For the most part, your Volvo’s AWD system is sophisticated yet simple and fairly robust overall. It does however have a few foibles to keep an eye on, especially in salt belt areas.
One of the failure modes of the system involves the angle gear collar/sleeve. Your AWD Volvo has a transverse mount engine, which means that when you look under the hood the engine lays sideways versus front to back like traditional rear wheel drive vehicles.
As a result, in order to power the back wheels, the power from the engine and transmission has to be redirected 90 degrees from the engine direction. To do this, Volvo uses an angle gear. It’s a simple device with a ring and pinion gear much like a traditional differential. This angle gear is bolted to the transmission and power is transferred from the transmission to the angle gear via the angle gear collar/sleeve.
The collar/sleeve is a simple splined tube that mates the output from the engine transmission to the input of the angle gear which then spins the driveshaft for the rear wheels. The collar is allegedly designed to be a weak link/failure point should you decide to overload the AWD system with too much power! I’m not certain of that, and the failure mode that I see most often does not usually involve lead footedness, but rather RUST!
In this case, the collar/sleeve splines have turned to rust powder! Once the rust has progressed to a certain point the power from the engine/transmission no longer gets transferred to the angle gear and, sadly no more all wheel drive for your Volvo! This process usually occurs noiselessly and without triggering any warnings in the system. Failure is most often observed during the first Winter drive where your previously unstoppable Volvo gets stuck and the nice folks helping you push observe that the rear wheels aren’t spinning with the front despite the mighty AWD symbol on your trunk.
Unfortunately, many times, it is the angle gear input shaft splines that rust instead of the much more inexpensive sleeve/collar. In this case the angle gear must be rebuilt or more likely, replaced. This can be quite expensive!
If you live in a rust belt area, you may want to have your AWD Volvo’s angle gear collar/sleeve inspected and lubricated to prevent this failure. If you suspect your AWD has failed you may have already fallen victim to this insidious rust issue.