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Thought on AWD Bevel gear failures

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
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1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
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Thought on AWD Bevel gear failures

Post by befarrer » Wed Feb 26, 2020 7:51 am

Has anybody done any monitoring of oil temperatures or attempted to cool the front bevel gear in the AWD Volvo's? Considering that a hypoid gear can make considerable heat, the bevel gear is located behind the engine, out of the air stream, under the turbo, the hottest part in the engine bay. I hit some parts under my car after a 80KM highway run in -10c weather. The oil pan was 60C, bevel gear was 85C, rear differential 40C. I have brand new tires, but once tires start wearing down slightly between rotations, and add another 30C of ambient temperatures, I can see the bevel gear oil getting well over 100C. I wonder if this is a contributing factor to the failures?
98 V70 GLT AWD
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Re: Thought on AWD Bevel gear failures

Post by jimmy57 » Wed Feb 26, 2020 9:47 am

The death of angle gears is almost always because the viscous coupling seizes and the rpm differential that the VC provides isn't there. So the AG gets the stresses and builds heat that it is not designed to accommodate. In my experience somewhere around 125K miles is when the VC has often seeped the oil out of the viscous gel and begins to seize. There is a VC test where you lift one rear wheel with gear lever in park (locks the final drive gear in trans and the final drive case is the drive splines for AG spline connector cuff) and a torque wrench on a lug nut positioned radially perpendicular ?? to wheel center should yield with a slow rotation at less than 80 ft-lb +/-. A failed VC will often take enough torque that you tighten the lug bolt and it jumps at something over 125 ft-lb in jerky motions. Or it doesn't move at all and you would break the lug bolt before it would move.

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Re: Thought on AWD Bevel gear failures

Post by scot850 » Wed Feb 26, 2020 10:49 am

What jimmy57 makes sense, he has a lot of experience, but I have not seen the VC failures he has seen, but my sample size is way smaller.

There are a list of issues that I would add to his suggestions and most are down to relatively simple maintenance or in most Volvo cases, lack there of!

1) Oil changes of the trans/angle gear/rear diff oils. Ask most owners when they were last done and I'll guarantee the regular owner will look at you like you have two heads. I have never been into a dealership and had the service/oil changes ever suggested unless I suggested it. The transmissions (auto) in the 90's Volvo said were 'sealed' for life which in reality was 'bollocks'. The regulars here know full well that 80,000km/ 50,000 miles is probably the max for a flush. The relative cost of the change, even at the dealer (here it about $500 CDN which is stupid) and with a synthetic oil should keep these transmissions in good shape. While not a scientific number, I would suggest every 5 years if you don't do big mileage. If you do it yourself you can increase this frequency. This relates to FWD or AWD's.
2) Angle gear - never had a suggestion from a dealer on changing this. I would suggest the same frequency as trans. I would guarantee most owners have never had this done.
3) Driveshaft/prop-shaft maintenance. More so the prop-shaft. Regular checking of the condition of the front UJ checking for movement and while there the rear seal for the angle gear should be checked for leaks. Then check for condition of the center bearing support, and although I have not seen it often, the rear UJ which seems to lead an easier life than the front one as the front one is right beside the heat for the catalyser.
4) VC (viscous coupling). AS per jimmy57 there is a somewhat sketchy test for the VC operation. The VC itself has no servicing other than checking for leaks from the clutch fluid and more importantly the front bearing which can literally collapse and can be sometimes heard by a 'whoose-whoose' or light grating noise when turning.
5) Rear diff. Again never had a dealer suggest changing this on an AWD. For the first time I have had the recommendation for our 06 XC70 to have the Haldex system serviced. Car is now 14 years old (although only 100,000 miles). That suggestion only came to light when I asked when it should be serviced. Now how good is 14 year old oil and the condition of the filter? Changing the oil in a P80 rear diff is challenging but not impossible. Again check for leaks at the diff axle seals. Again these do leak and take a fair amount of work to replace. A seized rear diff requires the dropping of the whole rear sub-frame as you have to separate the clamshell sub-frame to remove it and the bolts Volvo thoughtfully fitted from the top preventing easy removal. Great design!

If owners understood the cost of replacing these parts relative to the cost of maintenance cost then some might actually do it. Volvo does not help the process, and people understandably get scared when Volvo claim you have to use their ridiculous priced special oils only, but as we now know there are excellent aftermarket oils that are equally good for the purpose.

Some of the issues are just poor design, but the bigger issues are caused by lack of maintenance bad service advice from dealers. Add to that Volvos usually running for a long time after something should be fixed and becoming 'just old cars'.

Final points are again lack of understanding on the old AWD system and probably any AWD system of tires. Particularly the P80 AWD system if you have a tire failure except when the tires are virtually brand new, requires new tires all around. I have have seen many AWD cars, including Volvo where owners have fitted one new tire on a car with half-worn tires as they are cheap. This loads all the AWD system. Also lack of tire rotation adds to these issues.

Not trying to scare the crap out of folks, but maintenance is key here more than anything.

Last final point is driving style. The systems are fairly robust but like many things in life you get some drivers who are just rough on their cars. It is hard to tell how an old car has been driven, so it is always an unknown. All you can go by is maintenance history.

If you need any further advice feel free to PM me.

2000 V70 R - still being an endless PITA
2006 XC70
2003 Toyota 4Runner V8 Limited
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1993 850 GLT -Sold
1998 V70 XC - Sold
1997 Volvo 850 SE NA - Went to niece in California - Sold
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