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Discussion Starter · #1 ·
I got stuck recently in the Sierras on a dirt road when I backed over a large angular piece of granite that dropped the right rear wheel into a hole. All the other wheels were firmly on the ground and the chassis did not high center. I thought the AWD in the 5-speed would pull me out with no problem. But all it did was spin the right rear wheel and burn up the clutch. I managed to get out by slipping the jack under the chassis in front of the right rear wheel and jacking up the car enough to put some rocks and large branches under the wheel and the floor matte over the rocks/branches and up the rock, and I finally got out. But I was very disappointed in the car. So, what gives? Thanks. LRV
 

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It certainly isn't a 4X4 (with locking transfer case and differentials), but if the other three wheels had good traction, the car wasn't on a slope, there was nothing preventing the other wheels from moving, and no part of the body/suspension was on the ground, shouldn't the car have at least appeared to be trying to move (rocking) when the clutch was engaged?

Absolutely no movement would suggest that the center differential's viscous coupler wasn't working, but this is contradicted by the reference to the clutch burning up. If the clutch was overheating, it could only be because there was a significant load on it as it was being engaged, and if that's the case, it could only have come from the front wheels as the rear drive was free to spin (assuming the car does not have a limited slip differential at the rear).

Perhaps I'm misreading the original post . . .
 

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Discussion Starter · #4 ·
I'm the original poster. Every assumption that Plain OM makes is correct. See photo attached (I hope) But checking my original purchase material indicates that the car has "full-time AWD" and a "limited slip rear differential." So, would that explain why no movement with the right rear wheel hanging over the rock? If so, please help a mechanical peasant understand. Also, the left rear wheel was firmly on the ground, but did not move. And, from some Sub website demos and comparisons of variations on AWD, (see Subaru Symmetrical All-Wheel Drive | Subaru AWD, and
), it would appear that in my situation, the front wheels (and the other rear wheel) should have pulled me out. So, could center diff or viscous coupling you mention be inoperative? How does one check that? A Sub mechanic recently told me "off the record" that a solution to my predicament would have been to open the fuse box under the hood and to put a fuse into the vacant slot marked "FWD" thereby disabling the AWD and turning the car into just FWD. Then, with no power going to the rear wheels, the front wheels should have been able to pull me out. What do you think of that? I don't really want to repeat the experiment and try the fuse thing, but is this mechanic full of it, or legit? Thanks. LRV
IMGP0158a.jpg
 

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I'm the original poster. Every assumption that Plain OM makes is correct. See photo attached (I hope) But checking my original purchase material indicates that the car has "full-time AWD" and a "limited slip rear differential." So, would that explain why no movement with the right rear wheel hanging over the rock? If so, please help a mechanical peasant understand. Also, the left rear wheel was firmly on the ground, but did not move. And, from some Sub website demos and comparisons of variations on AWD, (see Subaru Symmetrical All-Wheel Drive | Subaru AWD, and Subaru AWD vs. competitors - YouTube), it would appear that in my situation, the front wheels (and the other rear wheel) should have pulled me out. So, could center diff or viscous coupling you mention be inoperative? How does one check that? A Sub mechanic recently told me "off the record" that a solution to my predicament would have been to open the fuse box under the hood and to put a fuse into the vacant slot marked "FWD" thereby disabling the AWD and turning the car into just FWD. Then, with no power going to the rear wheels, the front wheels should have been able to pull me out. What do you think of that? I don't really want to repeat the experiment and try the fuse thing, but is this mechanic full of it, or legit? Thanks. LRV
View attachment 24726
That looks like a heck of a ledge to try to pull that wheel out of. It's odd that the front wheels weren't spinning at all (did I read that right?), because one should have been. It sort of looks like even if they were, they would be on loose gravel with most of the weight shifted to the rear (and to one side at that), and I'm not sure you'd have enough traction to get out of that. Hard to tell for sure, though.
 

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Could be you were just planted so good it was like trying to drive off with the front bumper against a brick wall. In that case you'd just smoke the clutch, too.
 

