Why is my torque bind only when cold?

[SubaruFred]

The quick history is that I own a 93 Legacy wagon with 177,000 miles. I drained and re-filled the ATF 3 times and replaced the duty C. All 4 tires are identical as well as pressures.

 

The current problem is that I have very bad TB when the ambient temp is below 60F and the engine/trans are cold but almost none when ambient temp is above 70F or the engine/trans are warm.

 

The question is why I have bad TB with a cold trans but none when the trans is warm? Electrical resistance issue or hydraulic pressure issue?

 

Any ideas?

[eulogious]

Clutch packs are all gummed up, and the heat helps loosen them up. So when cold, they are more sticky, causing your TB. You need to rebuild the tail section of your trans to get rid of that. You could try some more drains and flushes, but ultimately you will probably have to replace the clutch packs.

[idosubaru]

basic questions for this issue:

 

1. does the TB go away with the FWD fuse installed?

2. is the AT light flashing 16 times on start up?

 

someone recently said they're torque bind went away with a front differential fluid change. honestly, i find that really hard to believe and am not sold on it, but if it really did happen checking the front diff fluid level and color takes 2 minutes.

[SubaruFred]

Clutch packs are all gummed up, and the heat helps loosen them up. So when cold, they are more sticky, causing your TB. You need to rebuild the tail section of your trans to get rid of that. You could try some more drains and flushes, but ultimately you will probably have to replace the clutch packs.

 

 

That sounds plausible, thanks. However I'd like to note that whenever TB occurs the TCU stores the same code 24 for a bad duty C. Once the fault is stored the TCU defaults to a 50/50 torquie split by not sending any voltage to the duty C. The only way to fix the TB after it occurs is by re-cycling the igntion switch to reset the TCU. When the trans is still cold, this doesn't work but as soon as it warms up enough, it does work. Something is telling the TCU that the new duty C has failed when the trans is cold but okay when the trans is warm. It seems to me that the biggest issue is the TCU refusing to send voltage to the duty C. I don't know what is causing the TCU to determine that the duty C has failed. Perhaps gummed up clutches as you suggest.

 

 

basic questions for this issue:

 

1. does the TB go away with the FWD fuse installed?

2. is the AT light flashing 16 times on start up?

 

someone recently said they're torque bind went away with a front differential fluid change. honestly, i find that really hard to believe and am not sold on it, but if it really did happen checking the front diff fluid level and color takes 2 minutes.

 

1) The cold trans TB occurs with and without the FWD fuse installed.

2) On the next startup following TB the power light always flashes 16 times. The short term code is always 24 for the duty C. If there wasn't any TB on the previous ign-on cycle, the power light doesn't flash and no codes are stored.

 

Considering that the front diff fluid may be 18 years old I'll just change it. It might not help but it couldn't hurt.

 

I'm leaning back to the idea of manually controlling the duty C with a switch but I'm concerned that supplying constant +12V to it may overheat and damage it. I could also try a new TCU if I can find one.

 

Thanks.

[johnceggleston]

since you have changed the duty c, and the TB is temp sensitive, i would suggest the the trouble could be in the TCU. there are a few threads of bad TCUs causing TB.

 

my son's 95 lego has intermittent TB . some times it does not occur until it has been driven for an hour. i couldn't find a solution so i pulled the rear section of the drive shaft. i left the fuse in and some times the FWD light comes on . but most times the AT Temp light flashes at start up.

 

maybe the TCU is the cause. get a used one and see if ti helps.

[johnceggleston]

found one, $25 shipped.

 

http://www.ultimatesubaru.org/forum/showthread.php?p=1048814#post1048814

 

i have no idea if they are compatible.

[SubaruFred]

since you have changed the duty c, and the TB is temp sensitive, i would suggest the the trouble could be in the TCU. there are a few threads of bad TCUs causing TB.

 

my son's 95 lego has intermittent TB . some times it does not occur until it has been driven for an hour. i couldn't find a solution so i pulled the rear section of the drive shaft. i left the fuse in and some times the FWD light comes on . but most times the AT Temp light flashes at start up.

 

maybe the TCU is the cause. get a used one and see if ti helps.

 

found one, $25 shipped.

 

http://www.ultimatesubaru.org/forum/showthread.php?p=1048814#post1048814

 

i have no idea if they are compatible.

 

Thanks! FWIW, my FWD dash light is always on when a fuse is installed in the fwd socket. TB or not has no affect on this. This makes me believe that the new duty C is okay. I'd like to try a new TCU next but I don't think a 96 TCU will work in my 93. I've seen TCU pinouts from 93 and 95 and they are completely different. AFAIK I need a TCU from a 92-94 Legacy 4EAT.

 

Do you think that the duty C can withstand a constant +12V since it was designed to spend most of it's time in 90/10 FWD mode? Short of finding a new TCU, a manual duty C switch control may be my last resort.

