Cooked Bevel Gear?

[Smurfs]

Damn, new to the Volvo XC, or any AWD for that matter . Bought the car as family car 2nd hand a year ago, and we are about to take a family holiday with the Caravan. I put on two new tires a week ago, anyhow they ended up on the rear axle, as I will be towing, and the front tires are good. Took the car on the open road today, and after 45km +-5km from my Dad's house, smoke, not good... Stopped immediately when I noticed/smelt the smoke. It seemed to come from under the engine, when I opened the hood. Got the car on the stands and it seems as though the smoke came from the Bevel Gear. I think it may have boiled, I took the car around the block after checking things out, and I now have a slight whining noise when I take my foot off the gas, and let it coast. I hope that I just have to top up/change the bevel gear fluid, or have a cooked it totally? What should I do, what whoudl I have checked out? The service agent's here in South Africa need a little spoonfeeding? Is there anything I can do to put my mind at ease? Damn, wish I had done 10hours research before changing tyres... :( Oh, we have a 2001 Model.

[BillAileo]

Smurfs,

I'm no expert, but I doubt your new tires would have anything to do with a problem with the bevel gear. Having differing tire diameters could adversely affect the viscous coupling which early XC70's had but I haven't heard of this causing bevel gear problems. About the only thing I can think of an owner doing in regard to the bevel gear is checking for the presence of gear oil and changing it. If you'd like more details on how that can be done check out the following posting on the resources section of this website:

http://www.volvoxc.com/resources/how-to/pdf/2001-v70xc-bevel_gear_oil_replacement.pdf

That posting deals with earlier XC70's but I believe the bevel gear is basically the same on later models.

Bill

[bwik]

You don't list what YEAR vehicle you have.

Anyway, buying "2 tires" because they are cheap or whatever is an absolute no-no. The 4 tires should be equal in circumference.

Especially, the larger circumference wheels should NOT be on the back. This will lead the AWD system to believe the fronts (with small wheels) are turning faster than the rears (with big wheels). THis will make the car send power permanently to the rear. THis will cook your AWD system in no time flat.

Your AWD is probably destroyed unless you are very lucky. Next time, it would be better to read the owner's manual regarding tire procurement and rotation.

In cars with a bevel gear, change all 4 tires at the same time with the exact same make, model and "batch number." Then, rotate back-to-front every 7,500 miles. This will avoid the expensive smoking. [thumbup]

[Smurfs]

Thanks for the replies so far. I am hoping that the bevel gear is just low on gear oil. Will keep you all posted. I agree with BWIK, as to what the failure mode is, only after inspecting the car myself did the penny drop. Now I understand the whole transmission better. The Bevel Gear just isn't made for continuous heavy duty. Basically the back wheels were trying to "push past the front", because they thought they needed to. I would imagine that you should have a temp sensor to detect overloading on the bevel gear, and then disengage the visous coupling, and put the car into front wheel drive mode only???

[BillAileo]

Smurfs,
You are wise to agree with Bwik. I'm afraid I have misunderstood the relationship of the VC to drivetrain problems. I had thought it was the component that would get cooked by diameter issues rather than the bevel gear....

Bill

[tranmkp]

you may be ok, exact thing happened to my daughters car (her spouse is not a gear head)

jack up the car and see if you can see seal leaks on the BG, if not then refill with synthetic and drive it till the BG is hot, drain and refill. YOu may have to do this 4 times to flush the case - Or you can pull it and drill a drain hole.

Be sure to check the vent hole is not blocked.

You should be ok.

[bwik]

Regarding rotation, keep this in mind.

The fronts wear faster. After 7,500 miles, the fronts are worn down and the "smoking gear" becomes a possibility. Rotate.

Next, you can do a longer, double interval until 22,500 miles. Think, at 15,000, the tires were exactly even again. Then at 22,500, the fronts are worn. Rotate.

Then, at 30,000, they are even. And 37,500, the fronts are worn. Rotate.

Goes like that. So even with the 7500 interval, you can do 15k intervals without a problem. Enjoy :)

[BillAileo]

Bwick,

Forgive my ignorance, but I'm still trying to figure out why the tire diameter would adversely affect the bevel gear rather than the VC. The following is a 1997/98 description by Volvo of the relationship of the transfer differential added to the transmission and the bevel gear (source romhttp://www.volvoclub.org.uk/press/pdf/1998V70AWD-PressDocV7-40.pdf):


"Volvo's transmission has been modified and extended to include a compact
transfer differential. The power is transferred between the transmission's
differential housing and bevel gear's tubular shaft via a splined joint. The bevel
gear's wheel and pinion and the final drive have the same numerical ratio between
the front and rear wheel."

