The gears in your car make it possible to drive up and down hills and pull heavy loads without losing power. Lower gears give you enough torque to get under way, while higher gears let you cruise along at speed without making your engine work too hard.

For your gears to work at their best, they need lubrication in the form of transmission fluid. This high-performance type of oil is designed to last for a long time but needs to be replaced ever 30 to 60 thousand miles. We’ll show you how it’s done at home, to help you save a lot of money.

What Transmission Fluid Does

The transmission in your car is a complicated beast, with hundreds of moving parts that all need to work seamlessly with minimal space between them. And they need to continue working flawlessly for thousands and thousands of miles. 

In order to keep your gears cool and to stop them from rubbing against each other, you need high performance fluids that work perfectly with your car. Think of it like a squeaky hinge on a door- without the right lubricant it will make a horrible noise because of metal on metal contact. If you let your transmission fluid get low or dirty, it could cause serious problems. 

Transmission fluid is some of the highest performing lubricant you’ll encounter on a day-to-day basis. It needs to be light and thin, so that it can flow around the many moving parts in your transmission. Modern gearboxes use fully synthetic fluids that don’t need changing often but should be looked at every 40 to 50 thousand miles.

Failure to look after your transmission fluids can cause a lot of problems in complex gearing systems that are costly to fix or replace. As a rule of thumb, fluids and oils are cheap but gearboxes aren’t, so it pays off to keep them running well with regular checks and maintenance

transmission fluid

Symptoms of a Failing Transmission

If you can spot problems with your car’s transmission early on, you’ll have a better chance of fixing them by changing the transmission fluid. Leaving an issue until it’s too late can be extremely costly, but how are you supposed to know when something in your gearbox is going bad? Here are a few ways to spot a bad transmission:

The fluid doesn’t look or smell right

The first and easiest thing to do to diagnose problems with your transmission is to check your transmission fluid. When it’s new it’s usually red and clear, with a sweet smell. If it’s black and smells burnt, you have a problem.

Strange noises

Noises in your car are always a good place to start when fault finding. If you notice something that sounds strange, don’t ignore it because it’s often a sign that something isn’t working the way it should. 

A high-pitched whine that changes as you shift gears, especially when you go into reverse, is a tell-tale sign of problems. A grinding noise coming from your transmission when you shift is another red flag to listen out for. 

Any persistent metal-on-metal sounds that you notice while changing gear should be investigated immediately. The same goes for an automatic transmission- try shifting in and out of first and second gears on your selector and listen for clunks, bumps or grinding noises. 

The Clutch is Slipping

You’ve probably heard of this term before, but let’s make it clear what it means: The clutch in a manual car is essentially two plates that spin together and connect the power from your engine to your wheels. A slipping clutch is when these two plates come together but don’t have enough grip.

You should notice a slipping clutch right away but try this diagnostic to make sure: 

  • Keep an eye on your rev counter when you change gears up 
  • Put your foot down and watch the rev counter fly up
  • If your speed doesn’t increase on your speedometer, it’s probably a slipping clutch

Even though an automatic transmission is a different beast altogether from a manual, it still relies on high-performance transmission fluid to work properly. If your automatic car is slow to pull away and revs high between gear changes, it’s a sign of a slipping clutch.

Strange Smells

Any burning smells coming from your car should be a cause for concern, but the smell of burning transmission fluid is quite noticeable. It probably means that the fluid is past its best, and not doing its job of keeping your transmission cool. 

How to Check Your Transmission Fluid

Checking your transmission fluid level is not much different to checking your oil level. Both systems have a dipstick that you find inside your engine compartment, but the transmission one is often better hidden than the oil. 

  • Check your service manual or owner’s forum to find the transmission dipstick on your car
  • Make sure you’re parked on level ground; this can have a huge effect on your reading
  • Switch on your car and let your engine warm up, don’t check your levels when cold
  • Pull out the dipstick and inspect the fluid for colour, consistency and smell- it should be clear, thin and not burnt
  • Clean it off with a clean rag
  • Replace the dipstick and pull it out again
  • Check the gauge and make sure your transmission fluid level is “full”
  • Replace the dipstick and switch off your engine

checking transmission fluid

How to Top Up Your Transmission Fluid

If you check your fluid and it’s reading lower than full, it needs topping up. 

Note: Check the type of transmission fluid for your car in your service manual before topping up. Never use fluids for a different car make or model because they can cause irreparable damage to your transmission. 

You’ll need a clean cloth and a narrow funnel for this job. Follow the instructions above for checking your transmission fluid, but before you replace your dipstick, carefully insert your narrow funnel into the inspection port. Top up with new transmission fluid a little bit at a time- keep checking the level so you don’t overfill. Replace the dipstick and switch off your engine. 

How to Drain Your Transmission Fluid

If you’ve accidentally overfilled your transmission fluid, or you need to replace the fluids completely after 40 or 50 thousand miles, you need to drain the transmission. 

Makes and models of car are all different, so make sure you consult your service manual before attempting this job. This is a guide to the most common method of draining your transmission fluid.

What you’ll need:

  • Rubber gloves to keep the lubricant off your hands
  • A good car jack and axle stands
  • A socket set with the right sizes for your car
  • An oil pan or suitable container to catch the old fluid

A step-by-step guide to draining your transmission fluids:

  • Park your car on flat ground and jack up the front end
  • Place your axle stands somewhere solid and let the jack down on them
  • Locate your transmission and the drain plug, and put on your gloves
  • Place your oil pan under the drain plug and use a hex key to loosen the plug
  • Remove the plug the rest of the way with your hands and let the oil drain out
  • Leave it to drip dry for a while, but don’t throw away the lubricant
  • Tighten up the drain plug with your socket
  • Measure how much transmission fluid came out into your pan and make a note of it

Note: Check if you need to replace your transmission fluid filter- some cars don’t have them. It’s a bigger job to fix but if you’re a keen DIYer it’s worth it to save a lot of money at a garage.

draining transmission fluid

 

Replacing Your Transmission Fluid

If you’ve drained your transmission fluid, then you’re going to need to replace it again. If you followed the steps above to drain it, you should have your old lubricant measured out. 

Note: In some cases, it’s not a good idea to replace your old transmission fluid for brand new product. Some old car transmissions have worn to the point that new lubricant just won’t work.

Why did you save your old transmission fluid? When you drain your transmission, not all of it comes out. It’s essential to know how much has been removed so you know how much to put back. 

Follow the steps above for checking and topping up your transmission fluid, and using your narrow funnel, refill your transmission fluid. Only ever use the manufacturer’s recommended lubricant here, especially in automatic transmissions.  

replacing transmission fluid

 

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