Automatic Gearboxes and Their Use.


 Automatic gearboxes have been around for a long while. Development of the Automatic gearbox has progressed at a pace as other modern vehicle systems. The modern Automatic Gearbox and its modern derivatives are becoming more common but some drivers are uncertain how to make the best use of them.

These notes outline the expectations of the examiner, they are designed to assist you get the most from your vehicle as you prepare for the advanced test. Included in these notes are details of the general construction and principles of the automatic gearbox. There are also brief descriptions of typical options and features. A reasonable understanding of these will enable you to make appropriate decisions based on the prevailing circumstances and the performance of your vehicle.

The modern automatic gearboxes can contain anything from three to seven different ratios and may also have other features such as ‘Overdrive, Economy, Sport’ or Ice modes. They may also have different names such as Tiptronic, Selespeed, Selectomatic, Easytronic, Steptronic and Multitronic. Some allow fully automatic mode and an optional manual mode.

(There is also another type called a CVT, Continuously Variable Transmission. (See separate Article).

The change mechanism can be mounted on the floor, on the dashboard or within the steering wheel/column. Manufacturers have invested heavily in these gear change systems and you should first refer to the owner’s manual for information and advice.

Basic Construction.

In the majority of cases the Automatic gearbox consists of 2 main assemblies; 1. Torque converter 2. Gearbox These are fitted to the car in place of a conventional clutch and gearbox.

The torque converter is a fluid (hydraulic) drive and consists of an impeller, stator and turbine. The impeller is connected to and driven by the engine flywheel. The stator is fixed to the casing and the turbine is connected to the gearbox input shaft. The impeller and turbine are both bowl shaped with the stator fixed between them. The impeller and turbine face each other. The impeller has integral blades; the stator and turbine have integral vanes. The torque converter is flooded with transmission oil called Automatic Transmission Fluid (ATF).

With the engine running, the engine driven impeller imparts energy to the oil inside the unit and forces it, by centrifugal force, out of the blades of the impeller towards the turbine. The energy imparted to the oil impacts on the turbine vanes and is used to drive the gearbox. Oil returning to the turbine passes through the stator. The stator also has vanes, which are designed to increase the velocity and energy of the returning oil as it passes back to the impeller. As the oil re-enters the impeller, the engine further increases the oils energy. This action has the effect of increasing or multiplying the torque transmitted by the engine to the gearbox. Hence, the name ‘Torque Converter’.

The Gearbox is a set of planetary or epicyclic gear sets or gear trains, interconnected and constantly in mesh, to give various gear ratios. The gear ratios are selected by hydraulically operated multi-plate clutches. The clutches are engaged automatically according to engine speed, road and load conditions.

Most are now computer controlled and in conjunction with the engine management system and sophisticate hydraulic controls give smooth gradual engagement of gears up and down the ratios. This fluid connection between the engine, through the torque converter and the gearbox, to the drive wheels means that leaving the gear lever in ‘D’ when negotiating a hazard is not the same as leaving a manual gearbox in top gear.

Gear selectors Gear selectors will vary according to vehicle manufacturer but the majority of gear selector positions are marked:

P  = PARK. This locks the transmission and prevents the car from moving. It must never be engaged whilst the vehicle is in moving, as it will cause serious damage.

R  = REVERSE. Reverse drive is selected

N  = NEUTRAL. As in a manual gearbox no drive selected

D  = AUTOMATIC. Drive using all forward gears from 1, through to 3 and 4 or more.

For normal driving the selector lever can remain in ‘D’ and allow the transmission to make automatic adjustments according to road speed, engine loading and accelerator position. Some may also be configured with another position or gate with a + and – symbol or position for upward and downward manual selection of gears.

When a selector of a conventional automatic gearbox is placed in 3 the gearbox will operate in automatic mode between 1st and 3rd gear only. When 2 is selected it will operate between 1st and 2nd gears only.

Basic Operation.

When the engine is running and D is selected the lowest gear is engaged and it tries to move the stationary vehicle. As the brake is released and the accelerator depressed the drive overcomes the load of the stationary vehicle and the vehicle moves forward. As speed increases, sensors on the vehicle and engine control the gearbox via the computer and it changes up through the gears.

It continually matches the gears to the road, load and speed conditions changing up and down to optimise the best performance. For normal driving the selector lever remains in D, which allows the transmission to make automatic adjustments according to road speed, engine load and accelerator position.

Options and Additional Features.

Safety interlocks:

Most modern automatic transmissions have various safety interlocks or devices to prevent operation under certain conditions. Ignition Interlock. This prevents the engine being started unless the gear selector is in P or N. It may also be linked to the driver seat belts, drivers seat cushion or drivers door.

