'Touchy-Touchy' Transfer Case
I thought this title would be appropriate for a short article about all-wheel drive systems commonly found on many GM trucks, such as the all-wheel drive vans and the popular Bravada and Envoy SUV. These trucks used to use a single speed transfer case that employed a clutch pack with a unique viscous clutch that would virtually lock up if the front and rear propeller shafts spun at different speeds. At low speeds, such as turning a corner or parking lot maneuvers, the clutch would allow a little slip so that the driver would not get the scuffing sensation of a locked-up transfer case. The clutch was simple and quite durable.
The newer design transfer cases use an electric motor to clamp or apply the clutch pack in the transfer case. This system uses a front and rear propeller shaft speed sensor monitored by a transfer case module. When the module senses a difference in the propeller shaft speeds, it applies power and ground to make the electric motor apply the clutch until the shaft speeds match, and then it releases and applies as needed.
This "apply and release" of the transfer case motor can happen in a fraction of a second. As you have probably guessed, the fluid requirements for these transfer cases are very specific. Seems simple enough, so why the "touchy-touchy" in my article title?
These transfer case designs were meant to keep both propeller shafts turning at the same speed. A set of tires that does not have the same rolling circumference or a speed sensor reading incorrectly will keep the transfer case clutch applied, and will put a constant and tremendous strain on the driveline (picture the transfer case trying to make different size tires turn at the same speed on dry pavement). It's not uncommon to find damaged front or rear differentials and burned-up transfer cases due to worn tires or even tires that are incorrectly inflated.
The best way to check tire circumference is to make a mark at the 6 o'clock position on all four tires. Roll the truck in a straight line about 10 car lengths. Line up the marks at 6 o'clock once again. All the tire marks must be within one inch of the
As long as your customers are aware of the importance of maintaining proper tire pressures and even tire wear, they should not expect any problems from the all-wheel drive systems. GM has released a technical service bulletin (No. 99-04-21-005E) regarding flushing the transfer case to try to relieve grabbing clutches that can be felt when turning or parking as the jerky sensation we are all familiar with on four-wheel drive vehicles. These important tire warnings apply to our automatic transfer case trucks as well.
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