February 26, 2018 at 3:46 pm #10248
What within a transmission would make a vehicle go in reverse when the gear is in drive?February 26, 2018 at 8:40 pm #10249
I have to assume that you are driving a Ford Taurus that was built sometime in the last decade because that does a great deal to explain your question. Getting to the root of the problem, it has to deal with the computer setup in your Taurus.
In every modern vehicle (car, truck, SUV), there are a number of computers controlling everything. Indeed, there are very few mechanical systems controlling things anymore. For example, the transmission is controlled by a computer called the Transmission Control Module (TCM). The TCM reports directly to the Engine Control Module (ECM). However, it also takes directions from the Powertrain Control Module (PCM), the Fuel System Control Module (FCM), as well as from the Brake Control Module (BCM), and others.
In turn, each of the other control modules not only listens to the others on the CAN (Car Area Network) but also to the ECM as they pass information and instructions back and forth.
Tying everything together is the CAN which serves to bind the engine electronics together so each control module acts when it has to or passes information on to other systems. You see each control module — there can be from eight to 15 controlling an engine — have a number of subsystems reporting to them. the subsystems are usually sensors. However, there may be a number of activators turning things on and off in response to the information provided by the sensors.
If this seems like a complicated information web, it is. Because it is and because it is electronic, there are times when orders from the boss (PCM) can be delayed or broken up. The transmission is a good example.
At one time, the transmission was mechanically controlled. You pulled down on the PRNDL lever and a cable in a carrier started the process that ended up in the car going into gear mechanically. Everything else was also mechanically controlled, including step-downs, gear choice and the like.
Now, the ECM, after listening to all of the subsystems on the CAN, as well as the sensors, determines how the transmission reacts. You start things off by selecting a gear, D(rive) for instance. Next, the ECM lines things up to answer your request. Let’s say a glitch happens in the “shift-by-wire” transmission. What happens then? It’s quite possible the indicator may read R(everse) while the transmission is Drive. That’s the answer to your question. It’s all the result computer control not reading things correctly.
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