSI). This pump is connected to the reservoir in the trunk.

Much of the debate in this thread focused on fluids and where they go and what they do. Both gear oil (Diaqueen LSD or equivalent) and ATF fluid (Diaqueen SP3 or equivalent) is used in the system. The consensus is summed up in these diagrams:





Some consensus info:

1) You NEVER need to change, drain, bleed, or otherwise service the fluid in the ACD/AYC pump circuit UNLESS lines where removed, the differentials where replaced, or the system experiences catastrophic failure. Just keep it in between the levels in the reservoir in the trunk. Bleeding the system requires the Mut3 factory tool and costs $$$ at the dealer, the procedure is a real PITA. Evoscan may support this in the future.

2) The fluid in the AYC / ACD pump circuit (fed by the reservoir in the trunk) is ISOLATED completely from the ATF fluid in the rear differential (clutch paks) and the gear oil in the transfer case (both in the clutch pak and the rest of the case). The fluids should NEVER mix.

3) A big thing that throws most people into confusion is that the ACD clutch pak sits in gear oil, and the AYC clutch paks sit in ATF fluid. Neither is pressurized. The fluids just sit there and lube the clutches, there is an air pocket at the top in ALL CLUTCH PAKS.

4) The ACD / AYC Pump (called the "Hydraulic Unit" in the service manual) uses high pressure to activate the ACD and AYC clutches.

5) If you notice the level of fluid fluctuate slightly in the rear reservoir for the ACD / AYC Pump circuits, it's normal. If it becomes completely filled and overflows or it runs completely out of fluid there is a problem, at this point the system will most likely throw codes and warnings.

6) Change your gear oil in the Tranny Case every 30k miles for normal use. Change it every 15k miles for severe conditions and track use.

7) Change your ATF fluid and gear oil in the Rear Differential every every 30k miles for normal use. Change it every 15k miles for severe conditions and track use.

8) Various problems plague the ACD / AYC pump (Hydraulic Unit) but most have to do with something called "Galvanic Corrosion". This occurs when two dissimilar metals with a fluid layer in between them form corrosion. There is no real solution to this problem in terms of preventative maintenance except changing the fluid in the Transfer Case and Rear Differential (gear oil and ATF) more often. DO NOT service/change/drain the fluid in the AYC / ACD pump in terms of preventative maintenance. Keep it at the correct level and make sure it's not brown and smelly (indicates BIG failure) and your good.
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"The evo is more car to take me to my job in action mode have more space for my childrens have SST and brembo for my wife and recaro seat for me is the perfect family car in this time (is more balance) is evolution to the future the subi is the same line from the past in other boby for teneger. Evo dad car."


Last edited by discogodfather; 09-03-2011 at 09:35 PM. Reason: Updated INFO
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Old 07-14-2011, 07:55 PM   #2
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Great post! Thanks for the information. In my 'research', I found out that they guys at CBRD and South Coast Mitsubishi do NOT bleed the system when doing a fluid drain/fill of the rear diff AYC section. I have chosen not the bleed the system because I simply do not trust my local dealer to do it correctly, and reputable shops and forums members have said it is not necessary. I think the real issue is this:

Quote:
Originally Posted by discogodfather View Post
This is because air bubbles get in when you fill and they can, THEORETICALLY, get into the pump and ruin it. Likely hood of this happening? Seems rather ambiguous at best, some reputable forum members say not likely, others have experience that say the pump will fail or at least loose service life.
Theoretically, yes. Necessary? Currently in debate...
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Old 07-15-2011, 03:46 AM   #3
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bump for comments? I think this is a good thread...
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Old 07-15-2011, 04:09 AM   #4
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Quote:
Originally Posted by orthojoe View Post
bump for comments? I think this is a good thread...
I agree. I would like to learn about this system as much as possible. Between the AWD system and the SST, I'm in a constant state of paranoia about something going wrong. Mostly since I know so little about them.
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Old 07-15-2011, 06:57 AM   #5
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great post! thanks
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Old 07-15-2011, 09:47 AM   #6
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I can confirm that using the gas pedal method, the pump comes on and allows fluid to flow through the ACD circuit. It does nothing for the two AYC circuits, however. There are three circuits total:

ACD - single line, single clutch pack circuit
AYC - dual line, dual clutch pack (right and left) circuit



My theory is that the MUT3 activates all three circuits at once to get fluid flowing. It's actually a dangerous thing if you really think about it. If this were EVER to happen while the car was running (all three circuits going at once) the entire drive line and differentials, tranny, and even the engine could be seriously damaged. The AYC works by clamping either left OR right, never together or the power would have no where to go and if the ACD clamp was going, BANG! It's kind of like flooring the gas pedal and standing on the brakes at the same time.

The MUT3 gets them all going and therefore bleeding can occur. It also runs a pressure diagnostic through the pressure sensor (maybe checking for certain fluctuations that mean air is in the system). The gas pedal method will only bleed the ACD, so you got some risk of major air in the rear diff AYC circuits. Even though they are separate circuits, they all mix at some point because the reservoir is the same.

There are three solenoids, one for each circuit. The way the pump works is that the motor energizes (it's just a single motor) and the solenoids act similarly to ABS units and they regulate pressure that is produced by the central pump. There is an accumulator that goes to 300psi (confirmed this from a British pump service place). There is a pressure sensor as well. I suspect the main pump runs just to charge the Accumulator and the entire system runs off the accumulator pressure (300psi) which is twice the circuit operating pressure (150psi). Thats why the pump comes on when you do the has pedal trick when you start the car, it needs some pressure to charge the accumulator.

