Up to Standards
- Subject: Operation of viscous couplings in transfer cases
- Units: BorgWarner 4473 and 4410
- Vehicle Applications: GMC Savannah and Chevrolet Express vans, Lincoln Aviator, Mercury Mountaineer
- Essential Reading: Rebuilder, Diagnostician, R & R
- Author: Mike Weinberg, Rockland Standard Gear, Contributing Editor
Working with viscous couplings
Viscous couplings have been used in automotive applications for many years in transfer cases, rear-end differentials, torque converters and radiator cooling fans. Although the concept is not new, the viscous coupling is not well understood in the repair industry and creates a number of diagnostic and repair issues. Part of this problem occurs because all viscous couplings are by necessity sealed units that cannot be inspected. The second part of the problem is a lack of understanding of how they work, principal causes of failure and the proper diagnostic path to follow.
A viscous coupling (Figure 1) consists of a metallic case that is filled with alternately splined steel discs resembling clutch plates (figures 2 and 3). A center shaft is splined to mate with the rear output shaft of the transfer case, and the metallic discs are alternately splined to the inner shaft and the outer case, as are the clutch plates in an automatic clutch drum.
The inner shaft is sealed at both ends to prevent the intrusion of transfer-case lubricant, and the coupling is filled with a highly viscous fluid. The fluid fill in the coupling is a silicone compound that is thermodynamic. This fluid fills about one third of the case.
The transfer case in which the coupling resides is a full-time unit, which means that it is always capable of splitting torque between the front and rear axles. These are “reactive” transfer cases, and the unit is always capable of creating a torque split between the two drive axles with no driver input.
The manner in which the viscous coupling reacts to transfer torque is caused by a difference in wheel speed between the two drive axles. When there is a slip, the alternately splined steel discs rotate but do not touch each other. This rotation causes the silicone fluid to expand very quickly in reaction to the disc movement, and the expansion causes the fluid to fill the entire case of the viscous coupling. The high viscosity of the fluid causes the disc to now rotate in the same direction, as the discs have to “shear” through this heavy, sticky fluid.
Since the drive sprocket for the chain is splined to the inner shaft of the viscous coupling, power is now sent to the front output shaft, sending torque to the front axle. When wheel speeds equalize front to rear, the discs stop turning and the fluid cools, so the coupling ceases to send power to the front axles. Understanding this theory of operation will lead you to a correct diagnostic routine.
Almost all transfer cases equipped with viscous couplings fail for one of two reasons. The No. 1 cause is mismatched tire sizes or pressures. Save yourself a lot of time and effort and learn to measure tire size on every vehicle you work on as part of your troubleshooting routine. The first step you make in any type of computer-related repair or diagnosis is to check battery voltage, as nothing you get from the computer will be worth your time if the battery voltage is not up to specs. The same applies to tire size; if you don’t measure you are just wasting your time. Measure means measure, not reading the tire sidewall label.
There are two ways to do this: easy and hard. The hard, slow way is to measure the tire circumference with a tape measure around the center of the tread. This means you have to pick all four wheels off the ground and measure and record each tire’s circumference, making sure the tape is in the center of the tire. Another way to do this slowly is to set the car on flat ground with at least 10 car lengths of open space in front of it. Mark each tire with chalk at 6 o’clock and then drive the car in a straight line with someone watching until the wheels make 10 revolutions. Stop the vehicle and make sure all the marks are still at the 6 o’clock position on all four wheels. If they are not, the tires sizes are not the same.
The easy way to measure all four wheels with spot-on accuracy is to use a “stagger gauge,” a large caliper that will read tire circumference directly. Center the stagger gauge on the axle and note the readings, and in less than five minutes one man can measure all four wheels accurately. At the same time check that all tire pressures are the same. We sell a good stagger gauge for about $50, and if you wish to make money in our field you should have one in your toolbox.
The viscous coupling is not a computer; it has no internal program or intelligence and merely reacts to its environment. If there is a difference in wheel speed it does not care whether the difference is due to slip or tire size; it only reacts and sends power to the front axle. If the tire sizes are mismatched the viscous coupling will kill itself trying to compensate. It overheats and locks up, and the driver comes into your shop with a complaint of wheel hop in tight turns. What actually happens is the coupling gets so hot it kills the seals on the inner splined shaft, which allows the transfer-case lubricant to fill the viscous case or allows the silicone fluid to escape into the transfer case, neither of which produces a happy outcome.
Other common causes of coupling failure are stripped axle splines or operating the vehicle with one of the driveshafts missing.
The second major cause of failure in these types of transfer cases is lack of lube. Most of the units are filled with ATF and are splash lubricated (Figure 4). Because these are full-time units the chain is usually moving, causing the ATF to form a mist and exit the transfer case through the vent. No one ever checks the fluid levels on their transfer case until it is too late. By the time you get to it, the chain is overheated and stretched, the coupling is dead and the front-case bushings are terminal. My suggestion is to have your customers come in for a fluid check every 10,000 miles. It binds your customer to you, it may produce other work you find on the vehicle and it preserves your warranty funds.
Unless you begin to measure tire sizes correctly you are bound to come up with a variety of mysterious, seemingly unsolvable problems. Active, computer-controlled transfer cases measure wheel speed very precisely on all four wheels. A difference of 1/16 inch in tire size is enough to set codes.
You will also see this tire-size discrepancy cause problems in rear and front differentials as well as the differentials in front-wheel-drive transaxles. What has tire size got to do with differentials? Elementary, Watson; the differential should not be working in straight-line driving, only on turns. However, if the tires are rotating at different speeds because of size differences, the side gears are also rotating at different speeds, adding to wear on the differential carrier and components. People have been known to drive for weeks on a doughnut spare until they get the time and money to replace a tire. How does that affect your warranty?
The last case to be made here is if your tire sizes are wrong to start with and you don’t know that, the new viscous coupling you just installed in your repair will have a life expectancy of slim to none, and it’s your dollar that buys a new one.
Now that we have a better understanding of the viscous coupling, we can take a look at what a transfer case looks like internally. BorgWarner has used viscous-coupling designs for many years. In 1990 it developed the 4472 transfer case, which GM used from 1990 to 1998. BorgWarner then brought out the 4473 in 2001 for the GMC Savannah and Chevrolet Express vans. At the same time BorgWarner provided the 4410 transfer case (Figure 5) for the Lincoln Aviator and the Mercury Mountaineer. Although the cases are different between the GM and Ford units, the internal parts are much the same. The viscous couplings are the same part number and have an aluminum case instead of steel like the one in the 4472. Both units use an open planetary differential from the viscous coupling to the rear output shaft to achieve a torque split of 35/65 front/rear to the drive axles.
When the rear case is removed for disassembly (Figure 6) you’ll see a short output shaft attached to an annulus (ring) gear for the planetary differential.
The Torx-head bolts (Figure 7) that retain the main case halves are self-tapping. Should you buy a new case half for replacement, the front-case holes will not be threaded; the bolts will self-tap as they are installed.
These units have full-time four-wheel drive with no low range and no external controls for the driver. They are compatible with antilock-braking systems, and the GM 4473 is interactive with brake-based traction controls. The viscous coupling is splined to the drive sprocket for the chain at the front of the coupling and to the rear output shaft at the rear of the coupling, allowing the viscous coupling and planetary differential to create the torque split front to rear.
These are very simple units, and when you understand the operation of the viscous coupling, they should be a good profit generator for your shop.