Car Repair: Automatic Transmission Troubleshooting

Automatic transmissions are generally beyond the reach of the home mechanic, being highly complicated and difficult to work with -- even more so than the engine itself, but there are things you can check and repair yourself.

Automatic transmissions are complicated, ever-evolving systems that are normally beyond the scope of the lay person to repair. The specialized tools required to perform most service operations are rather expensive and not commonly available in the average toolbox, therefore this article will focus on what you ARE able to do on your own. Please note: if your automatic transmission requires major repair work, it is wise to have it serviced by properly trained and certified mechanics and not attempt to perform the repairs yourself. It is possible to remove and install the transmission yourself to save expense, however.

There are five general conditions that commonly cause malfunctions in an automatic transmission: Mechanical malfunctions, poor engine performance, hydraulic malfunctions, inaccurate adjustments, and on board computer errors or related electronic issues are the usual suspects. To begin diagnosing your transmission, it's always best to start with the easy items.

Preliminary Diagnosis

Before continuing, be sure that the vehicle is recently driven to warm up the transmission and transmission fluid. The next thing you should do is locate some level ground to park your vehicle on. Now, check your transmission fluid level. If the level is low, add just enough transmission fluid to bring it to an acceptable level and check for leaks under the vehicle. If the fluid level is higher than normal, you will need to drain off the excess. Keep any oil you drain and check it for coolant contamination. If you see contamination from coolant, it's likely that a rupture has occurred in the internal radiator walls. If you find that the transmission fluid is very foamy, drain it and refill it, then check it for contamination from water / coolant (drain it again after it has been filled, and drive the vehicle several miles). Finally, check your engines idle speed (if you do not have a tachometer, most auto-parts stores have devices that can check the engine speed that they will allow you to use in the parking lot free of charge). If the idle speed is not normal, you will need to have the engine repaired before going any further with your transmission diagnosis!

Automatic Transmission Fluid Leakage

Transmission fluid is a deep red color. Be sure that what you are seeing as a leak is actually transmission fluid and not engine oil, which can end up in the transmission for various reasons (including air flow blow-by). The best way to track down exactly where the leak is coming from is to do a lot of cleaning, removing all the buildup of grime that occurs during normal operation from the surface of the transmission. Degreasing agents do very well for this task. After you've cleaned up the transmission thoroughly, drive the vehicle at LOW speeds (you want to avoid lots of air flow) for a few miles or a few minutes. You will not need to shift into reverse or go through each gear individually at this stage. When you're satisfied that any leak should have revealed itself, examine the transmission again. The most common areas of leakage are the fluid pan, speedometer connection, vent pipe, rear extension, filler pipe and the transmission oil lines.

Replacing the o-ring / gasket or simply tightening the bolts (as appropriate) may be enough to stop the leakage. If you do perform one of these repairs, be sure to monitor it carefully, especially after high speed driving or heavy shifting / towing situations.

Transmission Slips, is Rough or Noisy, Lacks Drive Power

Although there are many factors that may contribute to these symptoms, only one issue should really be of concern to the home mechanic: fluid level. If you determine that your transmission fluid is of the proper level, you will need to take the vehicle into a reputable dealership for diagnosis.

Transmission Won't Downshift, Pedal Completely Down

Remember to disable the airbag system on vehicles so equipped to avoid accidental deployment of the airbag. The throttle valve cable is the likely culprit here. The removal and replacement of the cable is fairly straightforward in most vehicles, although each make and manufacture offers its own challenges and obstacles. If you aren't sure where this cable is, and you have no desire to trace it out yourself, you can easily ask for someone at a dealership or service garage to show you. Replacement cables can be purchased at most dealerships for your make of vehicle or from your local auto-parts store.

Engine Starts in Gears Other Than Neutral and Park

Remember to disable the airbag system on vehicles so equipped to avoid accidental deployment of the airbag. If you automatic transmission is allowing your vehicle to start in gears other than neutral and park, you probably have a faulty neutral start switch. At this time, also verify your back-up lights are working properly as the systems are generally linked. Verify you back-up light fuses and bulbs.

You may replace the neutral start switch without a great deal of effort in many vehicles, although all makes and models provide different levels of challenge. Be sure to remove the cable from the negative battery terminal first, but check your owners manual -- some vehicles have anti-theft deterrent devices that could cause you a headache later on if you don't read your manual first. Installation and removal of the switch is straightforward, just be sure that your transmission is in neutral during removal and replacement. Finding a neutral start switch for your vehicle is easy enough; just contact your local auto-parts store.

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