Volvo: Rear Differential Noise, high mileage car, metal shavings

Question
Hi, first of all thanks for offering your services. Okay, I have an '87 240 sedan. Whenever I slow down (say I'm coming to a stop sign) there's this slight whining sound coming from my rear-end. It's like the gears or somthing is winding down until I stop. I called a place and they said it was probably my differential. I asked if I could lubricate it, but they said once it starts making noise I'm probably going to have to replace it. Is there any way to replace it cheaply, or what exactly is the differential anyway? Thanks for whatever help you can give me. -matt

Answer
The differential is the gear box on the rear axle that basically divides the power coming from the rotating drive shaft to the 2 back wheels.  If you look under your rear bumper, it's the steel tear drop between the two wheels right in the middle, about the size of a basketball.  When differentials start wearing out, they can make noise; but keep in mind that some noise is normal, especially in a high mileage car.  The first and easiest fix for this noise is to flush the differential with new lubricant (it uses motor oil).  Many times noise can be due to low or dry conditions inside the ball that cause the gears to make more noise; something that would be fixed by new oil.

If that isn't the case, and the noise remains the same, then you could do a few things.  First; ignore it.  Yes the car is old and wearing out, but drive until something actually goes wrong.  I say this because I've never really seen a rear differential fail in these cars.  It can happen, but yours would have to be in real bad shape for it to fail to the point that the car wasn't drivable.  Second; you could get the shop to flush the differential and replace the fluid, hoping it would be a quick and cheap fix.  If you go this route, finding out what stuff came out with the old oil would be key to assertaining the internal condition of the differential.  If a bunch of metal shavings appear, for example, then you're in bad shape.  Third; if reliability is key, you could have the shop either rebuild or replace the differential.  It would eliminate the problem, but that type of job would be very, very expensive.

It will come down to how much money you want to spend compared with how critical it is that you don't want to break down.  Personally, if I thought the sound was loud enough, I would probably flush the oil and replace it; then keep driving the car.  Keep in mind, though, that I usually take the cheaper and less reliable route when it comes to the mechanics of a car as inexpensive as what we drive.