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It is indeed quite a ledge to pull the car up and out of, but I would have still expected some movement of the car, sort of a rocking motion, when the clutch was being engaged and the released, or some wheel spin (other than the wheel in the hole) confirming that the other wheels were trying to pull the car forward, but couldn't overcome the ledge.

However, given the apparent drop of the wheel in the photo relative to the wheel well, I wouldn't be surprised if the wheel was hanging freely and some part of the chassis was on the ground. The rear bumper looks as if it might be hung on the slope behind (if not at the right rear corner, then further toward the center), thereby preventing the car from moving backward, and the rocker panel in front of the wheel is pretty low (the "air dam" in front of the wheel is down to the ground), suggesting to me that there could well be a suspension member or the differential support near the center of the car that's on the ground. (It also looks as if it would have been difficult to see if anything was binding.)

If the car was on the ground, and especially if a wheel suspension member was "hooked" behind the ledge as well, the car might not move at all. In addition, the RR wheel also appears to be right up against the ledge, and this would also prevent any forward motion -- in other words, the wheel would have had to go up before it could go forward. In this case, as the clutch is engaged, the RR wheel would start to spin, the viscous coupler would try to increase torque to the front wheels, but they still couldn't move the car.

After the car was raised a few inches, whatever was catching was free, or less restrained, and the car moved.

A Sub mechanic recently told me "off the record" that a solution to my predicament would have been to open the fuse box under the hood and to put a fuse into the vacant slot marked "FWD" thereby disabling the AWD and turning the car into just FWD.
This applies only to the 4-speed automatic transmission -- not cars with a manual transmission, which is what I believe you have based on your reference to the "clutch". In any event, using the fuse would not be necessary even with the 4-speed automatic.

Given the situation in the photo, I now tend to think it was just well beyond the capability of the car.
 

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Discussion Starter · #8 ·
OK, so a few more facts. No part of the chassis was resting on the ground. The right rear wheel was not hanging freely, but certainly without sufficient weight and contact to get traction. The vertical face of the rock showed I left quite a bit of rubber trying to get out. The front wheels were not on loose gravel and did not spin when trying to engage (and burning up) the clutch. I couldn't rock the car as the rear bumper was pushed up against (but not caught on) even larger rocks in the back. I've attached another photo showing how I got out after sliding the jack under the frame and lifting the entire car enough to push some rocks and large branches under the real right wheel and putting an old blanket over the stuff and the rock. With that help, the car pulled right out with little problem.

So, my question is exactly how does the center differential work on my car and how much torque is shifted to the front wheels or to the LR wheel in such a situation? I didn't get any sense that the front wheels were pulling at all.

As to the FWD fuse "solution", I'll go back to the mechanic about the fuse working only for a 4 speed auto. If I'd had an automatic would it have pulled out on its own? Why? Thanks for the comments. LRV
IMGP0160a.jpg
 

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OK, so a few more facts. No part of the chassis was resting on the ground. The right rear wheel was not hanging freely, but certainly without sufficient weight and contact to get traction. The vertical face of the rock showed I left quite a bit of rubber trying to get out. The front wheels were not on loose gravel and did not spin when trying to engage (and burning up) the clutch. I couldn't rock the car as the rear bumper was pushed up against (but not caught on) even larger rocks in the back. I've attached another photo showing how I got out after sliding the jack under the frame and lifting the entire car enough to push some rocks and large branches under the real right wheel and putting an old blanket over the stuff and the rock. With that help, the car pulled right out with little problem.

So, my question is exactly how does the center differential work on my car and how much torque is shifted to the front wheels or to the LR wheel in such a situation? I didn't get any sense that the front wheels were pulling at all.

