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How to Check Your Transmission Fluid

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How to change your automatic transmission fluid

Transmissions are one of the most essential and delicate components of your vehicle. Inside, they have many moving parts and complex mechanisms that must be perfectly lubricated. If not, they will cease to function properly and lead to costly repairs. In this article, we will give you some tips to keep your transmission in excellent shape and avoid potential breakdowns.

A Note About Modern Vehicle’s Transmissions – To Check or Not to Check the Oil

In the past, checking the transmission fluid’s level and condition was as easy as removing a dipstick. At some point, automakers introduced “sealed transmissions”. Most of these transmissions are supposedly service-less and don’t have a dipstick to check the transmission fluid level. There are even some contradictions between user manual and service manuals. Sometimes, the user manual doesn’t include any service or fluid change schedule for the transmission, but the workshop manual does. It wouldn’t be crazy to assume that gearboxes may have longer service intervals thanks to the advances in oil technology, engineering, and new materials.

However, no matter how sealed they are, modern transmissions can still leak oil due to broken or damaged CV joints or axle boots. In cases of AWD or RDW vehicles the cardan shaft’s seal are other sources of leaks. Besides, when talking about transmissions, “maintenance-free” sounds pretty optimistic. Transmission oil is not exempt from deterioration and contamination due to prolonged heat exposure and the wear of the internal components. Bad or low transmission fluid can lead to numerous problems. So it’s always important to check your transmission oil, no matter what the owner’s manual specifies.

It should be noted that there are also many types of transmission fluids as well. This article here does a pretty good job of explaining them.

How to check your transmission fluid
Photo by Garvin St. Villier

Symptoms of Low or Bad Transmission Fluid

Over time, transmission fluid deterioration can cause several problems on both manual and automatic transmissions. Low transmission fluid can also cause driveability problems and damage due to excessive friction and obstruction of the solenoids and valves in the case of automatic transmissions. Below, you will find a list of the most common problems associated with low or bad transmission fluid.

Manual Transmissions Symptoms:

  • Hard Operation: The shift stick is harder to operate than usual, and engaging and disengaging gears is a constant struggle.
  • Grinding: Grinding noises or buzzing coming out of the transmission could be a symptom of low or contaminated transmission fluid.
  • Slipping Gears: If the gearbox slips the gears out of place or you find it difficult to stay in gear, it could be a consequence of low transmission fluid.
  • Transmission Fluid Leaks: If you spot stains under the transmission or its components, check whether it’s transmission fluid. If it is, fix the leakage and re-fill the transmission.

Automatic Transmissions Symptoms:

  • Slow Operation or Rough Shifting: Slow shifting, rough and jerky operation, can indicate degraded or low transmission fluid.
  • Slipping Gears: Like manual transmissions, automatic transmissions may have gear slippage and fail to stay in gear due to low transmission fluid pressure or clogged valves or solenoids caused by insufficient or bad transmission fluid.
  • Noisy Operation or Whining: Unusual noises like whining, humming, or buzzing from the transmission may indicate low or bad transmission fluid.
  • Overheating: Low or worn transmission fluid can cause the transmission to overheat, leading to erratic operation. Overheating can also cause a burning odor and trigger a warning light on your dashboard.
  • Transmission Fluid Leaks: If you spot stains under the transmission or its components, check whether it’s transmission fluid. If it is, fix the leakage and re-fill the transmission. Additionally, most automatic transmissions have a carter or oil pan; check for signs of sweat or leaks that may indicate a faulty gasket.

Checking Your Transmission Fluid

Checking the transmission fluid in transmissions with a dipstick is as easy as finding the dipstick and checking the level as you would check your engine oil. If you are the lucky vehicle owner with one of these transmissions, just follow the user’s manual instructions to check whether you should check the fluid when the engine is cold or at operating temperature. Checking the fluid on dipstick-less transmissions is the real challenge, and we will show you how to check them.

Many transmissions have filler plugs, and others also have check plugs. Almost all transmissions also have a breather tube. These plugs are not meant for the end users, and often, you will see most of these plugs are of the hex type, so you just need a proper hex wrench to remove them. Some brands put more effort into keeping users out of the way, but if you are a DIY fanatic, you can find specialized tools to do the job.

Checking the Transmission Fluid – Dipstick-less Manual Transmissions

Manual transmissions have a check plug on their sides or the top. Get a workshop manual to learn how to check your transmission fluid. Transmissions without check plugs are more complicated, and the only way to measure their oil level is to flush the oil into a clean container and estimate how much oil drops. The oil shouldn’t be excessively hot, but it’s best to let the engine reach its operating temperature and drive the vehicle a few blocks before checking the oil for a more accurate reading.

