Transmission fluid must be monitored

Keeping an eye on transmission fluid is the most important routine maintenance task for owners of vehicles with automatic gearboxes.

Most cars have a dipstick that goes down into the fluid pan of the gearbox and which should be pulled out at regular intervals so that you can look at the fluid on it. It will tell you two things. Firstly, whether there is enough automatic transmission fluid (ATF) in the box. For the fluid level reading to be accurate, makers have specific instructions about what you should do before you take the reading.

Often they want the engine to be running and the gear selector moved through all its positions, then left in Park. But the owner's manual will state the correct procedure for your particular vehicle.

Secondly, and equally important, the dipstick will tell you the condition of the transmission fluid. Fresh ATF has a transparent bright red colour.

In use, the fluid slowly oxidises, changing colour to a slightly cloudy orange-red as it does so. By the time the fluid is more orange than red, it has lost some of its lubricating properties and should be replaced as soon as possible.

Excessive heat speeds up the oxidisation process, which is normally a very slow one. To ensure that the fluid doesn't overheat, some form of cooling is usually provided by the maker.

In older vehicles the fluid was often circulated through a pipe immersed in the radiator bottom tank where it was supposed to be cooled by the (equally hot) engine coolant. Not surprisingly, many makers now fit separate ATF coolers in the airflow behind the grill. If there is any chance that the fluid has overheated, perhaps while towing in mountainous terrain on a hot day, or because something went drastically wrong in the engine's cooling system, the transmission fluid should immediately be inspected on the dipstick. Badly overheated fluid will be brownish and may even have a burned smell.

To drain the fluid, the pan at the bottom of the gearbox normally has to be removed. It's a messy job, best left to a workshop with the necessary facilities. But it allows one to change the transmission filter as well. Check the old filter carefully for debris, which is an indication of impending transmission problems.

On most transmissions one only gets about half the fluid out when it is drained. The other half is in the torque converter and cooler lines, and cannot be drained without major disassembly. The fact that some fluid remains in the system is taken into account when the maker decides on fluid change intervals.

These should be strictly adhered to, even if the old fluid still looks perfectly good. Make sure that the transmission is topped up and refilled with the fluid stipulated in the owner's manual. Contrary to what some workshops may tell you, not all transmission fluids are the same, and certain cars are notoriously sensitive about the correct ATF.