My folks have an early 2000’s Dodge Grand Caravan with a four-speed automatic transmission. One of the things I’ve always wondered about it: why is it programmed not to go into lock-up mode until it is fully warmed up? Does revving the engine more when it is cold provide some advantage to either the engine or transmission?
My own car has a six-speed manual transmission, and I would tend to shift it a little earlier (and accelerate a little more gently) while it is warming up. I was under the impression that high revs were not good for a cold engine, because the oil is still thick and harder to circulate. Is that untrue?
If I have to borrow my folks’ van for whatever reason, I just tend not to drive it over 45 until I feel the lock-up engage.
The problem is that the torque converter is the only thing in the transmission that makes any real heat and so if you, say, start the van and jump directly onto the highway the transmission fluid will stay cold, but the fluid lubricates and performs better when it’s warm. So somewhat paradoxically, letting the transmission slip for a bit to warm it up will result in better efficiency overall. (Or at least that’s what the engineers who designed the van presumably concluded.)
On some cars, that’s not an issue because they simply have the transmission cooler built in to the radiator so the transmission fluid will warm up as the engine warms up, but minivans are really hard on their transmissions and so this van probably has a pretty beefy standalone transmission cooler instead.
Torque converters work on slippage (between the input and output turbines) which reduces as rotational speed increases. This allows the tranny to stay in gear at idle, with 100% slippage while the car is braked. Let off the brake and it creeps forward with maybe 90-95% slippage. Rev it up and slippage gets less and the car charges forward. At highway speed it may be down to 3-5%.
Continual pressure to get more gas mileage and produce less emissions led to the development of the lock-up torque converter, which has an additional mechanism to physically lock the input and output turbines together giving 0% slippage.
Even on some transmission-cooler equipped vehicles, even with towing packages, the lock-up convertor is put on hold until the temps come up. And the cooler probably causes the lock-up to be even later vs. non-cooler equipped trans.
It is my understanding that a torque converter (TC) working with cold fluid can lead to the engine ‘lugging’, which is a very bad thing for engines (rods/bearings). To prevent this lugging (bucking), when all conditions are right (fluid temp being top of list), the converter then ‘lock ups’. This is an event for the internal combustion engine which must be managed properly.
At the right temp, the TC will lock up at the right RPM and load, and it won’t lug the engine (and a cold engine isn’t experiencing ideal combustion until IT is warmed up, so everything needs to be hunky dory before that TC load is applied). Locking up the converter early, a risk of lugging, and an engine under strain before it gets its fuel/air ratio squared away (warm) isn’t desirable.
Goal is to avoid lugging and detonation (knocking).