[johnceggleston]

if the FWD fuse doesn't help, a switch won't either unless there is a wiring problem between the TCU and the and the duty c. but it might be helpful to see if it did help. maybe it is a wiring problem.

 

i think a new duty c will take a lot of abuse for a long time. but i wouldn't want to pay to replace it twice though.

 

it wouldn't be hard to find the duty c pin on the TCU connector and put juice to it. but i'm not sure i would want to back feed the TCU . it probably would not hurt , but who knows.

 

and of course, with the flashing AT Temp light, the TCU ''thinks'' the duty c is bad.

 

a thought, if you remove / disconnect the tcu, the car should be in limp mode, locked 4wd, TB, and only third gear. but what would happen if you put 12 v to the duty c then.? that might identify the possible wiring problem. and you wouldn't have to cut any wires to test it.

[SubaruFred]

if the FWD fuse doesn't help, a switch won't either unless there is a wiring problem between the TCU and the and the duty c. but it might be helpful to see if it did help. maybe it is a wiring problem.

 

i think a new duty c will take a lot of abuse for a long time. but i wouldn't want to pay to replace it twice though.

 

it wouldn't be hard to find the duty c pin on the TCU connector and put juice to it. but i'm not sure i would want to back feed the TCU . it probably would not hurt , but who knows.

 

and of course, with the flashing AT Temp light, the TCU ''thinks'' the duty c is bad.

 

a thought, if you remove / disconnect the tcu, the car should be in limp mode, locked 4wd, TB, and only third gear. but what would happen if you put 12 v to the duty c then.? that might identify the possible wiring problem. and you wouldn't have to cut any wires to test it.

 

 

With or without the FWD fuse installed, the main problem appears to be the TCU determining that the duty C has failed at which point the TCU stops sending +12V to the duty C. This results in a 50/50 split and TB that lasts until the TCU is reset by one or more ign on/off cycles until the TCU accepts the duty C as good. As long as the duty C receives +12V, there is no TB. I've tested this many times and the results are always the same.

 

My switch idea isn't to switch the FWD fuse circuit which would still rely on the TCU controlling the duty C but rather to cut the control wires to the duty C and replace them with a switch powered by an alternate ign-on +12V source. If this works I would have only 2 available torque split ratios (90/10 or 50/50). It's been done before but the cases I saw were later model 4EATs with different wiring harnesses. One guy went so far as to install a bank of resistors to simulate the duty C so the TCU wouldn't flash at startup.

 

I plan to test this ASAP but the hold up is finding the correct wires to intercept. I have no schematics and haven't found any for a 93 Legacy.

[motneal]

oNLY WAY TO FIX THAT IS Change the clutch housing for it as it is worn . The metal housing for the discs wears. There are no other fixes. 2 hrs on a lift and find one in the wrecking yard in a dead tranny. The metal part of the housing where it contacts the discs is worn. watch the plug in harness for the servos inside there.'

[SubaruFred]

oNLY WAY TO FIX THAT IS Change the clutch housing for it as it is worn . The metal housing for the discs wears. There are no other fixes. 2 hrs on a lift and find one in the wrecking yard in a dead tranny. The metal part of the housing where it contacts the discs is worn. watch the plug in harness for the servos inside there.'

 

I've seen photos of what you're talking about. Those grooves that wear into the clutch basket and hub and catch the clutch discs. That could possibly explain why the TCU reads a failed duty C and defaults to a 50/50 tq split. Re-building the clutch assy would likely fix all of my TB problems. However I have 2 problems doing this:

 

1) I don't have free access to a lift. I have to rent one at $30/hr.

2) Where can I possibly find a used clutch assy for a 93 Legacy that isn't as worn as mine? I suppose I could buy a used assy, re-condition it and then swap it in on the lift.

 

I was hoping to try the manual duty C control as a short cut solution. Since the clutches only stick occasionally, I would still have some TB when they're stuck but at least the TB would be gone as soon as they become unstuck. As it is now, once the TCU decides the duty C is failed, it stops controlling it and causes constant TB until I reset the TCU with the ign switch.

 

Thanks.

[idosubaru]

If this works I would have only 2 available torque split ratios (90/10 or 50/50).

i've done that before too, it's a perfectly viable solution, do it.

 

Pin #3 of 16 pin connector B33. 1990 TCU pinouts, which should be the same as yours:

 

http://www.main.experiencetherave.com/subaru_manual_scans/FSM_Scans/TCU_I-O_page1.jpg

 

http://www.main.experiencetherave.com/subaru_manual_scans/FSM_Scans/TCU_I-O_page2.jpg

[SubaruFred]

Thanks! It worked like a charm!

Edited October 11, 2011 by SubaruFred

[idosubaru]

Thanks! It worked like a charm!

awesome, good job. now make sure you or someone else, does not accidentally leave it on.

