Rear Diff, Viscous Coupling HELP

wammers said:

But there's nothing wrong with his VC. A seized viscous coupling will damage the centre diff, it will damage the front prop, it will damage the front diff sun and planet wheels. But it won't have a great affect on the rear diff because the rear diff is direct coupled through the drive. It will put weight on it yes but nothing like what the front diff has to take. A seized VC CANNOT destroy a rear diff in the way his was damaged without going through several front ones first. By the way how are you mate?

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Wammers, I'm going to have a pop at you here

I may be wrong, (unlikely, but its a while since I checked this out) but the last time I checked the VC, even though it's bolted on to the front of the transfer box, is actually simply limiting the slip between differential wheels in the centre diff so can't overstress one axle or another. In a schematic, the VC would still be between the differential gears inside the diff, not on the drive to the front axle. Its just 'coincidence' that it's on the front.

As far as I can make out, the P38 has a straight 50/50 torque split front to rear. The transfer chain drives the 'crown' wheel of the centre diff. The VC is just a link between front and rear diff gears, so the rear axle is not on a 'direct drive'. The rear will carry more stress (due to weight distribution, torque transfer, towing etc) but not more drive. A fooked VC will be bust in one of 2 ways, free or locked. Locked it can never put anymore than 50% to the rear, open it will transfer all the torque to the axle with the least grip. That will depend entirely on weight distribution. But the rear will never receive more 'drive' than the front unless you remove the front prop shaft.

As regards to how it will destroy more front than rear diffs, I don't know, maybe the rear axle is stronger (it certainly has a higher gross weight allowable).

This is one of the great dilemas of engineering design. In theory you should never be able to break the most expensive part of a drivetrain. The cheapest should alway go first, be it a half shaft or driveshaft component. But when LR did this with the early series halfshafts they were universally critiscised for 'soft' off roaders. We are now paying the price for that. In 4 years here I can't remember anyone coming on here with a bust half shaft, its all diffs. The Driveshaft knuckle sems to fail due to lack of grease rather than robustness. I think its another classic designed in fault to generate more cash for LR

SpudH said:

Wammers, I'm going to have a pop at you here

I may be wrong, (unlikely, but its a while since I checked this out) but the last time I checked the VC, even though it's bolted on to the front of the transfer box, is actually simply limiting the slip between differential wheels in the centre diff so can't overstress one axle or another. In a schematic, the VC would still be between the differential gears inside the diff, not on the drive to the front axle. Its just 'coincidence' that it's on the front.

As far as I can make out, the P38 has a straight 50/50 torque split front to rear. The transfer chain drives the 'crown' wheel of the centre diff. The VC is just a link between front and rear diff gears, so the rear axle is not on a 'direct drive'. The rear will carry more stress (due to weight distribution, torque transfer, towing etc) but not more drive. A fooked VC will be bust in one of 2 ways, free or locked. Locked it can never put anymore than 50% to the rear, open it will transfer all the torque to the axle with the least grip. That will depend entirely on weight distribution. But the rear will never receive more 'drive' than the front unless you remove the front prop shaft.

As regards to how it will destroy more front than rear diffs, I don't know, maybe the rear axle is stronger (it certainly has a higher gross weight allowable).

This is one of the great dilemas of engineering design. In theory you should never be able to break the most expensive part of a drivetrain. The cheapest should alway go first, be it a half shaft or driveshaft component. But when LR did this with the early series halfshafts they were universally critiscised for 'soft' off roaders. We are now paying the price for that. In 4 years here I can't remember anyone coming on here with a bust half shaft, its all diffs. The Driveshaft knuckle sems to fail due to lack of grease rather than robustness. I think its another classic designed in fault to generate more cash for LR

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The output shaft for the rear diff is splined into the diff planet carrier. Therefore when the morse chain turns the planet carrier it is direct coupled to the rear output shaft. This shaft is also splined into the centre of the VC. Power is transmitted from this shaft through the VC and into the diff and back out through the VC to the front output shaft. So that any difference in rotational speed between the front and rear output shafts causes the VC to slip and work the diff to prevent windup. So you are wrong on this occasion unfortunatly.

wammers said:

The output shaft for the rear diff is splined into the diff planet carrier. Therefore when the morse chain turns the planet carrier it is direct coupled to the rear output shaft. This shaft is also splined into the centre of the VC. Power is transmitted from this shaft through the VC and into the diff and back out through the VC to the front output shaft. So that any difference in rotational speed between the front and rear output shafts causes the VC to slip and work the diff to prevent windup. So you are wrong on this occasion unfortunatly.

