Another Look at Torquebind, Auto Trannys and TCU's

Flywheel, Clutch, Transmission, Axles, etc...

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Larry Witherspoon
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Another Look at Torquebind, Auto Trannys and TCU's

Post by Larry Witherspoon » Wed Feb 06, 2019 11:32 pm

Hello Legacy Central

It's been a few years since posting for me. I'm browsing for parts - maybe a 92 turbo wagon auto trans drive shaft

I replied to John Scoobiedoo22's recent post about "Parting Out 92 Legacy SS Auto", partly due to something described as "torque bind"

It appears my info was interesting enough that a suggestion was made to create a formal write-up and send to the Drivetrain folder, so here I am

I have not kept up with all the social media types, still stuck back in email and maybe text era, so not certain what "formal" is, and I maintain a pretty informal lifestyle so here's a copy/paste, with a few extra tidbits thrown in

I have seen the term "torquebind" used before by others, but have been uncertain about the specific symptoms

Just from the terminology I think it may be something I experienced some years ago

When turning the steering wheel all the way to lock, while moving slowly, as in a parking lot, it felt like the inside rear wheel was locking up, and skidding thru the turn.

I have a 92 Turbo Wagon, with an auto tranny, and now plenty of technical info on what could cause the condition.

I was particularly worried because I've had the trans upgraded by Level 10 Performance Transmissions https://www.levelten.com/aboutus.asp in New Jersey, and the torque converter upgraded by JMO Performance Torque Converters http://www.jmousa.com/us/, closer to me in Fullerton, so I was not ready for any kind of trans trashing.

I did two trans rebuilds for the torque bind problem, one at an independent trans, shop and one at the dealer.

Both found stuff and I tried to fix everything, from a specific solenoid replacement, to a worn rear housing, to even a changeout of my 4.11 limited slip differential - the dealer service tech said his Suby engineer friend swore was the problem.

Nothing Worked !

There is a service manual diagnostic tree which describes the problem, with several solutions, and a defective trans control unit (TCU) problem as last resort.

I kept mentioning this to the dealer after hundreds of dollars of attempts, then finally found a spare TCU on my own, and changed it.

PROBLEM SOLVED AND NEVER RETURNED !!!

I was pretty sour, but I had my 4.11 reinstalled and never went back to that dealer

I think something can happen to a trans control unit so in a sharp turn, maybe the difference in speeds of the rear wheels, makes it think there is a slip or loss of traction, and it locks the slow side

So none of the mechanical fixes worked, cause there was nothing really wrong except with the interpretation of some kind of signal input to the computer, which then did it's thing -

I would not have thought at the time, there was any info from the mechanical differential to the computer, but I know now there's LOTS

The 1st gen Legacy Turbo Automatic Trans computer is very special - It is independent of the Engine computer, whereas that is not the case for the normally aspirated auto trans computer. Don't know if that applies to other models and/or years, but for this torquebind problem, I believe it important for the BC-BF Turbo Legacy that a BC-BF auto trans computer must (should?) be fitted

I believe those MY91 - MY94 Legacy Turbo Automatic Trans Control Unit part numbers are:

31711 AA821, 31711 AA822, 31711 AB371, 31711 AB372, 31711 AB373, 31711 AB374, and 31711 AC051

The Legacy 91 Model Year Update Course gives the following tranny info to consider;

In order to handle the increased torque produced by the turbocharged engine, the 4EAT Transmission uses a larger Torque Converter. For additional strength, the input shaft has also been shotpeened. In both the Forward Clutch assembly and the High Clutch assembly, the number of drive and driven plates has been increased by one (1), while the retaining plate thickness has been reduced. The Low/Reverse Brake assembly has increased the number of plates by two (2), but the thickness has been reduced. The outer diameter of the Servo piston (band application) assembly has been increased in order to provide more band clamping force. Of course, the servo piston seals have also been enlarged to match the servo piston diameter. Due to the new piston shape the number of "O" ring seals has been reduced by one (1) Lubrication passages have been added to the Planetary Carrier assembly for increased gear durability. Finally the logic in the Transmission Control Unit (TCU) has been changed to match the mechanical changes to the 4EAT. And a new Select Monitor Cartridge (P/N 498348400) has been introduced which supersedes P/N 498347500 and P/N 498347501. This cartridge will also operate on the 90MY Legacy. The TCU and ECU do NOT interface on the Turbo Legacy

