I have probably said this before but I will say it again, the automobile is the most complex machine most of us possess. Modern day machines combine electrical, mechanical and electromechanical devices into systems that perform the magic we need to perform the many tasks we take for granted. We will probably throw away our printer when it stops working properly. We may even throw away our computer when it stops working. It is very unlikely that we will throw away our vehicle when it stops working. That is where your mechanic comes in.
There are basically two kinds of failure; mechanical failure or electrical failure. In many cases the challenge for your mechanic is determining the type of failure they are dealing with. Modern automobile electrical systems are complex to say the least but the mechanical systems recently have become no less complex.
Automatic transmissions to me always presented a challenge from the electrical vs mechanical failure mode. Digging into a transmission to find the mechanical faults usually means taking it out of the vehicle and taking it apart. That usually takes a lot of time and means it is going to cost a lot of money. Before I want to be taking out a transmission I need to be sure there is something wrong inside of it not outside of it.
A lot of what goes on inside a transmission is commanded from outside the transmission. In many vehicles the computer that controls the transmission is the same one that controls the engine. Before finding out why the transmission won’t shift gears when you expect it to the mechanic must ask himself, “Are the electrical inputs and outputs requesting the transmission to shift gears?”
The first place your mechanic looks is at their scan tool. That will show, in most vehicles, the inputs and outputs from the computer. Drive the vehicle and record the data. Is the shift in gears being requested by the computer? If the answer is yes, then why did it not happen? The output request to shift is there but a request does not always mean there is a response. Time to check the circuits that output signals. The output signal to shift gears usually turns a solenoid (electromechanical device) off or on. The electrical signal can be present at the solenoid but if the solenoid does not move then nothing happens. Everything electrical is good but the electromechanical device is not working.
In most cases the solenoid is in the transmission. Some disassembly required. If that solenoid works properly but the transmission still does not shift there is a whole chain of hydraulic mechanical pieces that make that shift in gears take place. Now the problem is deeper in the transmission.
Time to take out the transmission? Time to notify the customer! The cost of repair is escalating.