Gearbox problems: when a fluid service is enough

A gearbox that shifts badly is one of the more worrying things a car can do, because the repair has a reputation for being costly. Often the cause is smaller than feared, and sometimes it is the fluid.

Is gearbox fluid really for life?

Some manufacturers call their gearbox oil a lifetime fill. The oil still ages: it carries heat away, lubricates and transmits pressure, and with time it loses its additives and collects wear material from the clutches. Many gearbox makers publish change intervals of their own, and in a climate like the UAE’s, where the oil runs hot in slow traffic, the case for changing it is stronger.

What old fluid does

• Shifts become slow or harsh, most of all when the gearbox is hot.
• The car hesitates before taking drive when you select D or R.
• A shudder appears at steady speed as the torque converter locks up.
• On a dual-clutch gearbox, pulling away turns jerky.

A fluid and filter service, followed by the adaptation the gearbox needs to relearn its shift points, cures many of these.

When fluid will not help

• Slipping: the engine speed rises and the car does not pick up. Worn clutches do not return with new oil.
• A gearbox in limp mode, stuck in one gear with a warning on the dashboard.
• Burnt, dark fluid or metal in the pan: the wear has already happened.
• A leak from a seal, the pan gasket or the cooler lines: the level has to be put right and the leak repaired.
• Faults in the mechatronic unit, the valve body and electronics that control the shifts.

Automatic, dual-clutch and automated manual

A conventional automatic with a torque converter is the most tolerant and responds best to a fluid service. A dual-clutch gearbox, such as PDK, S tronic or DCT, has two clutches that wear and a mechatronic unit that is sensitive to oil quality. An automated manual has a single dry clutch that is a wear item, like the clutch of a manual car.

What a diagnosis involves

The gearbox control unit records its faults and its adaptation values, which show how far it has had to compensate for wear. The fluid level and condition are checked at the temperature the manufacturer specifies. A drive under the conditions that bring the fault on completes the picture.

Why the sequence matters

Starting with the fluid on a gearbox that only needs a service is the cheap route to a cure. Starting with the fluid on a gearbox whose clutches are worn spends money without fixing anything. Reading the data first tells the two apart.

Looking after a gearbox

• Change the fluid at the interval the gearbox maker gives, and sooner in hot, slow traffic.
• Have leaks dealt with early: a gearbox does not tolerate a low level.
• On xDrive, quattro and other all-wheel-drive cars, keep the tyres evenly worn; unequal tyres load the transfer case.
• Treat a new shudder or a delay as a reason for a check, not something to get used to.