As to the FWD fuse "solution", I'll go back to the mechanic about the fuse working only for a 4 speed auto. If I'd had an automatic would it have pulled out on its own? Why? Thanks for the comments. LRV
View attachment 24731

AT would have been better MT the gearing is way to high to tackle this type of technical surface. You couldn't spin the front wheels because you couldn't get enough power to them with the MT ie your gearing was too high and you had to slip the clutch to keep from stalling the car. MT the AWD system is very basic there really isn't anything to fail other than the viscus coupler but that won't matter in this case given you couldn't get enough power to the wheels to start with due to the MT and slipping the clutch.

Not to mention if the rear wheel was spinning enough to put some rubber on the rock - the viscus coupler was working given thats what sends power to the rear of the car. Again your issue would be lack of low gearing in the MT which is why the hard core off road guys in the outback prefer the AT the torque Converter makes up quite a bit for not having low gearing.

The only other possibility is a CV joint which has failed on the front end as a result the axle is turning but the front wheels are not. I highly doubt this though.
 

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too much for it to pull over, but like was said it is surprising the wheels didn't move.

the rear viscous type LSD's degrade as an open rear differential with age fairly often. so that both rear wasn't spinning doesn't surprise me. the fronts though that is strange.
 

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too much for it to pull over, but like was said it is surprising the wheels didn't move.

the rear viscous type LSD's degrade as an open rear differential with age fairly often. so that both rear wasn't spinning doesn't surprise me. the fronts though that is strange.
Not really - block a tire good and give the rest decent traction add MT with tall gearing and you just do not have enough power to spin two tires or three regardless. My guess is that the aWD system is just fine and the reason why he didn't spin the fronts, simply didn't have low enough gearing to get power to the wheels.
 

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Larry:

At any time (before lifting the car), did you ever have the clutch pedal fully up (clutch fully engaged), with the right rear wheel spinning and no other wheel turning?

If the clutch pedal was never fully released (with the engine revving), and I would understand not wanting to take it that far, then there's less torque transfer capability from the engine to the transmission and the drive train. In that case, there probably just wasn't enough torque transfer to the wheels to get them to pull the car, or at least give the sense that it was trying to move forward.

However, there's a huge difference between lifting a wheel, and car, vertically, and moving it forward. The lift requires far more torque because it has to account for the dead weight of the car in that corner. When just moving the car forward, the weight is distributed on four round wheels, which reduces the level of torque needed to get it going.

Think of a block of stone. It's impossible for you to lift it, or to push it along the ground. But put it on two rollers, and you can push it along (at least until one of the rollers moves out the back end). Now, take out one of the two rollers so that one end of the block is on the ground. Now it's again impossible to push it forward. If you could lift the lowered end, it would move on the one roller. But you can't lift one end -- it's too heavy. In other words, it's relatively easy to move forward on the rollers, but impossible to lift and move forward at the same time.

With the AT, it wouldn't matter if the fuse was in or out. The front drive is always connected, through gears, to the output of the transmission, and the rear drive is connected through the AWD clutch. In first gear (D), when the accelerator is pressed down significantly, the clutch can become fully engaged. This ensures that the front and rear drives are turning at the same speed. (The rear drive can never turn faster than the front in the 4-speed AT.) Both the front and the rear drives will get the full torque available from the engine. But, again, I'm not sure it could have lifted the right rear vertically out of the hole.

The MT AWD should be able to do the same as the AT except if the clutch pedal was never fully released, the full potential torque from the engine isn't available to the front and rear drives.

(I haven't taken into account the gear ratios of the transmissions which can also affect torque levels, but I believe that by design, there still would not be that much difference. The capability of the AT and MT should be equal.)

You were in an unusually difficult situation that called for the technique you used.
 

