Fundamental Question: What is "Torque Bind" and what is it indicative of?

On Feb 5, 2:57 pm, Hachiroku $B%O%A%m%/(B wrote:

You must be speaking about auto trannies. Someone here mentioned a Ford additive as helping with the torque bind issue. Hadn't heard about the Sea Foam.

the below is meant to be very general statements about the Subaru 4EAT transmissions.

The AWD is supposed to engage automatically when slippage occurs, then, if slippage is gone, the system returns to an 'open diffs' condition(there isn't really a center diff, it's a 'wet clutch pack'). Anytime the system stays in the AWD condition, BUT is on dry hard pavement, the drive train has no way to relieve stress. There's no slippage. The TCU/computer detects slippage as a difference in signals from the tone rings on the hubs. I think these same sensors are used by the ABS if equipped and maybe dynamic control or w'ever. Not sure on those last 2. If, for example, you have a larger or smaller tire on you car, the signals from the tone rings are outside the allowable threshold and the system reacts as if you were on snow or mud w'ever, and begins to alter the duty cycle of the pulses sent to the Duty Solenoid C (IIRC) that controls the clutch pack in the tail of the tranny. The clutches begin to engage more, apply torque to the rear diff. Depending on whether that diff is open or limited slip, one or 2 wheels in the rear will be able to apply more and more force to the road surface Once the tone ring signals agree, the system returns to 'open diff'. Naturally, if the TCU itself were damaged, or if the Duty Solenoid C is stuck, or perhaps damaged wheel sensors/wiring - could all cause the clutch pack to engage when it shouldn't . It should be pointed out that the system COULD fail in such a way that AWD never engages even when it should. In the past, there have been issues with a wear part in the transmission being made of aluminum causing eventual torque bind problems (that was in the 90s IIRC - replacement part, rebuilt trannies are steel). Also, some folks feel improper/ infrequent transmission fluid maintenance can cause the problem. If you experience torque bind, and try troubleshooting by using the FWD fuse in the fuse block under the hood, that can help point to where the problem lies. If using the fuse does NOT fix the binding (as felt by doing sharp cornering/figure 8s in a dry parking lot) then the problem is inside the transmission or the TCU - basically, the Duty C Solenoid is not able to respond to the FWD Only fuse's action. Some folks have had bad U joints feel like torque bind so that should be checked too. I think I read of someone that found a piece of steel road debris stuck to a wheel sensor once.

I may be off on a detail or 2, but someone will no doubt correct me if I'm misleading in any important way.

I don't think an odd-sized tire is enough to trigger the system, but the problem with that being the clutches are just rubbing against each other too quickly for the heat to dissipate properly, but either way, bad.

The additional failure mechanism is the clutch basket getting grooved and keeping the plates engaged, or from engaging.

Dave

The reason they give you the ability to force the system into FWD mode with the fuse trick, is precisely because you NEED to if you use the 'donut' spare. Subaru used to recommend no tire more than 1/4" difference in CIRCUMFERENCE than any other tires. That seems to work out to a tire that is about half worn out. Now, is Subaru just being overly cautious? Could you run a new replacement with 3 almost worn out tires and be safe? Suppose you ran the new tire deflated 2-3 psi? These discussions come up every 6 months or so. I'd say , it IS a risk to mix sizes of tires. The bigger the difference in size, model, wear w'ever - the bigger the risk to your transmission.

I run less expensive low-tread-wear tires (that are otherwise good at traction, heat speed rating, w'ever) beacuse I knoiw I may, someday, have to replace three 2/3rds worn tires because the sidewall got cut on one. With cheaper tires, less wasted value if that happens. Tire shaving can help with this issue. But it isn't easy to arrange for most people, it costs money, AND you are shaving off part of the value in a new tire.

(HELLO! Subaru! each dealership should offer free tire shaving!)

Ok, even liability in the states aside why would they want to do that for free?

Thanks for this description of the system - the best I have read.

My 20k mile 05 Forester automatic developed the binding problem, and neither of the two nearest "main dealers" had the slightest clue what it was or how to fix it.

The first said it needed a new rear axle, which even I knew was rubbish. The second thought he could fix it by driving round in circles on full lock, one way then the other. This wrote off both front CV joints at prohibitive cost (to me).

I took the opportunity to swap-out the whole gearbox/dry clutch assy when offered a low milage unit at a price roughly half the cost of one CV repair, so I never found out whether it was the gearbox or the centre clutch. (The old unit sold too quickly to investigate, and in any case was too big and heavy for me to work on).

The old unit had a clunk from overrun to drive, and had slight tyre scrub on full lock from day one, whereas the replacement is completely free of these effects (and does drive all four wheels on soft ground).

The purchaser of the old unit said it was a front diff fault, but he didn't want to pay too much for it "as spares" so could have been telling porkies. What do you think ?

I dunno about the problem you describe, but I like "telling porkies".

I dunno really. My wife's 03 Outback has a noisy front diff and I've read of complaints about front diffs - though it seems quite rare compared to other drivetrain problems.

Of course, that old tranny 'could' have had more than one problem.

hard to say.

I didn't say it doesn't damage the clutch, I said that the system is not activated, and the damage occurs without the system being activated.

The clutches are in close contact and will undergo significant friction and generate significant heat if the system is not activated.

Tire Rack will shave-to-spec before shipping.

Dave

That's cool that Tire rack will do that. I've done biz with them twice and been very happy. But many people use local tire retailers that may not offer shaving.

I don't see how the AWD 'system' can distinguish between faster tone ring signals due to actual slippage vs faster tone ring signals from - say - a deflated or undersized tire, but we can agree to disagree.

And I agree it's possible for components to fail or wear-out without the system 'engaging'. (When we say 'engaging' it depends on whether the slipping /undersized tire is on the front or rear axle. The TCU alters the duty cycle of pulses to the Duty Solenoid C to 'direct' torque away from the slippage and towards the non-slipping axle.)

Putting a scope on the signal to the Duty Solenoid C and letting a few psi out of tires in various locations on the car, then doing some sharp maneuvering on dry pavement, could be interesting.