overdrive on autos

[paulh]

You live and learn.

At 55mph the torque converter locks up, but overdrive comes in a lot earlier that that.

Following this reply I did some checks on my way to work and yes the o/d does operate at lower speeds than 55. I always thought that the sudden drop in revs was the o/d coming in but I stand corrected.
BUT IT'S STILL STAYING ON!!!!!!!!!!

[exmee46]

The overdrive is actually the fourth gear. So, with overdrive OFF you are only operating in three gears. With overdrive ON, fourth gear will usually engage somewhere around 30-40mph on a light throttle setting. Once a higher gear has been selected, the gearbox tries to hang on to it by making the torque convertor work harder.

Once you exceed about 55mph AND you have the O/D switched ON, the torque convertor lock-up clutch will operate, which has the effect of locking the gearbox inout shaft mechanically to the engine crankshaft - like a manual - eliminating the torque convertor losses which waste fuel. Once locked, it will stay this way down to about 50mph if you keep a moderate load on. If you take your foot off, the lock-up will drop out and then re-engage when you put your foot back down again, if the above conditions are met - the actual gear selected depends on road speed and throttle setting.

If the gear is too high for the roadspeed, then the torque convertor will be doing more work, generating more heat and wasting more fuel. This is the basis of the theory that says keep the O/D OFF in town (ie gears 1 2 & 3 only) so that the engine speed can match the low road speed more closely thus eliminating those torque convertor losses that you will get by being in 4th gear at too low a road speed.

It is only a theory though and its doubtful the difference is really measurable. Best you can say is that it exercises the relays and the gearboc controller.


Kev

[kitesurf_phil]

The overdrive is actually the fourth gear. So, with overdrive OFF you are only operating in three gears. With overdrive ON, fourth gear will usually engage somewhere around 30-40mph on a light throttle setting. Once a higher gear has been selected, the gearbox tries to hang on to it by making the torque convertor work harder.

Once you exceed about 55mph AND you have the O/D switched ON, the torque convertor lock-up clutch will operate, which has the effect of locking the gearbox inout shaft mechanically to the engine crankshaft - like a manual - eliminating the torque convertor losses which waste fuel. Once locked, it will stay this way down to about 50mph if you keep a moderate load on. If you take your foot off, the lock-up will drop out and then re-engage when you put your foot back down again, if the above conditions are met - the actual gear selected depends on road speed and throttle setting.

If the gear is too high for the roadspeed, then the torque convertor will be doing more work, generating more heat and wasting more fuel. This is the basis of the theory that says keep the O/D OFF in town (ie gears 1 2 & 3 only) so that the engine speed can match the low road speed more closely thus eliminating those torque convertor losses that you will get by being in 4th gear at too low a road speed.

It is only a theory though and its doubtful the difference is really measurable. Best you can say is that it exercises the relays and the gearboc controller.


Kev

I'm with you on this one.

Vote Kev X

[paulh]

I'm with you on this one.

Vote Kev X

I just got confused !