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Car Care: Clever automatic transmission delivers better economy
By Jack Biddle • 09/05/2015
Gear selection helps to lift car's overall performance
A reader wanted to know why modern automatic transmissions have the tendency to remain in some gears longer or change down unexpectedly, especially when descending hills.
The reader had bought a new automatic car and had noticed the difference and wanted to know if there was a problem he should be reporting back to the dealer, or should he be changing his driving technique were some of the concerns raised.
Let’s start by trying to provide an explanation as to just what is going on and why.
When first introduced into mass production, most automatic transmissions were basic and predictable.
Gear changes were determined by two main factors: road speed and throttle position. It was almost a battle between the two in fact and controlled by internal oil pressures within the transmission itself.
Light throttle and reasonable road speed meant up-changes happened fairly quickly while being more aggressive on the throttle resulted in changes taking a little longer to happen.
There was also the ability for the driver to “kick-down” to a lower gear if full throttle was used which was great when overtaking slower traffic.
If you can picture a valve with pressure applied to either end, and another pressure line directed to the centre of the valve we will attempt to explain how it all worked.
The pressure at each end of the valve is controlled by two different forces.
One is road speed while the other is throttle position. The faster the vehicle goes and the lighter the throttle, then road speed wins and the quicker the valve will move in a certain direction.
That allows a separate oil pressure feed to move through the centre of the valve and allow a particular gear to be introduced.
The change in oil flow direction also means the previous gear selected is no longer engaged.
If throttle pressure is greater than road speed, then the reverse happens and the gear change takes a little longer to engage. In older automatics there is no other variation in the gear change pattern apart from an external throttle cable adjustment. Up or down, gears changed because of those two main predetermining factors.
The introduction of on-board electronics opened up a whole new set of benefits for the automatic transmission.
Trying to “think” for the driver and changing the shift pattern as and when required on the modern autos, has been given several different names, but the one that simplifies it best for me is called “grade logic”.
As the name suggests, it allows the transmission to change its shift pattern depending on certain conditions and is controlled by an electronic transmission module and shift solenoids.
The two key elements of road speed and throttle position remain, but the introduction of electronics allows the computer to “read” both the road and the driver, plus other conditions such as outside air temperatures and to alter the shift pattern accordingly.
So picture a vehicle travelling up a long ascent and the driver accelerating and decelerating when encountering a combination of straights and bends.
The transmission will tend to “hold” a gear longer to stop the constant up/down changes based on feedback from various sensors connected to the control unit. Same but different travelling downhill; the transmission will once again pick the best gear ratio which can at times provide some engine braking assistance to the driver.
Once the vehicle comes off a long downhill run is where a lot of drivers can often detect a delay in changing to a higher gear. A slight increase in road speed/ throttle position by the driver can sometimes quickly help the control module to make the necessary change.
While many automatics have a manual option where the driver can make their own decisions on change-ups and change-downs, they are for many seen as a novelty that soon wears off. While the driving enthusiasts may love it, many other drivers simply rely on and trust the transmission to make the best call.
My best advice when asked this question was to get the transmission checked out by the dealer.
They can quickly hook up their diagnostic gear and check for any fault codes being held in the control module’s memory. At times, any in-service software updates can also be carried out by using this equipment.
Note: While there are several different types of electronically controlled automatic transmissions on the market currently, the same basic principles apply to help provide better overall performance and improve fuel consumption.