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Discussion Starter · #13 ·
OM: Thanks for the further response and sorry for the delay. When trying to get the car out, yes, I released the clutch peddle as far as I felt I dared with the engine revving. The car would lurch ahead just a tiny bit each time, but no front wheels spun. With the clutch burning, I felt certain that fully releasing the clutch would have just stalled the car. Yes, I understand that a lot of torque going somewhere would be needed to pull and lift the RR wheel out of the hole. So, then, would the "fuse solution" have worked? By putting in a fuse into the "FWD" fuse slot, would that have disengaged the center VC (or whatever it's called) so that the engine power would have gone only to the front wheels? If so, then that should have pulled me out since the chassis was not stuck and the LR wheel was firmly on the ground, correct? Someone else has suggested that with a MT, that the gearing can't get low enough to work in that situation and that an AT has lower gearing. Would that have made a difference? Also, exactly how does the center VC send power to the wheels with more traction, when the system is working properly? Thanks. LRV
 

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OM: Thanks for the further response and sorry for the delay. When trying to get the car out, yes, I released the clutch peddle as far as I felt I dared with the engine revving. The car would lurch ahead just a tiny bit each time, but no front wheels spun. With the clutch burning, I felt certain that fully releasing the clutch would have just stalled the car. Yes, I understand that a lot of torque going somewhere would be needed to pull and lift the RR wheel out of the hole. So, then, would the "fuse solution" have worked? By putting in a fuse into the "FWD" fuse slot, would that have disengaged the center VC (or whatever it's called) so that the engine power would have gone only to the front wheels? If so, then that should have pulled me out since the chassis was not stuck and the LR wheel was firmly on the ground, correct? Someone else has suggested that with a MT, that the gearing can't get low enough to work in that situation and that an AT has lower gearing. Would that have made a difference? Also, exactly how does the center VC send power to the wheels with more traction, when the system is working properly? Thanks. LRV
Clutch never released ='s not enough power sent to wheels. Turning the car into front wheel drive would have done nothing if the rear of the car was hung up enough where the front wheels would have simply just spun - assuming you had released the clutch all the way.

In this type of situation you need to let the clutch out and see if the car has enough gearing and power to hop out of the hole. Given you never did that - you have no idea if the gearing was beyond way too tall with zero hope of powering the car out of the hole. Having zero initial roll makes it very difficult to simply let the clutch all the way out but that really was the only option and if the car died without ever getting close to getting out of the hole - then you were simply lacking the right gears ie low gears to do the job. Hence why the AT is preferred by the OB guys who do pretty technical off road stuff over the MT.
 

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Further to what subiesailor said, given the situation, I very much doubt switching to FWD in an AT would make any difference. The wheel was just too far down a vertical surface. If anything, you would want power to the rear to allow the dropped wheel to help lift itself out.

I now suspect that even with the clutch released fully (which might have stalled the engine), the car still couldn't get the rear wheel to climb vertically, unless it was a "pop the clutch", "bounce the car", "up it goes" situation that probably could have damaged a lot more in the process. If it were an AT, with or without the fuse, I suspect the torque converter would take a major beating.

When the wheel was raised a bit on the rocks, instead of the tire/rock contact point being at the front edge of the tread, it was lower down. (See attached photos in posts 4 and 8.) In this case, the tire didn't have to climb vertically as much; the movement became more of a very steep slope, and with just a bit of rocking of the car, it could ride up and over the ledge.

These are not H1 Hummers or 4WD locked-everything military Jeeps that I've seen climb short walls, albeit at the front wheels only, with heavy lugged tires, and under specific demonstration conditions.

As noted before, I think raising the wheel was the right approach.
 

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In regard to relative gearing of the AT and MT, the gearing of the 5-speed manual transmission might actually be more favorable than the automatic.

In 1st gear, the MT ratio is 3.454:1, and in the MT OB the final is 4.111, for an overall ratio of 14.2:1. The 4-speed AT 1st gear is 2.785:1 and the final is 4.444, for an overall ratio of 12.37:1. So on a strictly mathematical basis, the MT would have a slight advantage.

Granted, the torque converter in the AT can "multiply" torque by a factor of close to two but this is only at the TC stall point, i.e., when the rpm difference between impeller and turbine is around 2200 rpm. When a wheel is spinning, as was the case here, there is little resistance fed back from the drive train and little difference in rpm across the TC. So the effect of the TC would probably be far less.
 
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