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Hmmm! I don't have the benefit of first hand experience here so I'm not going to flat contradict you

Buuuuttt, If you look at the diagram of the VC in the RRC section of RAVE you will see that the rear output shaft is splined into the rear differential gear, NOT the diff planet carrier. It then carries through the front front differential gear to the inside sction of the VC. The outside section of the VC, which is integral with the front output differential gear, is then splined onto the front driveshaft output.

So on paper I'm right, at least in the case of the RRC, which we are told is all but identical in operation to the P38 VC unit

Don't worry Wammers, you'll still be right the other 99.99% of the time

SpudH said:

Hmmm! I don't have the benefit of first hand experience here so I'm not going to flat contradict you

Buuuuttt, If you look at the diagram of the VC in the RRC section of RAVE you will see that the rear output shaft is splined into the rear differential gear, NOT the diff planet carrier. It then carries through the front front differential gear to the inside sction of the VC. The outside section of the VC, which is integral with the front output differential gear, is then splined onto the front driveshaft output.

So on paper I'm right, at least in the case of the RRC, which we are told is all but identical in operation to the P38 VC unit

Don't worry Wammers, you'll still be right the other 99.99% of the time

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Think you should stop looking at pictures and read the description. Transfer box description and operation page 8 of overhaul manual.

Hey, I was reared on comics!!!

But just for you I'll have a read and see if I interpret it as badly as you

Yep, I think the words match the picture, from RRC section of RAVE:

The front and rear output shafts 10 and
11 are splined to the differential output pinions. The
front output shaft 10 forms the outer member of the
viscous coupling, while the inner member plates are
splined to the rear output shaft 11.

Its only function is to limit the maximum difference in
speed between the two shafts, with the effect of
automatically locking the differential.

You'll note it says the rear output shaft is splined to the pinion, not the carrier

SpudH said:

Hey, I was reared on comics!!!

But just for you I'll have a read and see if I interpret it as badly as you

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Can't really see how. "Rear output shaft passes through differential and engages on planet gear carrier" can be misinterpreted to be honest.

wammers said:

Can't really see how. "Rear output shaft passes through differential and engages on planet gear carrier" can be misinterpreted to be honest.

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Well theres the problem, I've been looking at the RRC section as thats where the pictures were.

Looks like you're right and the P38 is very different to the RRC

wammers said:

Think you should stop looking at pictures and read the description. Transfer box description and operation page 8 of overhaul manual.

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Jeez, I just checked that section of RAVE, its completely different from the RRC. Thanks again for the education Wammers!

Well that proves your point about the rear diff being unaffected by the VC.

SpudH said:

Well theres the problem, I've been looking at the RRC section as thats where the pictures were.

Looks like you're right and the P38 is very different to the RRC

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You are right and Wammers is wrong on this one, the rear drive passes through the centre of the diff coaxially with the front output shaft and the VCU sits across the 2 shafts and limits the rotational difference between the front and rear outputs from the centre diff. With the VCU seized, it is the same as having no centre diff and transmission wind up results. IMO the reason the front diff fails first is because it is under more stress due to loads produced in turning. It would be perfectly possible to eliminate the VCU and still have 4WD, just no limited slip between front and rear, I suspect that with 4 wheel traction control the VCU would be hardly missed.

However you will not convince Wammers on this one as he has led forth on this subject many times and refuses to accept any answer but his own. I will at some point put photos up showing just how all this is arranged when I have got my transfer box off the gearbox and on the bench. I do have a very good drawing of the centre diff showing the shaft arrangement and the diff.

