5

I have a two questions related to the torque converter and shifting.

  • First, is it possible to lock the torque converter on first gear if I press the gas pedal hard?

  • Second, is shifting while the torque converter locked good for the car's transmission?

Thank you!

  • As far as I know most torque converters do not lock up until 3rd gear, regardless of throttle. Modern transmission are entirely controlled by the ECU, so shifting gears while the torque converter is locked happens all the time without issue, but I get the feeling that's not what you're asking? – MooseLucifer Aug 17 '16 at 22:50
  • I don't know about all transmissions but GM's 6L80 can lock in all gears, from 1 to 6 and does not unlock during shifts. – John May 17 '19 at 13:14
4

The torque converter locks when the engine rpm (impeller/housing speed of converter) and the transmission input shaft (turbine speed of converter) are roughly equal -- thereby improving efficiency for cruising, but also mitigating any torque multiplaction effect of the torque converter. With the TC locked, the engine is effectively directly coupled to the transmission input, and no longer uses a fluid drive.

This is controlled by the transmission control module (TCM), and usually is only activatd in top gear, and at steady cruise (low throttle position).

In some cases (my '95 Bronco, I think), this can happen in 3rd and 4th gear. In some cases (my '95 Bronco for certain), this can be defeated with a switch, and a light comes on (mislabeled "OVERDRIVE" in my opinion), which is useful when towing heavy loads and some converter "slip" is warranted.

Any input from the accelerator decouples the TC friction clutch, and the transmission reverts to a fluid drive scenario with torque multiplication. This also occurs while shifting (if any), as such a shift would be very harsh wihout the fluid damping effect of the TC. So in most vehicles, a downshift to 3rd would uncouple the TC clutch. I'm not aware of vehicles that lock up the TC clutch in lower gears, as this wouldn't make much sense. Lifting the throttle usually commands the transmission into the highest gear (for fuel efficiency) unless your TCM has a "sport" mode or some such that holds it in gear longer.

  • Is there a reason then to why my car's engine RPM drops a lot when going to 2nd gear from first if I do so when my first gear is at high RPM? When I upshift from the first gear when my engine is at lower RPM, it seems to not drop as much. I originally thought this is because the torque converter locks. If not, then what is causing this? – user3736336 Aug 18 '16 at 21:01
  • @user3736336At this point I would be checking transmission fluid levels, and perhaps investing in a transmission service and diagnosis. It would be helpful to know the make, model, year, and engine size of your vehicle. It's possible you have a bad torque converter, or failing stator bearings, or a damaged one-way clutch on the stator. However, regardless of this speculation, I'm fairly certain it is not a result of TC lockup going to second gear. – SteveRacer Aug 19 '16 at 2:22
  • @user3736336 - I would bet the reason is, if you are revving the engine out to higher RPMs, the transmission is being held in 1st gear, then when shifting it goes directly from 1st to 3rd gear. – Pᴀᴜʟsᴛᴇʀ2 Feb 23 '18 at 13:47
  • @user3736336 The reason is that the amount of slip in the torque converter is much less at high RPM than low RPM. With the engine above 3000 RPM, there will likely be less than 200 RPM of speed difference between the transmission and engine even at full throttle. At 2000 engine RPM, the speed difference could be huge under load. The transmission could be under 1000 RPM, or even stopped! – Alex Cannon Nov 27 '20 at 15:34
  • Imagine being in 1st gear with the engine at 2500 RPM and the transmission at 2200 RPM, going about 15 MPH with 300 RPM of slip in the torque converter. It shifts in to 2nd, now the transmission is turning at 1150 RPM and the engine drops down to 2200 RPM. There is now 1050 RPM of slip and loss of energy! You'll notice that the engine stays at a mostly constant RPM as the speed in 2nd gear comes up to 30 MPH. – Alex Cannon Nov 27 '20 at 15:38
0

The torque converter lock on a typical 90s 4 speed automatic won't engage unless the transmission is in the highest gear, and the throttle position is fairly low. The over drive cancel will make 3rd the highest gear and then it can lock in 3rd on most transmissions.

More modern 6 speed transmissions keep the torque converter locked even when shifting. The gear ratio differences are much smaller which allows for smooth shifting even with the TC locked.

Pressing hard on the gas will do just the opposite! It will make the torque converter unlock. This improves the drive-ability of the vehicle and makes shifting smoother. It also reduces wear on the bands because the torque converter can take some of the slip instead of having the transmission bands drag the engine to the new speed when shifting.

Shifting with the torque converter locked the won't damage the transmission because the transmission control unit is designed to not allow this to happen unless the transmission is designed to handle it. If you modify your transmission control logic then it's a different story.

Not the answer you're looking for? Browse other questions tagged or ask your own question.