I just don't understand how an automatic can idle at zero mph and have 550 rpms. I know it has something to do with the torque converter, but all of the liturature I have found has done nothing to help.

 

Here's a VERY detailed explaination - TCI Automotive - Torque Converters Explained.

How Stuff Works - Automatic Transmission is another one. I don't think it has specifics about the TC, but it does go into some pretty good information about the auto transmissions as a whole.

Let me know if more information is required.

 

Think of it as a pulley in the tranny. Great link Swartlk

 

Do you understand now?

If nto post up and I will try to answer it another way.

 

Nay, I do not understand. []

 

Well, if you just want a picture for your mind you can think of it like a clutch that is always slipping a little bit. If you push the clutch halfway in, a vehicle with a manual can idle at 500rpms in gear and still go when you push the gas. The difference, obviously, is that one is designed to slip slightly all the time and the other is not.

You can buy torque converters that "slip" by differing amounts... Usually the ones that come from the factory are relatively "tight" for fuel economy, but for racing purposes you can get ones that will let you push the engine to 2000 or 3000 rpms at a dead stop.

EDIT: This is a very simplified explaination. In reality torque converters behave very differently from conventional clutches, but hopefully it will help you get your mind around what's going on.

 

Basically a torque converter is a fluid link between the engine flywheel/crankshaft, and the tranny input shaft. The faster the engine spins, the faster the torque converter spins the fluid to grab the input shaft on the tranny.

Think of it this way. A torque converter is two big fans spinning in opposite directions. One fan is connected to the engine mechanically. The other fan is connected to the tranny mechanically. The tranny fluid connects the two fans together. Tranny fluid, like all liquids, does not compress. When the engine fan spins it makes the tranny fluid spin the tranny fan. This makes your truck move.