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Old 10-05-2011, 06:41 PM   #11 (permalink)
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Wow nice picts. So that age old argument of not using the parking brake could in theory also reduce the life span of the front diff no? Seems like loading up the pawl ie no parking brake would clearly add some additional wear on the gear driving the front diff right?
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Old 10-05-2011, 06:43 PM   #12 (permalink)
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The next question would be if you never service the front diff gear oil where would the most likely place of failure be? This primary gear? Is it the largest gear in the front diff oiled area?
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Old 10-05-2011, 07:39 PM   #13 (permalink)
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Haven't heard an argument against using the parking brake except in climates where rust and/or freezing might be a factor. But I certainly live in such a climate, and have always used the parking brake without problem. In fact, not using it, and not regularly moving the parking brake cables is more likely to lead to rusting and seizing, so that when it is needed, it doesn't work.

When Park (gear) is used, the parking pawl and front drive train take up the load (tendency of the car to role, which would depend on the slope). They should be able to handle the normal weight of the car when on a steep slope. What might cause this not to be true is if the drive train components are subjected to shocks which can happen if the car is bumped while in Park, or the parking pawl is engaged while the car is still moving (ouch!). In this case, what would give way first is a good question. There's the CV joints in the front axles; the front differential side, spider, ring and pinion gears, as well as the spider gear pinion shaft; the pinion drive shaft, the reduction gears; and the parking pawl itself.

The first photo below shows the parking pawl -- it's more visible here with the upper reduction gear and tranny output shaft removed. Note the wide tab on the pawl that the arrow points to. That's what does the work. (This and the next photo are from the VTD version, but as far as the parking pawl goes, it's the same in the 4EAT with MPT, and the CVT.)

The second photo shows the tranny output shaft from the VTD version, with the reduction gear, differential planetary gear set, and the splined hub for the AWD clutch. (In this case the clutch is used to lock the planetary differential.) We also see a notched ring behind the reduction gear. It's these notches that the wide tab on the parking pawl engages. Doesn't look like much, but essentially the same thing is used on millions (billions?) of cars world-wide with rarely a problem, in normal use.

There's no doubt in my mind that using the parking brake whenever parked is far better. First off, it adds two more wheels on the ground that are not able to turn, so in the event the fronts are on a slippery surface, the rears might still prevent the car from rolling, and vice-versa. Moreover, with the parking brake applied, if the car is bumped, the brake adds another level of braking to the car, and could very well prevent the front drive train from being stressed, or worse still, damaged, by the bump. And then there's the case of parking on a slope and finding it difficult to take the car out of Park. If the parking brake is applied with the gear in Park but before releasing the foot brake, the parking brake (if working properly) will prevent the load from being applied to the front drive train. The parking pawl might take on some load if the car rolls a bit, but it won't tend to get jammed as it might without the parking brake.

In regard to not servicing the differential, the impact of good lubrication is, in my view, uncertain. All the gears (teeth) are under "extreme pressure", especially when accelerating, and so wear on their contacting surfaces is going to be exacerbated. But there's also bearings that depend on the gear oil, and the spider gears rotate on the pinion shaft without any bearings, so again, wear can be a factor. When the gear teeth wear and the contact force areas change, there is more likelihood of a gear tooth breaking off and this can then cause collateral damage. But I think predicting what would give way first would be difficult. It's these case where a minor original manufacturing variance that would normally not affect anything for the life of the car become more critical factors. These could be anywhere among the parts that depend on the gear oil.

(p.s. The reduction gears are lubed by the ATF and are not affected by the gear oil in the differential.)

subiesailor: as you have a CVT, I've added a pic of the pawl from it. It's actually better than in the first photo -- much more close up and in color. It shows just how rugged the pawl, and the notched ring behind the reduction gear, actually are.

The upper right also shows the multi-plate clutch -- the reddish ring is on one of the the clutch plates.