[nipper]

oNLY WAY TO FIX THAT IS Change the clutch housing for it as it is worn . The metal housing for the discs wears. There are no other fixes. 2 hrs on a lift and find one in the wrecking yard in a dead tranny. The metal part of the housing where it contacts the discs is worn. watch the plug in harness for the servos inside there.'

 

 

What he said, there is more to torque bind then the clutch pack and the C solenoid. especially on pre 1997 1/2 - 4eats.

[nipper]

A better way of wording it, why do you loose your AWD (now 4wd) once the car warms up. When cold the fluid is thicker and seals are tighter. As things heat up, selas expand, fluids get thinner and there is some pressure loss. That pressure from the thicker heavier fluid was leaking around seals causing you to have TB. Once things heat up it goes away,m but so does your AWD.

 

Your housing is tired, and you need a new housing and clutch pack.

 

For the record gum gets gummier as it gets warm, so if it was a gum issue it would get worse with heat.

[SubaruFred]

awesome, good job. now make sure you or someone else, does not accidentally leave it on.

 

Not a problem, no one else drives the car and it will be left in 90/10 mode 95% of the time. The switch has a very visible LED to remind the driver that the proper fwd mode is selected.

 

A better way of wording it, why do you loose your AWD (now 4wd) once the car warms up. When cold the fluid is thicker and seals are tighter. As things heat up, selas expand, fluids get thinner and there is some pressure loss. That pressure from the thicker heavier fluid was leaking around seals causing you to have TB. Once things heat up it goes away,m but so does your AWD.

 

Your housing is tired, and you need a new housing and clutch pack.

 

For the record gum gets gummier as it gets warm, so if it was a gum issue it would get worse with heat.

 

 

I probably didn't explain my TB symptoms accurately enough or with sufficient detail. In 70F+ weather TB occurs only randomly and can always be cured by cycling the ign off and on again to reset the TCU. In <50F weather TB occurs constantly until the ign is cycled off and on after the trans has warmed up. If the TCU is not reset the TB is just as severe when the trans is hot as it is when cold. It always takes a TCU reset to fix it. It doesn't lessen at all as the ATF gets warmer. 50/50 AWD works the same, hot or cold.

 

Obviously something is causing the TCU to "fail" the duty C and it occurs more often in cold weather than hot but I can't say exactly what that is. It could well be a worn transfer clutch *********embly combined with higher ATF pressures when cold but the consistent "fix" is to simply reset the TCU. Rather than continue to turn my engine off and on again to reset the TCU, I felt that establishing manual control over the duty C would be preferable. I could have installed a switch on the TCU power circuit to reset it without turning the engine off but I wanted to try this method instead. Besides, I felt that the TCU did a lousy job of managing F/R torque split anyway.

 

This way I have FWD normally with AWD on-demand, on-the-fly.

Edited October 13, 2011 by SubaruFred

[SubaruFred]

Update: 2 days and not the slightest hint of any torque bind, cold or hot. If I flip the switch to stop sending voltage to the duty C I get immediate and severe torque bind. Enough to cause the rear tires to chirp as they slip while turning and very noticeable. Worn or not the transfer clutches really lock up solid.

 

I'm very inclined to blame the TCU for all of my TB problems. The TCU reported a failed duty C but when I replaced it, nothing changed. Byp*********ing the faulty TCU has made all the difference. I also noted that with the fwd fuse installed the duty C was getting 10V+ but without the fuse it only got 2.5-4.5V. I suspect that a problem within the TCU had reduced the voltage going to the duty C to a level insufficient to acheive a 90/10 split.

 

Thanks again Grossgary and others!

[johnceggleston]

I also noted that with the fwd fuse installed the duty C was getting 10V+ but without the fuse it only got 2.5-4.5V. I suspect that a problem within the TCU had reduced the voltage going to the duty C to a level insufficient to acheive a 90/10 split.

 

 

how do those numbers compare with the ones near the bottom of this page for the duty c, pin 3?

 

http://www.main.experiencetherave.com/subaru_manual_scans/FSM_Scans/TCU_I-O_page2.jpg

[SubaruFred]

how do those numbers compare with the ones near the bottom of this page for the duty c, pin 3?

 

IIRC with the gear selector in D I measured ~4.5V, in 1st gear it was ~2.5V and with the throttle wide open I measured less than .05V. I forget the measured resistance but it was within spec too.

 

I'm just speculating that since it presumably takes 8-14V to ensure a 90/10 split and less than .05V to ensure 50/50 that perhaps 4.5V is not enough to hold the solenoid closed for 90/10. I'm not very familiar with PWM circuitry but I'm speculating that something may have failed, causing the TCU to duty C output voltage signal to drop.

 

Then again it could be something else entirely. All I know for certain at this point is that bypassing the TCU appears to have fixed all of my TB issues.