SpudH said:

Yep, I think the words match the picture, from RRC section of RAVE:

The front and rear output shafts 10 and
11 are splined to the differential output pinions. The
front output shaft 10 forms the outer member of the
viscous coupling, while the inner member plates are
splined to the rear output shaft 11.

Its only function is to limit the maximum difference in
speed between the two shafts, with the effect of
automatically locking the differential.

You'll note it says the rear output shaft is splined to the pinion, not the carrier

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Oh deary me. Read the newer description version in P38 transfer box overhaul manual. Things are a lot clearer. The planet carrier is driven by the chain. The rear output shaft is splined into that, so the rear shaft is direct coupled to the propshaft. The rear output shaft passes through the sunwheel shaft and is also splined into the centre of the viscous coupling power is transmitted through the fluid back into the diff. Any slippage in the vicous coupling caused by different rotational speed of the two shafts is taken care of by the differential. If the viscous coupling is seized the diff CANNOT function. As we are talking P38 here it maybe advisible to get a copy of the manual for that transfer box. Borg Warner 44-62.

SpudH said:

Jeez, I just checked that section of RAVE, its completely different from the RRC. Thanks again for the education Wammers!

Well that proves your point about the rear diff being unaffected by the VC.

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Look again, I quote, THE PLANET CARRIER CARRIES 3 SETS OF GEARS which mesh in pairs to maintain the correct directional relationship between front and rear differential outputs. The rear output shaft passes through the differential unit, engaging with the planet carrier and protruding through the sun gear shaft to locate the VCU inner spline. The sungear shaft locates to the VCU outer spline.
Thus the VCU is simply in parallel with the diff and limits rotational differences between front and rear outputs. A siezed VCU cannot harm the centre diff as it effectively bridges it.

Datatek said:

You are right and Wammers is wrong on this one, the rear drive passes through the centre of the diff coaxially with the front output shaft and the VCU sits across the 2 shafts and limits the rotational difference between the front and rear outputs from the centre diff. With the VCU seized, it is the same as having no centre diff and transmission wind up results. IMO the reason the front diff fails first is because it is under more stress due to loads produced in turning. It would be perfectly possible to eliminate the VCU and still have 4WD, just no limited slip between front and rear, I suspect that with 4 wheel traction control the VCU would be hardly missed.

However you will not convince Wammers on this one as he has led forth on this subject many times and refuses to accept any answer but his own. I will at some point put photos up showing just how all this is arranged when I have got my transfer box off the gearbox and on the bench. I do have a very good drawing of the centre diff showing the shaft arrangement and the diff.

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Please read the description of operation of the Borg Warner type 44.62 transfer box page 8.

Datatek said:

Look again, I quote, THE PLANET CARRIER CARRIES 3 SETS OF GEARS which mesh in pairs to maintain the correct directional relationship between front and rear differential outputs. The rear output shaft passes through the differential unit, engaging with the planet carrier and protruding through the sun gear shaft to locate the VCU inner spline. The sungear shaft locates to the VCU outer spline.
Thus the VCU is simply in parallel with the diff and limits rotational differences between front and rear outputs. A siezed VCU cannot harm the centre diff as it effectively bridges it.

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And what does it say the planet carrier is driven by?

This is as clear as mud :doh:

Wammers, he may have a point, I just noticed the the drive input on the P38 is to the differential casing, not the planet carrier. I'm still trying to work out what difference that makes

the output shaft isnt directly driven but through diff ,if you remove vc from front vehicle will stand still and diff spin,it does work on the same principles as earlier bw but uses updated parts

SpudH said:

This is as clear as mud :doh:

Wammers, he may have a point, I just noticed the the drive input on the P38 is to the differential casing, not the planet carrier. I'm still trying to work out what difference that makes

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Edited for accuracy. I am blind.

jamesmartin said:

the output shaft isnt directly driven but through diff ,if you remove vc from front vehicle will stand still and diff spin,it does work on the same principles as earlier bw but uses updated parts

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The front one isn't the rear one is.

i think someone might be eating humble pie datatek and spudh were correct

jamesmartin said:

i think someone might be eating humble pie datatek and spudh were correct

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It would not be the first time. I find it makes me fat though. Yes i have had a closer look and it does seem that way.