Q: Does An Automatic Transmission Have a Clutch?

asked by on December 01, 2015

Does an automatic transmission have a clutch?

An automatic transmission does not use the same style clutch that a manual transmission uses. The automatic transmission uses a torque converter attached to the engine and transmission to serve as a connection between the two components. The torque convertor is turned by the engine and at the same time turns a pump in the transmission. The pump provides a fluid pressure that allows the transmission to not move the vehicle until the engine turns the pump enough to supply enough pressure to turn the input shaft of the transmission and enough pressure to the internal clutch packs to get the vehicle moving using fluid pressure.

The automatic transmission uses a series of clutches called clutch packs. These clutch packs are put together using many clutch discs and plates in a hub housing, with a top plate to compress them and a clip to secure them together. These clutch packs are used inside the transmission for the multiple gears that an automatic transmission would have. When there is no fluid pressure applied to the clutch pack, the clutches are allowed to turn without application of that gear. When the pressure is sent to a particular clutch pack using a control valve to turn the fluid pressure on or off, it will lock the clutches together and turn the entire hub and a particular gear ratio. Using pressure to different hubs allows different gear ratios to be used.

When there is a transmission that has what is called a lock up torque converter, it will house a clutch disc that can be locked in when a vehicle is at a steady speed above about 45 mph. The lock up will lock the engine and transmission input shaft together like a manual transmission would to increase fuel mileage.

The clutches in an automatic transmission are always covered in transmission fluid, unlike a manual transmission that stays dry. The clutches will get worn over time and may start to slip when enough material is worn off of them. This is why some gears may slip while others do not. They use different clutch packs and wear differently depending on use and loads carried. A clutch, once it slips, will only get worse and the material will start to contaminate the fluid, making it change to a darker color. Changing the fluid will only make the fluid cleaner and not fix the clutches. This is why servicing a transmission after it is slipping will not help it shift better.

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