One of the benefits of coming into the ownership of a automotive repair shop with less experience in the professional auto world is being able to describe to customers, in layman's terms, how complex systems on cars work. This is certainly the case when it comes to differentials. I am, admittedly, not an expert on the intricacies of differentials, but the good news is that there are thousands of articles online that can get you deeper into the weeds on that score. That said, I'm here to tell you that the majority of customers, when told that they need a new differential or to have the differentials service give us the 1000-mile stare and simply say, "ok". When this service is done they are none the wiser about what they just paid for. But it is good to have a general idea about these components because when they fail, they are expensive!
So, I think I'm safe in saying that in general people understand the universal concept of gears and gearsets. After all, we've all had that toy that exploded when we were kids and inside were little plastic cogs that alluded to how power is transferred. Differentials are essentially balls of gears (great name for a band by the way). To step in the other direction for just a minute, transmissions have all sorts of gears inside them, and it's all about ratios: smaller gears drive bigger gears for certain speeds or matching gears drive each other or larger gears drive smaller gears. The various speeds that the cars goes are all determined by these ratios resulting in "torque". Differentials use this same concept of gear ratios except that they have an added wrinkle: because they are applying power to the wheels they have to have gears that are constructed in such a way that allows for turning, which is to say that one wheel might be turning faster then the other at times, notably while turning. Therefore, while transmissions are for the most part linear gears, differentials use opposing gears that have that ball-shape. Lost yet? Again, I'm not someone who is speaking in technical terms, we could go there, but I would have to call in the experts too.
Alright, so who cares? Why should anyone care about a lame old set of gears that may or may not exist on my car? Here's the short answer: money, money, $$$. Rebuilding diffs (the shorthand in our world) is pricey to say the least. It can be thousands of dollars to put new gears and seals in these mini-transmissions so it's worth knowing what they're up to. Why do they fail? Mostly this is due to the neglect associated with changing the fluid that keeps these puppies healthy. If that fluid is changed regularly it could be 300,000 miles before they need any major work. But, because they have no dipstick and they can be tricky to check (must be done from underneath the vehicle), they are subject to oversight. We often tell customers: hey, time to change the fluid - which results in the blank stare. We then say: if you don't it could be an expensive repair - it usually results in the green light to do the work. But, the lowdown on this is that this fluid should be changed every 30,000 to 60,000 miles. However, before you go rushing to the website to make an appointment, be aware that differentials are present on rear-wheel drive vehicles, all-wheel drive vehicles and four-wheel drive vehicles. If you have a front wheel drive vehicle, the differential is likely part of the larger transmission assembly called a transaxle and they could share fluid, but not always. If you don't know, ask us or your mechanic. They can tell you. But, whatever you do, don't shrug it off as another upsell. That's definitely something you would regret later.