Hope this is formal enough and helps somebody, and look forward to supporting or dissenting reply's to this message

Happy New Year and I'm enjoying the rainy for a change So Cal weather - hope the rest of you aren't having it too bad

Larry Witherspoon
Torrance (Southwest LA county) California
310-429-4269
Larry Witherspoon
ssspoon@aol.com
Torrance (Los Angeles)
92 Turbo Legacy Wagon
310-429-4269

scoobiedoo22
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Re: Another Look at Torquebind, Auto Trannys and TCU's

Post by scoobiedoo22 » Thu Feb 07, 2019 6:16 am

Great post Larry, thanks for sharing your experiences with the group...glad to have a small part in all this!
1991 Legacy SS 5-Speed

1999 Impreza 2.5RS Coupe

carfreak85
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Re: Another Look at Torquebind, Auto Trannys and TCU's

Post by carfreak85 » Thu Feb 07, 2019 4:42 pm

Excellent and very detailed info! Thanks for sharing, but oh my god, lime green text on a blue/grey background, MY EYES!!!
1975 Superstar Wagon
1984 GL-10 Turbo-Traction Wagon
1984 GL-10 Turbo-Traction Hardtop
1987 RX coupe
1987 RX Type RA
2002 WRX sedan

Legacy777
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Re: Another Look at Torquebind, Auto Trannys and TCU's

Post by Legacy777 » Thu Feb 07, 2019 6:54 pm

Hi Larry,

Thanks for sharing this information. I would like to clarify though that there is at least one wire connecting the ECU & TCU on the turbo Legacies (at least according to the wiring diagrams). There is the engine tachometer output signal from connector B56 pin 16 that goes to the ECU. It's possible this could be a typo in the manuals, but I don't think so. You could check and verify this to see if there is a wire at that pin on the ECU and then check if connector B44 pin 5. The wire color should be black with a blue stripe.

On the non-turbo cars there is another wire for the barometric sensor in the ECU going to the TCU. This would be on the ECU's connector B48 pin 16 and go to the TCU's connector B44 pin 9. The wire color should be red with a yellow stripe.
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

If you need to get a hold of me please email me rather then pm

Larry Witherspoon
First Gear
Posts: 81
Joined: Fri Oct 18, 2002 4:08 am
Location: Torrance (Los Angeles), California

Re: Another Look at Torquebind, Auto Trannys and TCU's

Post by Larry Witherspoon » Fri Feb 08, 2019 3:46 am

Hi Josh,

That is curious info, and thank you. I also do not think it is a typo in the manual. I think a wiring diagram would have too many checks before publication, whereas a bad word or two could sneak by.

Conversely, all that last paragraph trans info is taken right from the Legacy 91 Model Year Update Course, released as a supplement to the factory repair manual; that's not me trying to interpret something on my own.

I guess with confusing info like this we have to go back and try to interpret how the two statements might not conflict. For this I submit that the engine tachometer output signal is used by both the TCU and ECU, so what you see is a pass thru, without one affecting the other, as what must be intended by the word "interface"

<< The TCU and ECU do NOT interface on the Turbo Legacy >>

Carfreak85 I apologize for the eye attacking font color - I thought I was taking the extra step to make it more legible ! Hope this works better for you
Larry Witherspoon
ssspoon@aol.com
Torrance (Los Angeles)
92 Turbo Legacy Wagon
310-429-4269

Legacy777
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Re: Another Look at Torquebind, Auto Trannys and TCU's

Post by Legacy777 » Mon Feb 11, 2019 6:41 pm

Larry,

You would be surprised at how many errors are in the factory manual....and honestly I've lost count. The 90 FSM's I have are from a Subaru dealer and they have new pages/sections taped into the various books because the original was wrong.

Either way, I would tend to agree that the tachometer signal is most likely not being molested or changed by the ECU and it's primarily a pass through.
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

If you need to get a hold of me please email me rather then pm

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