Workings of an auto 'box

[Pulaski]

Please can you settle an argument.

Automatic 'boxes generally have several numbers beyond the R/P/N/D marks. There are usually, though not always (so I hear) one fewer numbers than speeds in the box.

Please can somebody explain what the numbers signify? ..... is it merely a option that the driver has to lock in to a particular gear? Or do they have some other meaning as in the US they are commonly called the "low gears". .... are they really any lower?

Any other thoughts on how/ when these gears should be used would also be welcome.

 

[ivymike]

The number signify the highest gear that will be used by the transmission when that setting is selected.

Example: 1997 Honda 4-speed Automatic
"Prundle" (or P-R-N-D-L) display reads P-R-N-D-3-2

If I select D, the transmission will pick whichever of the four forward speeds it deems appropriate. If I select 3, the transmission will not select the top (highway) gear, but will choose one of the remaining 3 gears. If I select 2, the transmission will not select the top two gears, but will pick either (1) or (2) depending on conditions.

Occasionally, such as when climbing a long hill, it is nice to lock the transmission out of top gear so that it doesn't continually shift up & down & up & down. In such a situation I'd pick 3. (the honda transmission is usually pretty good without input, because it has an inclination-sensitive switch to let the tranny know it's on a hill)

Occasionally, such as when going down a long steep hill, it's nice to engine-brake to keep the vehicle speed under control. In this situation I'd pick 2.

 

[TheBlacksmith]

My new F-150 (with electronic trans) has D, 2, 1. There is an overdrive off switch to lock it out of overdrive. Selecting 2 starts the truck off in second gear - poor man's four wheel drive, similar to the C-6 3 speed auto inmy old F-250.

Blacksmith

 

[ivymike]

The physical mechanism behind "grade logic" is nothing but a little mercury (or similar) switch that tells the trans controller when you're on a hill. The control strategy is altered when the trans believes that you're on a hill.

A side effect of the type of switch that they used is that it's acceleration (deceleration) sensitive, and will downshift if you're braking hard. Sometimes that's exactly the behavior you want, but it can really throw you off if you're braking to achieve a desired deceleration rate and suddenly the car starts engine braking as well. Once you get used to it, you can learn to press the brake just hard enough to initiate engine braking, and then put your foot back on the gas pedal while the car is still rapidly slowing down, and wait for the proper time to accelerate again. (not sure of the exact type of switch, but I've definitely observed this effect)

 

[Pulaski]

Many thanks Mike, that was very helpful, especially as it answered the question in my favor!

Regards,

Pulaski