The transmission uses a special hydraulic fluid under pressure to engage a series of forward gears and one reverse gear, basically transferring the power from the engine to the rear wheels. It has to do this constantly and smoothly every time the vehicle makes a trip. Sometimes the transmission can malfunction or indicate some warning signs before total failure. An observant vehicle owner will know what to look for by learning how to interpret these warning signs, and as a result, avoid costly repairs down the road.
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Dashboard Lights and Sensors
If the vehicle has an OD (overdrive) indicator dashboard light that flashes on and off, it means a problem has been detected inside the transmission. OD lights exist on only a few models as standard equipment, like the Ford Explorer. The best way to solve the exact component fault or nature of the system failure will be to to have a trouble code scanner connected to the vehicle. A special code will pinpoint the problem by the use of a code book. The code scanner might pick out a faulty electrical sensor, thus eliminating the need for a visit to the transmission repair shop.
Electronic transmissions use speed and shaft sensors to monitor shifts in the transmission controller. Electrical fault codes with these components happen when a sensor has shorted or a solenoid ends up in an "open" position, which means no current. Erratic shifting can result, as well as the transmission shutting down and going into "limp" mode, which selects only 2nd or 3rd gear as a default. Limp mode allows the driver to get to a repair station without further damaging the transmission. A code scanner will detect these problems.
The transmission fluid level can be one of the most overlooked maintenance chores, responsible for the majority of transmission problems. A low fluid level in a vehicle that turns sharply can cause a temporary "slosh" of the fluid to one side of the transmission case, resulting in a hard shift when the vehicle straightens out. The dip stick should be checked for the proper "hot" level with the engine running. Fluid should always be kept at its proper limit.
Dirty or contaminated transmission fluid can cause hard shifting. Transmission fluid that has been contaminated by water, age, or sludge build-up cannot flow properly through the transmission channels and valves. Black fluid, or fluid that has a burnt smell to it, indicates fluid that has lost viscosity and lubricating qualities. Overheating results from dirty fluid, expanding and wearing the bearings, which make the transmission work harder to shift the gears.
The transmission filter sits in the pan sump and filters all particulate matter and debris in the fluid. When the screen or filtering device becomes clogged, the pressure flow from the pump becomes restricted, causing a high back-pressure condition. Hard-shifting can result. Transmission filters and fluid need to be changed at regular service intervals; every 30,000 miles can be considered an average interval.
Many transmissions have vacuum modulators connected to their transmission housing. The small diaphragm on the modulator has a vacuum line that receives vacuum from the intake manifold. The shifting characteristics of the vehicle depends on the vacuum modulator's ability to read the amount of engine vacuum being sent to it. If the vacuum line becomes loose at its connection point, or the diaphragm fails inside the modulator, the transmission will not shift properly. The vacuum line and modulator should be inspected for proper working order.
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