Troubleshooting C6 Transmission Problemsby John Stevens J.D.
Ford’s C6 transmission was first produced in 1966 and was used primarily in trucks and for high-performance big block engines. The C6 consists primarily of a torque converter, a planetary gear train controlled with a band, clutches and a hydraulic control system. The three-speed automatic was built with durability in mind, but problems can occur, particularly if the transmission has been improperly serviced or not serviced at all. Common problems include a failure of the transmission to propel the vehicle forward, overheating and leaks.
No Drive in Forward Speeds
If the vehicle will not move forward when the transmission’s gear selector is in either the first, second, or drive positions, check the manual linkage adjustment on the driver’s side of the transmission first. If the linkage is out of adjustment, the transmission may not be in gear, although the gear selector may otherwise show that the transmission is in gear. If the transmission has recently been disassembled, the problem is likely a lack of fluid or a missing valve body ball. The C6 operates on hydraulic pressure, necessitating a sufficient amount of fluid. Always check the fluid level with the transmission warm and on a level surface. The valve body is located just above the transmission’s filter, and contains a number of passageways through which a small metal ball travels as the transmission changes gears. This metal ball is commonly misplaced due to its small size. Finally, ensure that the rubber vacuum hose from the engine is connected to the vacuum diaphragm located at the back of the transmission on the passenger’s side.
The C6’s fluid constantly requires cooling for it to cool the transmission. If the transmission gets hot enough, the seals within the transmission will sustain damage. Check the fluid level first, as it is the more common cause of overheating and because the transmission’s dipstick is easily accessible underneath the hood. If the transmission has sufficient fluid, examine the cooler lines. To cool the transmission, two brass lines connect the side of the transmission to the bottom of the radiator, where it is then cooled independently of the radiator fluid. These brass lines are easily damaged, and are commonly bent if installed with a standard wrench instead of a flare-nut wrench. Trace each of the two lines from the transmission to the radiator, inspecting the lines for any sign of damage, which will reduce the fluid’s ability to travel between the radiator and the transmission. Damaged lines should only be replaced with a flare-nut wrench.
Fluid leaks can quickly destroy a transmission, particularly if the lost fluid is not regularly replenished. Fluid leaks are typically caused by a defective cork gasket or rubber seal. The most common source of a fluid leak on a C6 is at the transmission’s pan, located on the bottom of the transmission. Engine oil should not be mistaken for transmission fluid, with its distinctive pink color. Between the pan and the transmission is a single cork gasket. Pay particular attention to the area surrounding each of the C6’s 17 pan bolts. Located on the driver’s side of the transmission is the black speedometer cable with its single rubber O-ring, as well as the two cooler lines. At the front of the engine, between the engine and the transmission, is the torque converter. On the engine-side of the torque converter is a drain plug, which will sometimes leak. At the back of the transmission is the extension housing. The extension housing is the metal portion of the transmission that the driveshaft enters. Upon disassembly, the extension housing is unbolted and separated from the body of the transmission. Between the housing and the transmission is a gasket. Inspect the bottom of the gasket for signs of leaks.
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