Four-wheel drive vehicles have a transfer case to engage power to all four wheels, then transfer the power back to just two wheels. The transfer case--as with all transmission components--has a tremendous amount of stress applied to it as it manages and transmits all of the engine's power to the drive shaft. Transfer cases in most contemporary cars and sport utility vehicles use a chain to transfer the power. The combination of chains, transfer case, fluid and vacuum lines all can lead to problems.
A transfer case is separate from the transmission case; the transfer case requires its own supply of transmission fluid. The fluid serves two purposes: to lubricate the chains and linkages, and to cool the fast-moving metal parts. If the fluid is low the transfer case can overheat, and cause parts to seize and not actuate properly between two- and four-wheel drive modes.
A vacuum line actuates the transfer case and changes the linkages from two- to four-wheel and back, depending on the driving conditions. If the vacuum line becomes worn and develops leaks, the transfer case will not get the full vacuum power needed to properly operate the mechanism. Check lines for leaks by simply listening with the engine running--hissing noises indicate a faulty line.
As in a bicycle, the transfer case has a chain on sprockets to transmit power from the engine to the drive shaft on to the front and back wheels. Changing sprockets changes the driving mode. The chains endure a tremendous amount of strain and can eventually stretch or develop a broken link. When this happens the chain will not stay on the sprocket and can lead to complete transmission failure. If you feel stalling or delays in changing drive modes, it could indicate a weakening chain. The only repair for a bad chain is to replace it.
Transfer cases are made of either magnesium or aluminum alloys. Although able to withstand the heat the chain and linkage generate, the metals can be damaged easily by rock or road hazard strikes. When this happens the fluid will leak away quickly and the unit will seize. When the transfer case is damaged, the remedy is the same as the chain: replace the case. Also, when the case is damaged there is an excellent chance there will be considerable damage to the mechanism.
- "Automatic Transmission Diagnosis and Repair: Total Service Series (New Total Service Series)"; The Nichols/Chilton Editors; 1998
- "Transmission Repair Book Ford 1960 to 2007: Automatic and Manual"; Max Ellery; 2003
- Photo Credit gear stick image by Freddy from Fotolia.com
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