Brake interlock. This prevents the gear selector being moved from P to any other position unless the driver depresses the footbrake.

Gear Selector interlock. Prevents R and P being selected unless a button or plunger on the gear selector lever is fully depressed. It may be integrated with the brake interlock so the R or P cannot be selected whist the vehicle is in motion.

Audible warnings. Some vehicle manufacturers fit audible warnings, which sound when:

a) The ignition key is turned to stop/off with the gear selector NOT in P.

b) When the driver’s door is opened and the ignition key is NOT in the stop/off and/or P selected

c) When Reverse is selected.

Downshift Inhibitors. This feature is part of the automatic gearbox hydraulic system. It prevents a manual downward shift to a lower gear when the road or engine speed is too great. The gear selector lever will be able to be moved by the driver but the gearbox will not change down until the appropriate lower road or engine speed is reached.

Economy devices.

Overdrive. Similar to an overdrive unit of a conventional gearbox and generally used on 3 speed automatic gearboxes. It normally has a manual on-off switch to give more flexibility to the driver.

Lock-up clutches. An automatic gearbox is not as efficient as a conventional clutch and mechanical gearbox. There are internal losses in the hydraulic drive of the torque converter. This is why an automatic gearbox is not as economical. To counteract this inefficiency some manufactures are now fitting lock up clutches. The lock up clutch allows the impeller and turbine of the torque converter to be mechanically locked as one unit producing a solid drive similar to a conventional clutch. This improves fuel economy. The lock up clutch is engaged automatically when a preset cruising speed is reached. Typically this can be from 40 mph upwards. If the accelerator is pressed gently to increase speed it remains engaged. However, if the accelerator is pressed quickly the lock up clutch will disengage and the torque converter will operate normally. When an appropriate speed is reached it will re-engage again. Lock up clutches may be fitted to 3rd and 4th gears of a 4 speed automatic gearbox or on the top 3 speeds of a 7-speed gearbox.

Auto-Neutral. Some later automatic gearboxes (e. g. General Motors) are fitted with a device that automatically puts the gearbox into neutral when the foot brake is applied at traffic lights or road junctions. The selector still remains in D. As the foot brake is released the drive re-engages. This action takes place before the footbrake is fully released so that the vehicle does not roll backwards or forwards when on a slope.

Kick down. This feature allows the driver to accelerate rapidly when the accelerator pedal is depressed sharply past a preset built in resistance. When the accelerator is “kicked down” the gearbox changes down to an appropriate lower gear for rapid acceleration of the vehicle. This can be quite alarming for the inexperienced driver as the gearbox may drop down to 1st or second gear. The gearbox will change up again when the accelerator is released.

Sport. This feature allows the gearbox to maintain a specific gear ratio longer up the rev range. It is normally selected by the driver via a button on the gear selector lever or next to the gear selector lever. On some modern automatic gearboxes, which are designed for economy rather than performance, the kick down facility can be quite sluggish. It can be very advantageous to select the sport facility as it usually reacts quicker than kick down. This gives added flexibility.

Ice or Snow. This feature is used for winter driving in icy and snowy conditions. It is selected by pressing a button alongside the gear selector or on the dashboard. This locks the gearbox in an intermediate gear (usually 3rd gear) reducing the risk of wheel spin when setting off. Setting off is much slower as the gearbox has to move the vehicle from rest in 3rd gear. The torque converter manages this with ease and no excess strain is placed on the gearbox or drive components. The feature automatically reverts to normal operation when a preset road speed is reached (normally 40 mph.). On manual/auto transmissions it only works when in automatic mode. If manual operation is selected it disengages.

Use of Automatic gearboxes.

Many drivers of automatics leave the gear selector in D and never consider other options built into the gearbox even though there are times when this might be clearly desirable to optimise flexibility and control of the vehicle. In many cases this is due to ignorance of the potential benefits of using the full range of the gearbox. If it is necessary to manually change down and limit the range of gear ratios the gear lever or change mechanism should be moved to the required position. The owner’s manual will describe the most appropriate method. Selecting the ratio is done within the system of car control when the desired speed has been reached and that speed is within the range of the ratio chosen.

Most modern systems will override the lever selection and prevent a change to a lower gear if the engine revs or the road speed are too high. Manually selecting a set of ratios may be in response a particular hazard where there is a need for more control through use of the accelerator. This will prevent the gear changing up automatically, which may result in the vehicle ‘running on’ and increasing speed when this is not required. Manually locking a ratio also provides the flexibility to control speed during and after an overtaking manoeuvre, as an alternative to a ‘kick-down’ or when approaching an area of uncertainty. However, when the specific or general need for flexibility has passed the ‘D’ (Drive) option should be reconsidered.

