Floor jack placement
Jacking up a vehicle by placing a floor jack under the differential (pumpkin) of a vehicle has been standard practice for many years and as far as I know, never caused any damage. Now we read in owner’s manuals that this is not an acceptable practice. What’s the deal? Has it always been bad practice or are rear axle housings now of lesser quality?
Far be it from me to advise anyone to disregard any warning in an owner’s manual, because they must have had a good reason for putting it there in the first place. I would only jack up any car according to the way they tell you to do it in the manual. That being said, I have routinely jacked up many older classics by their differential housings on cars with solid rear axles with no ill effects. And I have not seen any admonitions not to do so in their shop manuals. But then I am talking about cars built 50 years ago.
In the last 30 or 40 years, carmakers have worked to make cars as lightweight as possible to help with fuel mileage. It may well be that the axle and differential housings on modern cars are not designed to take the car’s whole rear weight at the center of the housing. In normal use the wheels and hubs are supported by springs and shock absorbers a few inches inboard from them. But if you jack the car up by the differential, they will hang down as far as the springs will let them. That means that the weight of each wheel is cantilevered from the pumpkin where the axle housing attaches if you jack from the center.
Another situation that the engineers may be taking into consideration is that if you jack up a car from the differential, it will be prone to tipping one way or another. You would never want to get under a car jacked up from the differential housing. You always need to drop it on jack stands before doing any work on it.
If all you are doing is changing a tire, it is much safer to lift one corner and leave the other three wheels on the ground. But if you are going to be doing a brake job or something similar, you will need to jack the car up and put it on sturdy jack stands at each corner to be safe. Jacks alone can collapse or tip over when pushed even a little bit, with catastrophic results. I lost a friend in high school due to jack failure.
Be careful, and think about what you are doing. If the shop or owner’s manual says don’t do something, don’t do it. It is also necessary to check the shop manual for the correct jack points on any unit-bodied car if they aren’t apparent, because you can actually warp the structure if you jack up the car at the wrong point.