[plain OM]

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 . . .

 

[plain OM]

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.

 

[plain OM]

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.

 

[plain OM]

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.

 

[plain OM]

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.