Candidates may also choose to retain a lower ratio, for instance within a built-up area, to improve control through the accelerator. However, the upper ratio selected should be appropriate to the circumstances. Selecting 3 in a 5 speed automatic box may be suitable for urban driving but 4 may be the better and more flexible choice for winding rural roads. Leaving the gearbox in ‘D’ may be appropriate for open bends where the flexibility of a lower ratio is not considered necessary.

Candidates should remember that, as with a manual gearbox, selecting a specific ratio on an automatic box should take place when the correct speed for the hazard has been attained. As with a manual gearbox secondary braking should be avoided. Some police forces advocate a more direct manual use of the automatic box, particularly in ‘pursuit’ or ‘response’ situations. However, for the purposes of the RoSPA test, where that degree of flexibility and maximum performance is neither required nor necessary, the manual over-ride facility must not be used excessively.

When immediate maximum acceleration is required the ‘kick down’ facility may be beneficial. This is engaged by pushing the accelerator pedal to the full throttle position, overcoming the built-in resistance. This causes an immediate down shift into the gear for maximum acceleration. When the accelerator pedal is released the gearbox will automatically change up.

When stationary in traffic, even for many minutes, it is not necessary to move the gear lever into neutral because the torque converter absorbs the engine’s propulsion force but does not transmit it all to the gearbox. No wear is taking place. Some people believe that more wear will take place if you engage neutral then re-engage a drive gear when it is possible to move off. This is not the case as the clutches that engage the gears are so well designed, built and accurately controlled that any wear that could take place is minimal. Most gearboxes will automatically select first gear when the vehicle becomes stationary.

While guidance may be given regarding the correct actions in certain circumstances, not all eventualities can be covered. The following is intended as general guidance in some common sets of circumstances. You should apply the footbrake when moving the gear lever from P when the vehicle is stationary. You may wish to set the parking brake as many vehicles will ‘creep’ and some vehicles may have a tendency to lurch, particularly if fitted with an automatic choke.

Stops in traffic, at junctions and at traffic lights leave the selector in D. There is no need to move into neutral, as no damage will result. You may wish to set the parking brake if the pause becomes a wait but your decision will depend on the circumstances. In wet weather at night you may want to set the handbrake and select N to prevent excess glare to any vehicles behind you.

 AT ROUNDABOUTS: Leave in D unless a lower set of ratios has already been selected, in which case it may be appropriate to leave it in that ratio.

ON BENDS: Single bends can normally be negotiated in D. For a series of bends consider locking the vehicle into a suitable lower ratio prior to the first bend, and on exit from that bend, when the accelerator is eased to set the vehicle up for the next bend, the vehicle will not automatically change up and the driver will have the benefit of engine braking which will give better control.

OVERTAKING: Depending on the circumstances, use, either a planned, predetermined lower ratio lock or the ‘kick down’ or sport mode. If acceleration is needed followed by deceleration to fit into a gap, manually selecting a lower gear or ‘locking’ the ratio before the start of the manoeuvre may be beneficial.

STEEP HILLS: When descending steep hills in DRIVE, the vehicle will tend to maintain or drop into its highest ratio. This will result in excessive use of the brakes. Although brakes on automatics are larger than on their manual counterparts, ‘brake fade’ can still create a problem. However, manually locking a low ratio provides compression braking to enhance flexibility and braking control, particularly when towing a trailer/caravan. Conversely, when ascending a steep hill, manually locking a ratio may also provide better control and improve smoothness if the vehicle is hunting (changing down then up again) between two ratios.

GENERAL: In unusual circumstances when the gearbox is continually changing up and down between two gears, manually selecting an appropriate ratio may prevent undue wear of the gearbox components and give a smoother ride. On twisting roads ‘Sport’ mode will often hold a gear sufficiently thus reducing the need for a manual over-ride. It is not necessary to either kick down or change down manually for a hazard simply because one would change down if driving a car with a manual gearbox when negotiating that same hazard. The modern automatic gearbox is designed to select the correct ratio for the speed and throttle setting, and it does so very well. A manual intervention should be a considered option and planned to give a specific advantage or benefit according to the circumstances encountered. It should not be undertaken solely to demonstrate to the examiner that you know how to do it. As with all aspects of driving, this technique is not carved in tablets of stone. There may well be other occasions when you judge it necessary to manually over-ride the gearbox and, if that is the case, then do it. But do not make excessive use of the manual holds and return to ‘D’ when the need has passed.