A side effect of air in the system (and a dead giveaway that bleeding is necessary) is a big fluctuation of fluid in the reservoir in the trunk. If it's filled to the max line and you drive and get it hot after some spirited cornering with the ASC off (one push) and it rises to the top- big air in the system.

The reason I am nerding out on this system is my fascination with everything AWD and the fact that the ACD/AYC is the system that makes the X the X. It's a performance thing- if we treated our SST like we do our ACD/AYC systems the tranny would be completely fubar and shifting like crap with burned fluid and bits and pieces of the clutches in the pan and filter, etc. Yet when it comes to the ACD/AYC system we pay little attention. Probably because its so hard to tell a difference while driving- but my theory is high mileage Evo's that have had the system abused will just not perform nearly as well as something that is bled and flushed with clean ATF. It's only 3 pints in the entire system from what I can tell. Does it mean that much and is it worthy of nerding out on it, probably not, but I want 100% performance from the AWD all the time on the street and track, just the way we expect it from the engine and tranny, etc.

I'm working towards a new method of dealing with all these headaches, it involves using the gas pedal method but disconnecting the return line from the res and allowing the fluid to purge out of the system while new fluid is fed into it. No under car, no braking the vacuum seals, no bleeding. Should be able to be done by 1 person too. I will post a how-to with pics when I am done, hopefully the theory works.

The Japanese and the British have had AYC since 1996 and I have been studying the old pumps from pictures. The Brits service these things all the time and offer rebuilding services that don't look to be too expensive. They also seem to have individual parts, but mostly for the old Evo4-9 AYC not the Evo X S-AYC.
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"The evo is more car to take me to my job in action mode have more space for my childrens have SST and brembo for my wife and recaro seat for me is the perfect family car in this time (is more balance) is evolution to the future the subi is the same line from the past in other boby for teneger. Evo dad car."

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Old 07-15-2011, 10:29 AM   #7
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awesome write up...
disco you are correct in assuming that the pump charges the accumulator and the actual pressure to the system is supplied from the accumulator rather than the pump, or else the accumulator wouldnt be needed.

an idea for bleeding the AYC circuit, if it is possible to measure the voltage/amperage that operates the two solenoids, could you could reproduce the same voltage with a small 12v battery and resistors?
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Old 07-15-2011, 11:07 AM   #8
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Yeah, that makes sense. From what I know about ABS and those old hydraulic brake boosters (from the 90's) the accumulator was basically the pressure reservoir and that fed the system. In other words the motor pump does not pressurize the actual circuits it just handles the accumulator.

As for the 12v battery stuff that's what they do in Britain to service 7/8/9 etc AYC's. Look at this nifty box someone rigged up.



That's all good but it means getting under the car and I would rather wait on some genius like Tephra to read and decode the roms that handle the MUT3 process in Evoscan before I made a box or s test rig myself. There is a pressure sensor that governs things so if we just nuked the thing with a battery we might overpressure and blow it up. Add to that I just don't know what the MUT3 is doing when it bleeds.

Here is an interesting idea: these solenoids have to be open by default, right? I mean, when you do the gas pedal trick, and turn the power ON, it must close the AYC circuits and leave the ACD open, therefore allowing flow. Maybe just disconnecting the AYC solenoids (thus leaving them open) and doing the gas pedal method? Hopefully no codes.......

I'll test all this stuff this weekend and hopefully have some answers. My goal is to not get under the car, but I don't know if the system can induce enough flow to purge all the old fluid from the Res return line- it might take forever and be a dead end.
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"The evo is more car to take me to my job in action mode have more space for my childrens have SST and brembo for my wife and recaro seat for me is the perfect family car in this time (is more balance) is evolution to the future the subi is the same line from the past in other boby for teneger. Evo dad car."


Last edited by discogodfather; 07-15-2011 at 11:09 AM. Reason: forgot pic
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Old 07-17-2011, 05:31 AM   #9
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So I found the circuit diagrams in the service manual and isolated the different wires the the AYC/ACD pump.



The circuit seems pretty simple, there are:

1) (1) AYC proportioning valve - this probably controls the pressure to the AYC
2) (2) AYC directional valves - these control whether the pressure goes to the right or left clutch pack
3) (1) ACD proportioning valve - this probably controls the pressure to the ACD

My theory is that if you open your diffs up (make sure the drain plugs are out) and then power the pump motor whilse making sure both the "proportioning valves" aka the solenoids are powered (meaning they are open) the system should pump fluid to both the ACD and the AYC. Not sure how the directional valves would work or if they need to be powered to be "opened" too.

We know now that the gas pedal routine of turning the ignition to the "ON" position and flooring the gas pedal turns the ACD proportioning valve "on" or "open" and pumps a little fluid out.

I will connect a datalogger to the ACD circuit and see what kind of electronic control it's getting. Hopefully it's as simple as a small voltage between 1-12v, and it's not some duty cycle type signal. The theory is if we can mimmick that signal to the AYC proportioning valve it will also pump fluid (may also have to figure out whats up with the directional valves too).

Hopefully when you open the diffs up there is not a lot of need to regulate the pressure the pump produces (since the flow allows pressure to bleed off).

More when I get a chance to log and test-Thx.
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"The evo is more car to take me to my job in action mode have more space for my childrens have SST and brembo for my wife and recaro seat for me is the perfect family car in this time (is more balance) is evolution to the future the subi is the same line from the past in other boby for teneger. Evo dad car."

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Old 07-17-2011, 06:17 AM   #10
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Interesting... If Evoscan could run this operation as a actuator then it would be possible to bleed them... not sure if Hamish was ever informed out this...
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