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