symptoms of low transmission oil
Engine Troubleshoot

7 Signs Of Low Transmission Fluid

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Transmission fluid is just as important as engine oil when it comes to keeping your vehicle running smoothly. If there isn't enough fluid in the transmission, then you're going to run into problems with how it operates.

The symptoms of low transmission fluid include difficulty changing up and down through the gears, slower gear changes, grinding noises when changing gears and transmission overheating.

Most transmissions these days are sealed systems filled with transmission fluid that is supposed to last the lifetime of the car. This means that if you suspect the transmission fluid is low, you must have a leak in the system.

In this article I'll run through the symptoms of low transmission fluid and explain the cause and effects it can have on your transmission.

What are the symptoms of low transmission fluid?

The fluid in the transmission of your car has many jobs. It is there to lubricate the internal gears of the transmission and to reduce wear and tear as they move against one another. It also plays an important role in keeping the transmission from overheating. 

It is also important to have the right amount of fluid in the transmission. This is because the fluid is used to create the pressure needed to physically change the gears by triggering the correct valve at the correct time. If there isn't enough fluid in the valve body, then the gearbox won't perform as it should.

The most common cause of low transmission fluid is a leaky transmission. It's usually fairly easy to spot a leak coming from the transmission. There may be drops of fluid on your driveway, or on the plastic underbody of your vehicle.

There are also some symptoms you should watch out for, as these can point to low transmission fluid and you should really get it checked out as soon as possible to avoid expensive damage to the transmission.

1. Your Car Has Trouble Selecting Gears

If you find that your vehicle is not changing gears when it should, then this can be caused by low transmission fluid. 

The fluid is there to lubricate the gears and to keep everything running smoothly. If there isn't enough fluid in the system, then you will notice a problem with the speed and timing of the gear changes.

The inside of the transmission is made up of many different gears arranged in what is called a planetary gear set. To change the gear ratio (change gears) one or more of the gears is stopped or started using clutches and brakes.

planetary gear

A planetary gear set from an automatic transmission

To change gears, the fluid in the transmission is pressurised to activate the appropriate clutch or brake using a valve.  It is this pressure that in effect changes the gear by locking one or more parts of the planetary gear set. 

If there is not enough fluid in the transmission, then there will be a delay in the build up of pressure needed to engage the gear clutch. If the pressure is really low, it may not be able to select a gear at all.

2. The transmission overheats regularly

If there is not enough fluid in the transmission, then you run the risk of it over heating. As the transmission fluid is pumped around the inside of the transmission it heats up by drawing heat from the internal components it comes in contact with. The hot fluid is then usually pumped via transmission cooling lines to the car radiator where it is cooled before returning to the transmission.

The temperature of the transmission is monitored by a transmission fluid sensor. If there is a problem it will usually trigger a check engine light on the dashboard. An overheating transmission will usually be recorded by the ECU as error code P0218. This can indicate a problem with the fluid level, or if it is restricted by blocked cooling lines.

3 A Noisy Transmission While Driving

Strange noises coming from the transmission can be a sign that there is something not right. Most of the time, these noises can be traced back to the short and long term effects of the transmission operating with too little fluid.

A noise that is present when moving, that sounds like a bad wheel bearing (a constant whirring) is usually caused by low fluid levels. You may also notice that the gearbox will slip gears regularly. What is happening is that the fluid is too low to build up the required pressure to activate the clutch that holds the gearbox in the correct gear.

Sometimes a noisy transmission can be caused by wear and tear of the internal gears because of low or bad transmission fluid. This type of noise is usually only present when changing gears as there is added friction between the gears and the turbine shaft.

4. The transmission slips gears

Gear slippage is when the vehicle unexpectedly changes gear while you are driving. This is usually accompanied by an increase or decrease in revs as the engine speed catches up with the change in gear.

The most common cause of gear slippage in an automatic transmission is low fluid. The fluid is used to build up the required pressure to change and hold the correct gear. If there isn't enough fluid then it will take longer to build up the necessary pressure to change gears, and there is also the possibility of the pressure dropping and the gear slipping.

5. The transmission is slow to change gears

A slow gear change as you accelerate can be caused by a number of problems in the transmission. Sometimes it is caused by a faulty solenoid or clutch valve, but more often than not it is caused by low fluid levels.

