We all know what an automatic transmission is, basically—very basically, at least—so I'm not going to bore you with the details of how it converts power from your engine into power at the wheels. But you should know that the process involves plenty of hydraulic fluid, as well as gears and clutches.
If you don't know why your car’s transmission is slipping, I just might have the answer, though you may not want to hear what I have to say.
The cause of a slipping transmission will depend on the type of transmission you have in your vehicle. There are three types of transmissions: automatic, standard (or
“manual”), and CVT (continuously variable transmission). I will discuss the two most common types, automatic and standard.
Why is my Automatic Transmission slipping?
If you have an automatic transmission and it "slips" while you are driving it—that is, the car engine revs without the power going to the wheels—the most common cause (though not the only possible cause) is low transmission fluid. If the slipping is caused by low fluid, it will get worse as the transmission gets hotter.
Why is my Manual Transmission slipping?
A standard (“manual”) transmission uses fluid too, but leaks aren’t a common issue; a standard transmission could lose all its fluid and never slip at all, though eventually it would lock up while driving down the road. If you have a standard transmission, and it’s slipping—the engine revs but it doesn’t transmit power to the wheels—the problem is usually in the clutch.
If your Automatic Transmission is low on fluid
The next question is, why? Probably because you have a leak. Maybe you have noticed red transmission fluid in your driveway or your parking spot at work, but it didn’t occur to you that it came from your car. The cause of your leak is probably a failure of one of the seals that keep the oil inside the transmission (though there are other places the transmission might leak). How many seals the transmission has depends on whether you have a front-wheel-drive, four-wheel-drive, or rear-wheel-drive car.
If you do have a seal that is leaking, and you catch it in time before it damages your transmission, there’s a chance it might be relatively cheap to repair. Seals themselves are not expensive parts, but depending on where they are located, it might take a lot of labour to replace them.
If you have the leak checked out by your mechanic, and he tells you it’s just your axle seal, it could just cost you for a couple hours of labour, and you could be back on the road in a few hours if the parts are readily available. Axle seal failures are common, and the dealer will usually have those seals in stock. But if it's your input shaft seal, plan on leaving your car for a day or two, and expect the labour cost to be upwards of eight to 10 hours.
Checking and topping up Automatic Transmission Fluid to prevent slipping
If your transmission is leaking, you will want to fix it. Meanwhile, if you must drive a car with a leaking transmission, you should top up the fluid.
Check your owner’s manual to see exactly how you should check your transmission fluid level. Some manufacturers will want you to check the transmission fluid level when the engine is warmed up and running with the transmission in park; other manufacturers such as Honda will have you check it with the engine warmed up but not running. So be sure to read your owner's manual. Don’t guess!
If your transmission fluid level is low and not showing on the dipstick, add the proper transmission fluid. Each car manufacturer uses a specific fluid. If you use the wrong fluid, you could damage your transmission internally, so again, check your owner's manual. Fill it to the top line on the dipstick and then drive it to your mechanic.
Note: Sometimes when an automatic transmission is low on fluid and you top it up, air pockets develop inside the transmission that keep the fluid from getting to all the parts of the mechanism. After adding the fluid, I suggest that while the car is running, you move the shift lever through the different gears on the shifter selector, and then check the fluid again. Running the shifter through the gears redirects the fluid to different parts of the transmission and removes the air pockets. You may have to do this procedure several times, or even drive the car around the block, and then recheck the fluid level.
I do not recommend using additives that are supposed to stop leaks; they can plug up the transmission. Use the automatic transmission fluid your car manufacturer makes.