The new CVT uses a different design of reduction gear, but the purpose and location are essentially the same. The lower reduction gear drives the front differential pinion drive shaft, as in the earlier transmissions.
Attached Thumbnails
Blown transmission? Drive shaft? Axel?-vtd-parking-pawl.jpg   Blown transmission? Drive shaft? Axel?-vtd-gear-set.jpg   Blown transmission? Drive shaft? Axel?-cvt-parking-pawl.jpg  

Last edited by plain OM; 10-05-2011 at 08:01 PM. Reason: add p.s. and CVT pic
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Old 10-06-2011, 01:36 AM   #14 (permalink)
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I got her back from the shop today. The end result: a broken axle. The passenger side axle failed on the inner joint. My best guess is the axle failed at speed, channeling all of the power to the path of least resistance which would be the broken axle. This explains the noise I heard as well as the lack of power and jump in revs. Since the wheels were all still spinning the same speed VDC never kicked in. The issue with it not going into park is explained by the fact that I put it into drive and the broken half of the axle began to spin and since the breaks are on the wheel and not the axle it was still spinning when I put it back into park. (Definitely not healthy for tor the parking pawl.)

The guys at AWD Auto in Kirkland did a great job of fixing her back up. The repacked and rebooted a used axle for me which came to $330 installed.

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Old 10-06-2011, 07:34 AM   #15 (permalink)
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The issue with it not going into park is explained by the fact that I put it into drive and the broken half of the axle began to spin and since the breaks are on the wheel and not the axle it was still spinning when I put it back into park.
If the gear shift lever was in the Park position, but the car still rolled, (which I what I understand was the situation), and assuming that the parking pawl mechanism wasn't broken (which appears to be the case), the pawl was set properly. The transmission was in "Park". The front drive system has more than enough friction and drag to stop it as soon as the transmission shifted out of a drive gear.

But as was outlined above, the parking pawl stops the reduction gear from turning. The gear is permanently and mechanically connected to the front differential pinion drive shaft, which is connected (engaged teeth) to the ring (crown) gear of the front differential. So if the parking pawl is engaged, none of these can turn.

With a drive system that is intact, when the differential ring gear can't turn, then for one wheel to turn, the other wheel must turn equally and in the opposite direction. This is a fundamental characteristic of a differential. But because both wheels are fixed on the ground, this can't happen -- neither wheel turns, and the car can't move. All it takes, normally, is the parking pawl being set to prevent the reduction gear from turning.

However, what happens if one wheel is disconnected from the differential, as with a broken axle? Now, even though the differential ring gear still cannot turn (nothing has changed between the pawl, reduction gear and pinion shaft), if one wheel tries to turn, the (broken) axle on the other side of the differential now can turn (in the opposite direction). It's no longer connected to a wheel fixed to the ground. So there's nothing to stop the still-connected front wheel from turning while the broken axle on the other side turns in the opposite direction. As neither wheel is being stopped by the brakes, they are free to turn, and the car rolls.
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Old 10-06-2011, 10:25 AM   #16 (permalink)
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Originally Posted by FOX- View Post
I got her back from the shop today. The end result: a broken axle. The passenger side axle failed on the inner joint. My best guess is the axle failed at speed, channeling all of the power to the path of least resistance which would be the broken axle. This explains the noise I heard as well as the lack of power and jump in revs. Since the wheels were all still spinning the same speed VDC never kicked in. The issue with it not going into park is explained by the fact that I put it into drive and the broken half of the axle began to spin and since the breaks are on the wheel and not the axle it was still spinning when I put it back into park. (Definitely not healthy for tor the parking pawl.)

The guys at AWD Auto in Kirkland did a great job of fixing her back up. The repacked and rebooted a used axle for me which came to $330 installed.

-FOX-
Inner joint- interesting. I figured if it was an axle it would have been an outer, and *that* you would have seen looking under it.

Dave
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Old 10-06-2011, 10:58 AM   #17 (permalink)
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The really weird thing is there were no symptoms prior to failure other than the split boot. I knew the boot was split and was waiting for it to start making some noise before I replace the whole axle.

It really happened in an ideal situation as far as total failure goes. I had just driven 90 miles to pick up my girlfriend who had her mom's car. I was following her, we were going to drop off her mom's car and then drive the 100 miles back to Seattle. It did fail at freeway speeds but there was very little traffic and I was able to shift into neutral and coast across two lanes onto the shoulder with no issues. My girlfriend saw me drop out of traffic and she pulled over onto the should as well 100 yards down the road. She called AAA and I called the shop and with-in an hour we had the car on a roll-back heading to Kirkland 95 miles away. She has AAA+ which covers up to 100miles of towing. I'm glad it didn't fail further from home, while my girlfriend was driving the car, or worse on top of the pass with no cell service and sub freezing temps.

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