Most likely your transmission is computer-controlled. As such, the car can be driven with a scanner connected that displays live data. All of the scanners used by dealers, and many of the better aftermarket ones used by independent shops, have a "record" feature. Many dealers also have scaled-down versions that can be left with their customers for days until the problem occurs. When it does, you press the "record" button. Later, that can be played back by the mechanic to see what the computer was seeing to base its decision to downshift. Since that data travels through the scanner's memory, the recording actually starts a couple of seconds before you press the record button so it catches the event.
This is typically not a transmission mechanical problem. It is almost always electrical and is usually caused by a sensor or the need to update the software in the computer. Hyundai is the most customer-friendly company at letting independent shops, (and car owners), access their web site for computer updates. Toyota and Chrysler are next but they do charge a small fee for access. BMW and GM are at the bottom of that list of customer-friendly manufacturers.
The dealer is not always the most expensive place to get your car fixed. Whatever problem you can think of, they've already seen it or heard of it, or they have a help-line to call. With all the stuff they are required to spend their money on, it's only natural they have to charge more per hour, (usually), but they will generally take less time to fix your car because they are very familiar with it.
If, by chance, your transmission is not computer-controlled, there will be a "kick down" cable going to it from the throttle body. That cable tells the transmission throttle position. That, along with road speed are the two things hydraulically-controlled transmissions look at to calculate shift points. Sometimes that kick down cable can stick causing the transmission to think you're pressing the gas pedal. At lower speeds that can initiate a downshift too soon. The clue can be found in what's happening to car speed when that down shift occurs. If it down shifts as the car is slowing down, it's kick down-related. If the downshift occurs as the car is picking up speed when coasting downhill, it's road speed-related.
A whole new nightmare has been added to cars with the addition of the extremely dangerous "throttle-by-wire" system that put Toyota in the news a year ago. We all knew that was ripe for lawsuits and deaths but every manufacturer has gone that route favoring complicated, unreliable computer controls and sensors to replace the simple throttle cable. The cruise control and transmission functions are incorporated into that system. That is one system you do not want an independent shop working on. Many shops will not even look at some cars with those systems because they don't want to be a party to future lawsuits.
If there is a problem related to that thottle-by-wire system, it will be recorded by Toyota, but only from your visit to a dealer's repair department. Independent repair shops don't report anything to the manufacturers. When the manufacturer starts receiving notices of the same repairs being needed on many of the same model, they investigate the cause and come up with a fix. If it's common enough, but not a safety concern, they will issue a service bulletin to make it easier for the next person to solve that problem. Every manufacturer produces hundreds of service bulletins each year; some every month. It can takes months or years for those bulletins to work their way through the systems used by independent shops. Dealers get them as soon as they're published. Thanks to the internet, you can even do a search yourself to see if there's any bulletins that might apply here.
When a problem looks like it could be safety or emissions-related, they will typically issue a recall. That means they've identified a problem, they have a solution, and they want you to being the car in for a free repair or update. Recalls make news. Service bulletins don't, but they can make solving problems like yours a lot easier.
Friday, March 2nd, 2012 AT 12:30 AM