An example of an automatic transmission valve body

When the transmission control module decides to change to a different gear, it activates the correct solenoid that operates a particular valve. When the valve opens, the transmission fluid is forced into a tiny channel where it increases in pressure and activates the clutch corresponding to the desired gear.

If the fluid levels are low, then there may not be enough fluid to increase the pressure in the valve channel. Or there may be a delay in the build up of pressure, causing a delay in changing gears.

6. A surging or jumpy when changing gear

A transmission that is low on fluid will not operate smoothly, and this can translate to jumpy gear changes and a surging engine if the gear changes are delayed or fail to engage.

This is related to slow gear changes, and the lack of pressure build up in the valve body. If the gear clutch is slow to engage it will affect the timing of the transmission and the ability to change gears smoothly.

7. Transmission failure

If you continuously drive a vehicle that is low in transmission fluid then you may end up with total transmission failure. The transmission may be too damaged to operate properly due to excessive wear and teat caused by a lack of lubrication.

It may also fail due to the inability to change gears properly because of a lack of fluid pressure. Sometimes, the ECU (or powertrain module) will prevent the vehicle from starting to protect the transmission from severe damage.

There are several sensors located in the transmission that keep track of the input and output shaft speeds. If they detect speeds that are wrong, then his can indicate a problem with the transmission.

Some common error codes that may be logged in the ECU include P0729, P0730, P0731, P0732-P0736. If one or more of these codes is logged in the ECM, then you will normally see the check engine light on the dashboard. Most vehicles will either not start or will be restricted to very low speeds to prevent further damage to the transmission.

How To check for a transmission fluid leak

It can be difficult to spot a transmission fluid leak on newer vehicles. Most have plastic underbody panels that cover the transmission and the engine, making it less likely for fluid to drip onto the pavement.

To check for a transmission fluid leak, you will need to safely jack up your car and support it on axle stands. You should only do this if you have the right tools and know exactly what you are doing. Otherwise get a mechanic to do it for you.

  1. Start by removing the plastic underbody panel that is covering the transmission.
  2. Using a torch, check the transmission sump for leaks. Check around the edges of the sump for leaks in the sump gasket. If it is leaking there will be fresh or dried fluid along the edge of the sump. Transmission fluid is red when new and turns brown/black over time.
  3. Next check the transmission filler nut for leaks.
  4. Move to the front of the transmission and check around the input shaft seal, this is a common place for leaks to start.
  5. Check the back of the transmission for leaks at the tail seal, another common source of leaks.
  6. Locate the cooler lines coming from the transmission and check along the lines and at the joints for leaks.

If you find a leak, or suspected leak, then clean away the fluid from the suspected leak area and take the vehicle for a short drive to try to recreate the leak.

How to check the transmission fluid level

Most newer vehicles come with a sealed transmission that does not require you to check or change the transmission fluid.

If your vehicle is older it may have a transmission fluid dipstick that allows you to keep an eye on the fluid, which helps to prevent low fluid problems with the transmission.

To check the fluid on an older vehicle you can simply remove the dipstick to check the fluid level and the condition of the fluid. If the fluid level is too low, you can top it up using the correct fluid as recommended by the vehicle manufacturer.

If your vehicle has a sealed transmission, then my advice would be to leave it to a specialist transmission mechanic to check the fluid level and condition.

The process usually involves the use of specialised tools to monitor the condition of the fluid as it is pumped around the transmission. The transmission itself is put into a diagnostic mode while various readings such as temperature and pressure are taken from different parts whilst selecting different gears. 

Cost of fixing low transmission fluid

The cost of fixing the cause of low transmission fluid can vary greatly. The transmission is a complicated part of the vehicle, and often requires specialist knowledge to repair. 

A simple transmission sump leak that requires a gasket change and a fluid refill will usually cost around $150-$250. If the sump itself is warped or cracked then this can significantly increase the cost of repair with the cost of a new sump ranging from $50 to $300. 

If the fluid is leaking from one of the main transmission seals located at the input or output shafts, then the majority of the cost is going to be in the labor involved  in fixing it. You can expect a repair like this to cost in the region of $200 to $300.

If the transmission needs to be replaced with a new or rebuilt unit, then you are looking at a cost of $800 to $1500 for a refurbished transmission, and over $2500 for a new transmission.