The same source describes the function of the viscous coupling as follows:

"The viscous coupling, which is located ahead of the rear differential, appears as
a long tube filled with silicone oil and contains a large number of discs. The
viscous clutch is designed to use the silicon oil's viscosity to transfer power via
the discs to the rear wheels when the front wheels start to spin. Every other disc is
connected to the front wheel part of the shaft and the alternating discs are
connected to the rear differential. When the discs revolve at a different rate the
silicone oil in which the discs rotate becomes more viscous and thus increases the
pressure. As this pressure increases, torque is transferred to the rear wheels.

If the viscous coupling is subjected to intensive slipping, the silicone oil and
discs become very hot. At this point, the coupling locks-up to prevent any
slippage."

I can see how differing diameters would cause severe overheating problems for the VC but I'm missing how that translates into bevel gear (which does not serve a differential function) failure unless a locked up VC causes such excess torque on the gears in the bevel gear that they would fail ??

Bill

[Smurfs]

Bill

You are spot on, the VC was locked up by "slippage" and the gears in the bevel gear have meshed into each other harder than they should, Excess Torque.

This caused the excess heat, and caused the bevel gear fluid to vent through the vent plug. The Smoke was definately the fluid out the bevel gear getting a bit too hot. I will start taking it apart tonight to assess the full extent of the damage. I just don't understand why there is no temperature warning on the bevel gear, It could let you know when you overload the AWD, and help to avert disasters. This would be Especially useful for those of us silly enough to put the new tyres on the front.

Oh, I read the manual again last night, and nowhere does it mention that new tyres must be put on the front axle. It also doesn't explicitly state that all four tyres must be replaced at the same time, there are vague mentions of mixing tyre dimensions and brands may cause damage to transmission.

[Omega5]

Yah, I'm with you on that one Bill, it does sound a little strange that the problem would propagate to the bevel gear instead of the coupling.

But I can kind of see how it can happen. Once the coupling VS or else (makes no difference) decides to "lock" and drive both rear and front axle together, then because they have the same ratio, they naturally expect the axles to spin together as the same speed. BUT ...the front tires were smaller then the backs, they cannot and this difference in RPM will result in torque or stress on SOME component .. sux that the BG got in the way of this one. It does make sense that once the coupling locks the problem propagates to the next component in line. Something has to give eventually ... i would personally prefer that the tires would have gotten shredded rather then the mechanical components, but hey. Naturally the more expensive of hte two will give up first.

[Smurfs]

Well, after flushing (drain plug installed), and checking everything. Took her for a ride yesterday morning, and the slight whining is a little quiter, but it looks as though the worst is confirmed, the bevel gear has seen better days. I imagine I am going to look for a differential/gearbox repair speciallist before excersising the Volvo replace option. Phoned yesterday, and the new bevel gear is ZAR13500 (US$ 1800) ex labour. Apparently it is a really easy job to do :) I just wish there was a temperature sensor on the bevel gear to give you a little fore warning of imminent damage/failure. I will definatley be retrofitting one onto the bevel gear when I get it fixed/replaced. Anyone else interested?

[jdj2]

Smurfs,

Just to keep your options open, there is a salvage yard up here in Namibia (Spares Center - Windhoek - Phone # 264-061-226347) that has a wrecked V70XC. Looks like a rollover (all to common here) but they have already pulled the powertrain out of the chassis for inside storage.

It might be possible to get a used bevel gear unit this way for a more reasonably priced repair.

Good Luck!

[dalius]

My 2003 XC70 has a cooked bevel gear as well. My family is in Cape Town, on holiday from Pretoria. The "Transmission Service Required" message came on, yellow warning light came on. Took car to Swedocar Volvo in Cape Town. They said all the bevel gear (front diff) fluid leaked out through faulty seal and the gear is cooked :( Unfortunately, the warnings did not come on until it was too late. It was a serious leak.

To add to the troubles, Volvo South Africa does not have the part in stock - it has to be ordered from Sweden direct, 2-3 weeks delivery time. Can't find a used one from a scrap yard either. Looks like I may have to put my family on a plane and ship the car back home. Vacation over :(

This is the first Volvo I've ever owned and I am deeply saddened having bought it. AC went out first week after buying it. A few weeks later, developed suspension problems - had to replace tie rod ends, control arms. One tie rod end came off while going 100km/hr, nearly killed my entire family. Now this. Ughhh, so far Volvo gets an F in my book. And the Volvo dealer/service shops are totally incompetent (at least here in SA) - F minus. Unless things change drastically, this is the last Volvo this family ever owns.