Engine and transmission oil cooler
Ring gears, planet gears and sun gears visible in this disassembled gearbox

Q: I have a turbocharged 2008 Volvo S60 T5 2.4cc. As ambient temperature during the hot season is high, I plan to use air-to-liquid auto transmission and engine oil coolers. What are the pros and cons of original/factory-style liquid-to-liquid oil coolers vs air-to-liquid oil coolers? I want to keep the ATF and engine oil temperatures in good range. Daud A. Carlos

A: I’m going to give a long-winded answer for the benefit of our readers, and to help them understand more about automatics and how they work. If you want to skip straight to the answer, just read the last two paragraphs.

Most of us drive automatics for ease and convenience, especially in our gridlocked city streets. While it may be simple to drive, by no means is the automatic transmission a simple device. In fact, it is closer to black magic.

Which is why many mechanics simply steer away from anything to do with auto gearboxes. Until a customer shows up with a faulty auto gearbox, that is.

Then your friendly neighbourhood foreman reaches for the Yellow Pages (or more likely, the search function on his phone) and looks up the nearest auto gearbox specialist. I have a little dog-eared notebook with the numbers of the specialists’ numbers (besides other suppliers) myself. But it is not that difficult to understand what your autobox does. In the end, it is just a method to change gears just as you would a manual gearbox. Only that other parts replace the missing clutch-and-manual gear lever.

The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in the automatic transmission, the same set of gears produce all the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission.

The workings of an autobox consist of:

• The aforementioned planetary gearset;

• A set of bands to lock parts of a gearset;

• A set of three wet-plate clutches to lock other parts of the gearset;

• A hydraulic system that controls the clutches and bands; and,

• A large gear pump to move transmission fluid around.

These parts allow you to check your Facebook while driving around in the traffic jam.

No, don’t do that, please.

PLANETARY GEARSET

The planetary gearset has three main components - a sun gear, planet gears and the ring gear. Amazingly, we can get the four forward gears and one reverse gear for our transmission from just this one set of planet gears.Explaining how the gears are chosen is a subject even Einstein failed, so, I will not attempt to explain it.

BANDS

Not “The Beatles”, but the bands in a transmission are, literally, steel bands that wrap around sections of the gear train and connect to the housing. They are actuated by hydraulic cylinders inside the case of the transmission. Hydraulic pressure, routed into the cylinder by a set of valves, causes the pistons to push on the bands, locking that part of the gear train to the housing. So, the bands are the culprits that lock certain parts to others in order to choose the gears.

Clutches

In a typical transmission, there are four clutches. Each clutch is actuated by pressurised hydraulic fluid that enters a piston inside the clutch. Springs make sure that the clutch releases when the pressure is reduced. Unlike a manual transmission clutch, the autobox clutch is multi-plate and runs in transmission oil.

HYDRAULICS

The Maze, pump, torque converter, pumps and Governor The Maze, or Black Magic. The real brain of the automatic transmission is this maze of passageways. This maze routes fluid to all the different components in the transmission. It is so complex but it does the job with help from these components;

THE PUMP

The pump is usually located in the cover of the transmission. It draws fluid from a sump in the bottom of the transmission and feeds it to the hydraulic system. It also feeds the transmission cooler and the torque converter.

TORQUE CONVERTER

The torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission, replacing the conventional clutch.

If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still requires only a light pressure on the brake pedal.

THE GOVERNOR

The Governor is a valve that tells the transmission how fast the car is going. It is connected to the output shaft, so the faster the car moves, the faster the governor spins.

Inside the governor is a spring-loaded valve that opens in proportion to how fast the governor is spinning - the faster the car goes, the more the governor valve opens and the higher the pressure it lets through.

VALVES AND MODULATORS

Some cars have a simple cable linkage connected to a throttle valve in the transmission. The further the gas pedal is pressed, the more pressure is put on the throttle valve. Other cars use a vacuum modulator to apply pressure to the throttle valve. Then, there is the manual valve; what the shift lever is hooked up to.

Depending on which gear is selected, the manual valve feeds hydraulic circuits that inhibit certain gears. So, in third gear, it feeds a circuit that prevents the overdrive from engaging.

THE SHIFT CIRCUIT

Shift valves supply hydraulic pressure to the clutches and bands to engage each gear. The shift valve determines when to shift from one gear to the next. It is pressurised with fluid from The Governor on one side, and the throttle valve on the other.

The shift valve will delay a shift if the car is accelerating quickly. If the car accelerates gently, the shift will occur at a lower speed. Very clever, isn’t it?

OIL COOLERS

Most automatic cars come standard with a separate heat exchanger for the transmission oil just below the radiator. Given the high temperatures you can find in a radiator, often just under the boiling point of water, a short cycle through this heat exchanger doesn’t reduce the temperature of the automatic transmission fluid all that much - only by about 10 to 20 per cent.

Still, with few exceptions, this does the job when you’re only doing everyday driving. Some cheaper cars have none. Not good.

So, now we come to the answer to your question. Because overheating is the main cause of automatic transmission failure, it may be worthwhile having a transmission oil cooler installed. This is because an automatic transmission takes extra stress in hot weather, driving hard or if you are carrying heavy loads. These extra stress can raise the temperature in the transmission. Even at just a few degrees hotter than the temperature it was designed for, transmission fluid can become less viscous, therefore, not doing its job of lubrication as well as it should, and reducing the lifespan of the autobox.

An auxiliary cooler can keep transmission fluid at optimum operating temperature longer.

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