Simply put, the transfer case in a four-wheel drive (4WD) or all wheel drive (AWD) vehicle is the component which allows the engine power to be directed to all four tires. Transfer cases are generally mounted just outside or next to the transmission. A traditional vehicle generally has a drive shaft that delivers power to just two of the wheels. The transfer case adds and connects a second drive shaft to the second pair of tires.
Transfer Case of Yesterday (Traditional Truck Type)
The transfer case of yesterday is a big, cast iron, crude, heavy gear box that bolted on the back of the transmission that used a set of gears to connect the front drive shaft to the rear drive shaft. They were generally controlled by a large shifter sticking out of the floor board and you also manually locked the hubs at both front wheels. You shifted into four wheel drive only on an as need basis. In older four wheel drives traditionally the rear wheels were the primary drive tires and the front wheel only came on board when you went through this ritual.
Modern Transfer Case of Today (Automatic Engaging Truck Type)
The modern transfer case is much lighter, built of generally aluminum or magnesium. They use a chain to connect the front and the rear drive shafts in the more common truck type configuration where the rear wheels are the primary source of delivering power to the pavement. What is significantly different is the methods of control and the addition of an electrically controlled clutch. No longer do you need to go through the ritual listed above, the vehicles computer monitors for wheel slip and when it registers wheel slip, it signals an electric motor mounted on the transfer case which actuates and connects the front and rear drive shafts through a clutching system. This is much easier to operate but does present challenges when it malfunctions. Although it sounds fairly simply, a good portion of our repair industry does not know how to diagnose these issues. The consumer will purchase a replacement transfer case when all they needed was a transfer case actuator motor, control computer, new tires or …. (See article on tire replacement & tire size challenges with automatic engaging all wheel drive systems)
Modern Transfer Case (Manual Engaging Truck Type)
Much like described above, with the exception of the transfer case not utilizing a clutching mechanism and engagement manually happening through a button or shift lever. Much like people preferring a “land line” over a cell phone, we find some people prefer the manual type operation where they feel some control. Maybe this is because it seems more objective or concrete in it operation. Either way, both have pros and cons and it is simply personal preference.
Modern Transfer Case (Full Time All Wheel Drive, Car or Front Wheel Drive SUV)
Quickly becoming very common since more SUV are moving to front wheel drive and more cars are utilizing all wheel drive. Different from both of the above, this is where the primary source of placing power to the pavement is through the front wheels and the rear wheels are secondary. The transfer case is generally much smaller and has no clutching mechanism. The manufacturer of the vehicle has generally moved the clutching mechanism in most cases to the rear differential. The transfer case in this application is mounted to either the right or the left side of the transmission between the drive axle and the front wheel. Sitting by itself, it looks more like a differential than a transfer case but is still described as a transfer case because of its function. Like the other automatic engaging transfer cases, tire replacement should happen as a complete set of four tires unlike the two at a time people are accustomed to. We see customers daily that have to buy a new transfer case because they did not follow this rule. (See article on tire replacement & tire size challenges with automatic engaging all wheel drive systems)
Signs You May Have a Transfer Case Problem (Traditional Type)
- Difficulty Shifting Into Different Ranges (4-Low, 4-High, 2wd)
- Noisy In All Gears
- Jumps Out of 4-Low Range or 4-High Range
- Fluid Leaking From Vent or Seals
- “Service four wheel drive light” (usually yellow)
- Vehicle goes completely into neutral while just driving
Signs You May Have a Transfer Case Problem (Automatic Engaging Type)
- Binding sensation when turning
- Popping noises on acceleration either in a turn or in a straight line
- “Service four wheel drive light” or “all wheel drive light” or “traction control light”
- Vehicle completely goes into neutral while just driving
A frequent transmission miss-diagnosis worth mention is the transmission is rebuilt or replaced, when simply a purse strap got caught on the transfer case shifter pulling the transfer case into neutral when exiting the vehicle. We see this one from time to time. This usually happens to an inexperienced transmission technician and they only figure it out after they have rebuilt or replaced your transmission. The honest one will tell you, the dishonest will simply shift the transfer case back into gear and you get a transmission you didn’t need.
Maintenance!! - Maintenance!! - Maintenance!! - Maintenance!! - Maintenance!!
The transfer case almost always gets forgotten. People say, “I never use the four wheel drive, what does it need maintenance for?” This is a wives' tale. The transfer case is always rotating and has power or torque going through it. This is regardless or not if you use the four wheel drive. Transfer cases are expensive and you definitely don’t want to buy one because transfer case services are relatively cheap. They usually hold less than two quarts of oil.
The transfer case fluid level & condition should be checked at every regular oil change to make sure it also has plenty lubricant (this is very important, transfer cases hold so little fluid they will run out, even if you never saw a puddle on the ground.
The modern transfer case, especially the type with clutch mechanisms requires more frequent maintenance than a traditional type transfer case.
Specialty or Exotic transfer case fluid is something you need to be aware of. In old days transfer cases were simply filled with gear oil or automatic transmission fluid (ATF). Some mechanics never got the memo from the transfer case manufacturer. We see the wrong fluid in transfer cases daily. Sometimes we catch it when it is already too late and someone has to buy a new transfer case. You need to be aware as a consumer that you are working with a knowledgeable Phoenix, AZ repair shop. We can all look the same and sound the same. Even doctors have been known to amputate the wrong limb from time to time.