Talk about the seven wonders of the world, well we have eight wonders, they forgot to include or add automatic transmissions. An automatic transmission is truly a wonder in terms of theory and engineering. It is much like the human body, meaning it has a variety of systems that should operate in harmony to produce the desired result. As in, moving, shifting and all the wondrous things your transmission does by itself.
Something that adds to the wonder of how an automatic transmission works is that there are actually several different ways a transmission can produce gear ranges internally. While the actual shift ‘feel’ may seem to be the same from car to car, there are a couple ways of achieving the necessary gear ranges.
First I will choose a TF727 3 speed automatic transmission to describe what components apply to make the different gear ranges. The Dodge 727 is a common transmission and uses a band that wraps around the 3rd gear clutch drum to make second gear. FYI: The Chevy T-400 has a different set of components that produce second gear, we will cover that in the future.
I am not going to describe the hydraulic theory, it’s over most rebuilders heads too, but the mechanical theory may be interesting to you.
Once the shifter lever is put in drive we are set to go forward. From there here is what applies and un-applies in order to make your gear ranges.
1. The forward clutch pack applies and the sprag in the back of the case mechanically holds, thus we are now in first gear and ready to take off.
2. When the transmission is ready to shift to second gear the intermediate band (2nd gear) applies using the 1-2 shift servo. The band actually stops the 3rd/Rev. clutch drum from spinning. Much like a band brake, it squeezes the outside of the drum until it stops, all in a fraction of a second so you will not get a smooth seamless shift.
3. In order to shift into 3rd gear, the second band has to fully release or return to the off position, in order for the 3rd/reverse clutch drum to apply properly. The release of the second gear band has to be instant, other wise the third gear shift will bind up causing a jerking sensation or a hard bump. On the other end of the spectrum, it may have a slippery feeling since the shift is taking to long. Timing is of the utmost importance here to produce a quality shift without wearing components.
The valve body’s functional condition, wear on the clutch plates in the third gear drum and the band adjustment all play into this, but that is to heavy (to much) to discuss now.
4. Reverse is a combination of the reverse band and the third gear clutch pack being applied at the same time. Simply put, the combination of the reverse servo applying the reverse band as the third/ reverse gear clutches apply at the same time = reverse.
I’m not going to get into passing gear or anything more complicated. This is a very common method of what applies in a 3 speed band type automatic transmission.
Pretty simple, eh? That is just the mechanical side of operation too. I am not so sure how important this info is when it is time to replace your transmission. I am sure that GotTransmissions.com has earned their reputation as the leading transmission supply company in North America by providing the necessary information to our customers in terms they understand. It’s my job to help you understand what is in your best interests through our blog. Enjoy.
