• Staff

How does an automatic transmission know when to shift?



Let’s take a second to pay homage to the 20thcentury geniuses that designed the automatic transmission, because its next-level stuff. In principle, the way they work is actually quite simple ---once its been explained. So for this article, we’ll stick to that, and not worry your head (our ours) with the complex details.

So how does an automatic transmission know when to shift, especially when it often needs to shift at different RPMs? While its true that modern vehicles use sensors for most of the of work, the automatic transmission has been around since the 1940’s. Until the use of onboard computers late in the 20thcentury, shifting was controlled mechanically. And even when computers and sensors are used, the things they monitor are still the same. Here’s how it works

Mechanical magic

The first thing to understand is that the automatic transmission is actually a hydraulic machine. That means its inner workings are driven by hydraulic pressure.

Let’s start with actually happens when your transmission shifts. The gears are engaged or disengaged via clutch packs. These are stacks of friction plates that form a connection when they are squeezed together, and end that connection when they spread back apart. They are regulated by valves, called shift valves, that move in response to pressure.

The pressure, of course, is a response to the two things that matter most when timing a shift of gears ---engine output and vehicle speed. These two factors each generate pressure on one side of the shift valve, so that valve moves according to the demands being placed on the vehicle, and thus activates the right clutch packs, which activates the appropriate gear.

The way that engine output is typically signaled to the valve is by use of a vacuum modulator, which responds to pressure in the intake manifold. As you throttle up, more air rushes into the manifold and increases pressure.

On the other end, vehicle speed is monitored by a component called a governor. The governor is inside the transmission housing, attached to the output shaft, which means it turns along with the drive train. So as the wheels turn, so does the governor, and its input signals the shift valve to shift or delay a shift at a certain RPM.

That’s why your car takes longer to upshift when you’re driving uphill or accelerating aggressively. The engine is being tasked with more work, and to do this work, torque is needed. The lower (larger)gears provide more leverage. Once enough momentum is gained, a smaller gear can be used, which runs the output shaft faster.

The final piece of the puzzle is throttle position. When you accelerate aggressively, the car needs to stay in each gear longer before upshifting. Your desire to reach speed sooner is communicated to the transmission by how wide you open the throttle, AKA how far you push the gas pedal. Throttle position is linked to internal components in the transmission with cable linkage or other means like sensors.

All you really need to know about your transmission

The most important thing for vehicle owners to take away from all this is that the fluid in your automatic transmission is a hydraulic medium. If the fill level is low or the fluid quality is bad, the transmission will not function properly. This may or may not be something you notice, but your transmission definitely will notice. It will increase wear and tear on your transmission, which is one of the few components in your car that can cost 5k or more to replace.

We recommend checking your ATF every time you fuel up, as well as getting your transmission serviced whenever the fluid becomes tainted or discolored. For a full breakdown on what your transmission needs from you, see this post.

If you live in the Des Moines, WA area and need service or repairs on your transmission, call ABC Auto at (206) 395 5300 or fill out our contact form, here.

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