  • Locate the Check or Filler Plug: Check a workshop manual to see which plug you need to remove to check the oil level.
  • Get the Vehicle Ready: Depending on the plug’s position, you can check the oil level on the floor, or you will have to put it over jack stands to access it. Either way, ensure the vehicle is adequately leveled for an accurate reading.
  • Check the Oil Level: Remove the check or filler plug and ensure the oil level is clearly visible and near the hole’s bottom.
  • Check the Oil Condition: Inspect the color and consistency of the oil. Clean manual transmission oil should be transparent with a clear brownish tone and contain no particles or debris. If it looks dirty, dark, or even black, you find metallic particles or other contaminants, and it has a burnt smell, you should change it.
  • Top-Up if Needed: If the oil level is in good condition but is low, top it up using the specific transmission fluid recommended for your vehicle.
  • Reinstall the Filler Plug: Reinstall the filler plug securely but without over-tightening it.

Time it with an oil change

This can be a challenging task, and in most cases, it’s essential to have a good scan tool to read the engine oil’s temperature. If you don’t have a scan tool, you will need an infrared temperature gun or other reliable way to measure the transmission oil temperature.

This procedure is almost as complex as changing the engine oil, so if your vehicle is over 65,000 miles and you don’t recall or have information on whether the transmission oil has been replaced, we encourage you to kill two birds with one stone and change your transmission fluid right away. Usually, changing the oil involves replacing other critical elements, like filters and gaskets, and cleaning the magnets on the oil pan that trap any metallic debris caused by wear and tear to prevent it from clogging the valves or damaging other components. A proper fluid change can make a real difference and extend your automatic transmissions service life. 

Checking the Transmission Fluid – Dipstick-less Auto Transmissions

However, if you are curious or want to check your transmission oil yourself, below you will find how to do it in most models. Automatic transmissions are sensitive, so we advise you to have a workshop manual handy or find a video on YouTube to see your vehicle’s specific process.

  • Jack Up Your Vehicle: Use jack stands to raise your vehicle in a leveled position and take all the precautions to work safely underneath.
  • Warm Up the Transmission: Start the vehicle and let the engine run to warm up the transmission fluid and ensure it’s evenly distributed. ATF (Automatic Transmission Fluid) is susceptible to temperature changes and changes its volume with the temperature. That’s why, to ensure a correct reading, it’s essential to measure the level at a temperature specified by the manufacturer*.
  • Stay Alert to the Fluid’s Temperature: If you can access a scan tool, find the channel where the ATF’s temperature is displayed at the TCM (Transmission Control Module) and monitor the temperature as it goes up. There is a margin of about 35 to 40 °F to have a good reading, so it’s important to monitor the temperature and be ready to remove the filler or drain plug when the time comes. If you don’t have access to a scan tool, you can use a temperature gun and point it at the transmission’s oil pan. It’s not as accurate as measuring it with a scan tool, but you can get a pretty accurate reading.
  • Remove the Plug: Once the transmission reaches the specified temperature, remove the inspection or fill plug, depending on your transmission’s model.
  • Measure the Oil: As you remove the plug, check whether any fluid leaks out. If no fluid leaks out, it’s probable that the fluid level is low and needs to be topped up.

If a small amount of fluid drips out, the fluid level is ok.

  • Reinstall the Plug: After checking the fluid level, immediately reinstall the plug with the engine off, and torque the plug, following the manufacturer’s specifications.
  • Inspect the Oil: Once the plug is secured, inspect the oil. Depending on your transmission’s fluid type, the fluid should be red or light brown. ATF and CVT transmission fluids are typically reddish, while dual-clutch transmissions use a brownish fluid similar to the one used by manual transmissions. If the fluid looks dark brown or black, or if you notice signs of foaming, you should replace it immediately.
transmission gear and oil changes
Photo by cottonbro studio

Wrapping Up

Checking the transmission fluid is very easy when they have a dipstick and is pretty straightforward for dipstick-less manual transmissions. Automatic dipstick-less transmissions are more tricky and require more tools, knowledge, and time. 

Find out your transmission service schedule, and if you struggle to find it, remember that transmission fluid should be changed every about  60,000 miles. Again this depending on the fluid and the transmission. For automatic transmission, it’s advisable to change the oil every 45,000 or 80,000 miles. Most automatic transmissions will also need a filter change during their service life. In conclusion, stay alert for leaks and/or strange behavior and noises. If you don’t find a maintenance schedule in your vehicle’s service manual, ask your trusted mechanic. They should be able to provide a reasonable maintenance plan to keep your transmission in excellent shape.

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