Automatic transmission problems: the early warning signs

Slipping

You press the accelerator, the revs rise, and the car does not pick up speed to match. It can feel like the gearbox briefly dropped into neutral between gears, or like the engine is running away from the car on a hill.

Inside the box, clutch packs and bands are applied by fluid pressure. Slipping means one of them is not holding, either because the pressure is not there or because the friction material is worn. Low or tired fluid is the first thing to rule out, because it is the smallest bill and it makes every other cause worse.

Harsh, late or clunky shifts

A thump going into drive or reverse, a pause before the car takes up, or a gear change that lands with a bang rather than a nudge.

Modern automatics are run by a computer that adjusts how each shift is made as the gearbox wears. When it runs out of adjustment, or when a sensor gives it bad information, the shifts go harsh. That is why a scan comes before any conclusion. A faulty speed sensor or an engine misfire can make a healthy gearbox shift badly.

A shudder at light throttle

A vibration like driving over rumble strips, usually at a steady speed with a light foot, that goes away if you accelerate harder or lift off.

That is often the torque converter's lock up clutch grabbing and releasing instead of engaging smoothly, and fluid condition plays a big part in it. Dual clutch gearboxes have their own version: a judder when pulling away slowly, which points at the clutch packs.

Limp mode

The transmission warning light comes on, the car holds a single gear, and it feels gutless. That is deliberate. The computer has seen a fault serious enough that it has stopped shifting to protect the gearbox.

Limp mode is designed to get you somewhere safe, not to be driven on for a week. The stored fault code is the starting point, and it is worth having it read before the codes are cleared by anyone.

Fluid on the driveway, or a burnt smell

Automatic transmission fluid is usually red when new and darkens with age. Red or brown drips under the middle of the car, rather than the front, often mean a transmission leak, commonly from a pan gasket, a cooler line or an output shaft seal.

A burnt smell from the fluid, or fine dark particles in it, tells you the box has been running hot or wearing inside. It is information worth having before anyone decides what to do next.

Checking the fluid is not always simple

Plenty of modern automatics have no dipstick. The level is checked through a plug, at a set temperature, with the car level, and the right fluid is specific to the gearbox. The wrong fluid can cause exactly the shifting problems it was meant to fix.

So if a gearbox is behaving strangely, resist the urge to top it up with whatever the service station has. Have the level and the condition checked properly first.

Why early matters

A gearbox that is starting to slip, caught early, is often a conversation about fluid, a sensor, a leak or a software adaptation. The same gearbox driven until it stops going into drive is a conversation about a rebuild or a replacement, and the gap between those two bills is large.