In a manual car, when the clutch is fully dis-engaged there is a direct physical connection between the engine and the wheels (unless the clutch is slipping, in which case it needs attention)
But as I understand it - an Automatic uses a torque converter, which uses fluid and centri-fugal force to convert engine power into wheel power.
Does this mean that an automatic transmission (without any mechanical ‘lock’ mechanism) never achieves full connection?
Explaining in as simple a way as possible - what happens when the car changes gear? (does something happen in the TC to loosen the connection while the gear changes, and then tighten it again when the change is complete?)
Unless the auto-tranny has a “lock-up” torque converter, which is very common these days, there’s not any direct mechanical connection between the engine and the wheels. On our 1989 minivan, the torque converter “locks up” at highway speeds, directly connecting the engine and wheels–and you can watch the RPMs drop by several hundred.
But on the classic auto-tranny, say a Turbohydramatic 350, there’s never a direct mechanical connection. When an automatic changes gears, the torque converter just “slips”.
My understanding (pending correction from the pro car types) is that many torque converters have a “lock up” feature, which above a certain speed locks the torque converter input and output, just a like fully engaged clutch.
My own car (a 96 Pontiac Bonneville) and my wife’s last car (92 Pontiac Grand Am) both had this feature if I’m not mistaken. They both seem to lock at about 40 to 45 mph.
So what are the chances of an eight year old car having the type of TC that locks?
And do the types that lock stay locked when you’re flooring it? (between gear changes)
The car in question is a Jaguar S type 3 litre v6 (petrol) (2000)
Manually put your tranny in the next to highest gear (if it has 1, 2, D - put it in 2) then accelerate. When the tach gets close to redline, put it in D and you’ll see the tach drop a couple of thousand RPM. Stop accelerating at a safe speed - say 55MPH and watch the tach. If it drops noticably again, that is when the that is when the TC lockout engages. I had a 91 Chevy S-10. If I stepped it down a little at highway speeds, I could see the tach jump up about 600 RPM like it had shifted down. But I knew I was still in drive because if I floored it (or manually put it in 2 with the lever) the tach would jump up another ~2K RPM.
Torque converter slippage ranges from 100% (car idling in gear while stopped) to ~3-5% (cruising at highway speed). This is the main reason why a car would get less fuel mileage with an automatic transmission than with a manual.
Lockup torque converters effect zero slippage. Typically the lockup only operates in 3rd & 4th gears (in a 4-speed automatic) at road speeds of over 35 mph. Lockup converters have been used at least since the 1980’s.
The lockup can stay engaged during a gear change, as the change is effected by applying/releasing clutches and bands in the planetary gearset. This can be done with the input shaft (directly connected to the converter) speed staying constant, which is a key characteristic of an automatic transmission.
A friend had his lock up. “Really lock up” while on a trip. The engine would stall when he came to a stop and it was one of those cringing times when he would shift into gear to continue on.
He was able to continue the trip when a service tech told him witch wire to disconnect on the transmission to disable the lock up.