There are plenty of reasons why a smooth, firm-shifting automatic transmission can suddenly start slipping out of gear and hesitating between shifts. The following offers a few reasons for transmission slippage, as well as a few ways you can potentially fix the problem yourself.
Low Fluid Level
The vast majority of slippages can be traced back to low transmission fluid levels. Low fluid levels can cause the transmission to hesitate in between shifts, making your vehicle seem sluggish and unresponsive. In many cases, this could be remedied simply by adding the proper amount of transmission fluid.
Measuring the transmission fluid level properly can be a tricky task, so here's what you'll want to do for an accurate reading:
- Make sure the transmission is warmed up. You may have to drive the vehicle a few miles before the fluid warms up properly.
- Park the vehicle on level terrain. Measuring your fluid on an incline will only result in inaccurate readings.
- Leave the vehicle running at idle in Park. The fluid has to be circulating throughout the transmission and torque converter to get an accurate reading.
- Carefully look between the ADD and FULL marks on the dipstick. Your transmission's specified fluid should be in between these marks.
If there's not enough transmission fluid, slowly add the fluid specified for use in your transmission until it reaches the FULL line. Be careful – overfilling the transmission can be just as problematic as running it with too little fluid. In addition, check the transmission for leaks, especially around the rear main seal and transmission pan.
Low Fluid Pressure
Automatic transmissions rely on carefully calculated amounts of pressure to actuate various gears, valves and clutches. A decrease in fluid pressure can cause the transmission to slip out of gear. There are several issues that could result in a low fluid pressure problem and, as a result, negatively impact your vehicle's overall performance:
- The transmission filter could be plugged with debris. On most vehicles, changing the filter is a simple matter of dropping the transmission pan and detaching the filter using simple hand tools. It's a good idea to inspect the pan for metal shavings and other foreign debris prior to cleaning and remounting the pan.
- The transmission pump itself could be worn out. When the pump is no longer capable of adequately supplying the transmission with fluid, it could cause the transmission to slip, overheat and exhibit other behavior associated with transmission failure.
Failing Valves and Sensors
The inside of a typical automatic transmission is bristling with control valves, sensors and solenoids, all working together to provide smooth, yet crisp shifting. However, any one of these valves, sensors and solenoids can fail unexpectedly. Given the complex, time-consuming nature of the average automatic transmission, it's usually a better idea to replace the entire transmission instead of repairing it in piecemeal.
On many vehicles, a transmission problem stemming from a faulty valve, sensor or solenoid is often accompanied by a "Check Engine Light." With the help of an OBD-II-compliant scan tool, you can easily pinpoint the sensor, solenoid or valve body responsible for the failure. However, most replacements are time-consuming affairs that require removal and partial disassembly of the transmission itself, making it a complex job that is often left in the hands of transmission repair specialists and other repair shops.
Having the Wrong Fluid
Transmission fluid can be best described as a "witches' brew" of friction modifiers, lubricants and other additives, all which are necessary for your transmission's smooth operation. Most manufacturers have tailored their fluid requirements for use in specific transmission models, which makes it all the more dangerous to use the wrong fluid.
Putting large amounts of the wrong fluid in the wrong transmission can easily kill it in short order. On the other hand, you may be able to remove or even cope with minute amounts of incompatible transmission fluid.