Torque Converter Lockup?

[Mint Q45A]

First of all: Excuse my ignorance....I keep finding references to the Torque Converter Lockup, but unfortunately I am not sure what it is and how it works....Can someone give me a quick explanation? If applicable, I would like to learn how to tell whether it is working correctly or not....Thanks....

[MinisterofDOOM]

The torque converter's fluid connection is less efficient than a mechanical connection, so most torque converters have a secondary mechanical connection as well, and that's the lockup. When cruising above certain speeds (which vary by gear) the TC will engage a mechanical connection that overrides the fluid connection. You can actually feel it when it happens. It helps to improve fuel economy and to maintain consistent RPM during highway cruising.

One way to tell is to drive on the freeway and switch between acceleration and cruising. As you stop accelerating and level off the speed, you should feel the RPMs level off to match, and you'll be able to see it on the tach as well; that will indicated the converter locking. You might want to try setting cruise control for 60 or 65 and then stepping on and off the throttle. As the car settles back into cruise, you should notice the TC locking, and as you apply throttle, you should notice it unlocking. It ALMOST feels like a super-fast, gentle shift, except without the drastic change in RPM and power.

[Mint Q45A]

Thanks for the info.

[maxnix]

It's a low capacity mechanical clutch that locks the TC to the transmission input shaft.

[Mint Q45A]

I came across some additional info on TC Lock:

Here is how a torque converter (TC) works, in layman terms. Which is the only way I can understand it. With a manual transmission, the clutch sits between the engine and the transmission. When the clutch is engaged (foot off the pedal) the engine and transmission are connected. When the clutch is disengaged (pedal depressed), the connection between the engine and transmission is no longer effective, so the transmission can be shifted and the engine is no longer connected to the drive train. In an auto transmission, the TC takes the place of the clutch. The TC has a shaft with several impellers (blades). The exposed end of the shaft slips into splines located in the engine's flywheel. The other end of the shaft, with the impellers, is inside the TC. The TC contains transmission fluid. The impellers are turning inside that fluid. At idle, not a lot happens. As the RPMs rise the swirling fluid begins to force the "Output" of the TC to drive the transmission. If the transmission is in gear, the car begins to move. The higher the RPM the more pressure, and the car moves faster and the transmission starts doing it's thing. In days of yore, that was it! There was always some slipping of the TC and its swirling fluids. Resulting in poorer mileage. In modern cars, something inside the TC actually LOCKS the TC and transmission together. And the drive between the Engine and transmission is more solid, similar to the way a clutch does. This generally only happens in the overdrive gear on most cars. Such as 5th gear with Honda's You can observe the TC operation by watching the tach. Say you are in 4 gear at 40 mph with a steady foot. A light pressure in the throttle will cause the RPM to rise slightly even before the car begins to gain speed. The TC wasn't locked and was allowed to slip more resulting in more engine rpm. At 60 mph when the car is in Overdrive and the TC is Locked up, a light additional pressure on the throttle won't cause the rpm to rise instantly because the engine and transmission are locked together and nothing is slipping. The rpm will rise only as the car gains speed. If you press a little too hard you will see the tach jump 300-500 RPM. That is the TC unlocking and the drive is now depending on the swirling fluids. A bit more throttle and the transmission will likely downshift to a lower gear resulting in another 800-1000 gain in rpm. You can really observe this on a long up hill with Cruise Control activated.

[Q45tech]

The TCU looks at the engine rpm vs the after TC shaft rpm via the " turbine sensor" to evaluate what to do along with speed sensor.

TC efficiency is the name of game [MPG efficiency] and this varies with input vs output speed rotaional difference.

There are great charts in FSM which show what happens vs TPS load factor in each gear.

Lock up TC can improve MPG by up to 10%..........the more the car weighs the better the improvement.

If you have a Consult or equivalent you can view the injector open time and eventually calculate how much fuel is squirted per unit of time.......much like what a MPG gauge does.

If you are cruising along at 1.8 milliseconds [60mph] and floor it you will see the fuel go to 10-11 milliseconds and the rpm immediately double so using 10x the fuel per second, minute..................simultaneously the TC unlocks before you have reached the 40% point of pedal long before the downshift occurs. Also the increase in plenum pressure [from more open throttle plate] causes the fuel pressure regulator to go from 34 psi to 44 psi.