Red fluid on the ground? Don't drive it
If you see a red trail or a puddle, or the car starts slipping, jerking or refusing to go into gear, stop somewhere safe and switch off. RAA's advice for a transmission leak is not to drive the car until it's been inspected. Fluid on a hot exhaust can smoke or burn, so if you see smoke or smell burning, get everyone well clear. Call roadside assistance if you have it. If anyone is hurt or in danger, call 000.
How do you know it's transmission fluid?
Transmission fluid is dyed so it can be told apart from engine oil. It's usually red or green, and most of it darkens with use, so colour alone isn't reliable. An old leak can look brown or black. It's usually thinner than engine oil and often leaves its spot around the middle of the car, though it can show up further forward if the leak is at a cooler line.
Power steering fluid is red too, RAA notes, so a red puddle at the very front could be that. A burst high-pressure power steering hose is a fire risk, so a mechanic needs to see that one quickly as well. See power steering fluid leak.
Manual gearboxes have their own thicker oil. It tends to seep rather than run, and it can have a strong smell. Most of what follows applies to both, though the repairs differ.
Where do transmission leaks usually start?
| Why it leaks | What's usually involved | |
|---|---|---|
| Driveshaft seals | RAA says they leak where the driveshaft leaves the transmission, often after driveshafts have been out for other work. | Pull the driveshaft, fit a new seal, refill with the right fluid. |
| Cooler hoses and lines | RAA says they can split and leave a bright red trail. On many automatics the fluid runs to the radiator to shed heat. | New hose or line, refill, check the level. |
| Pan gasket or sump plug | An old gasket, or a plug not sealed after a fluid service. | New gasket or washer, refill. |
| Front seal, between engine and gearbox | Fluid appears around the join between the two. | A bigger job: the gearbox usually has to come out. |
Why a small leak matters more here than on an engine
An automatic uses this fluid to do almost everything: it lubricates the gears, works the valves and clutches, and drives the torque converter. Run it low and the gearbox can slip and overheat, and the damage can outrun the original leak quickly. That's the difference between a seal and a rebuild.
A lot of late-model automatics have no dipstick, so you may not be able to check the level at home. RACV says regular servicing normally covers it, and fluid that drops quickly or needs frequent topping up should go to a mechanic.
The right fluid matters as well. Different transmissions need different fluids, so don't stray from what the owner's manual specifies. A generic top-up can do more harm than the leak.
What to do now
Park it and leave it
If it's safe where it is, leave it there. If you're on the road, get to the nearest safe stop.
Photograph the spot
Take a photo of the fluid and where it sits under the car, plus any trail on the driveway. A sheet of cardboard under the car overnight helps if it's slow.
Note how it's been driving
Late shifts, slipping, a flare in revs, a burnt smell or a warning light all help the mechanic.
Ask before you top up
Don't guess the fluid or pour in additives. Check the owner's manual for the type, and ask the mechanic what they want you to do.
What the mechanic will check
They'll lift the car safely, clean the area, and trace the leak to the highest wet point. They'll check the fluid level and condition, using a scan tool on sealed units, and read any stored gearbox codes. Dark or burnt-smelling fluid suggests the transmission has been hurt, and it's better to know that before paying for a seal.
If coolant in the radiator looks pink or milky, tell them. On cars where the fluid runs through the radiator, a fault there can let the two fluids mix.
A good written estimate names the seal or line, the exact fluid going in and how much, and says if the gearbox has to come out. Ask whether a road test is included after the repair. For sealed automatics and CVTs, look for a mechanic who regularly does transmission work, since filling and checking these takes the right scan tool and procedure.
What you can do, and what's the mechanic's job
You can
- Leave the car parked, and photograph the leak.
- Note any slipping or late gear changes.
- Find the fluid type in the owner's manual.
Leave to the mechanic
- Lifting the car safely and tracing the leak.
- Replacing seals, hoses or the pan gasket.
- Checking the level and refilling with the right fluid.
Our take
Don't test your luck. We'd leave the car parked, send the job today and say in the note that it's leaking and hasn't been driven. If it needs moving, ask the mechanic how they'd like it done, because some automatics shouldn't be towed with the drive wheels on the road. A small repair done now is the outcome you're after.