Checking the fill level of transmission oil, the right way
Knowing how to properly check the fill level of automatic transmission is key to proper preventive maintenance of the transmission.
There are two categories of automatic transmissions which we shall base our discussion on.
1. Automatic transmission with dipstick
2. Sealed automatic transmission without dipstick
The methods used in checking the fluid levels of these to types of transmission are different.
We shall look into the procedures for both types of transmission differently.
Automatic transmission with dipstick
This types of transmission has dipstick similar to engine oil dipstick but it differs in the sense that the engine oil dipstick has minimum and maximum level marks while the transmission oil dipstick has cold and hot marks.
Front wheel drive automatic transmissions have their dipstick at the front driver side of the engine while the rear wheel drive cars have their dipsticks at the rear passenger side of the engine.
Let's go straight to checking the correct transmission oil level now.
° When the car has not been started for the day (preferably in the morning), start the car while the emergency brake is engaged, cycle through all the gears (P R N D) pausing on each gear for about 5 seconds.
° GENTLY pull out the transmission oil dipstick and wipe the oil off with a clean towel. Pulling the dipstick out gently is very crucial to getting an accurate reading when you dip the second time or subsequent times.
° Dip the stick again, pull stick out gently ensuring that you keep it in vertical position to keep the oil mark clear.
If the oil mark is below the cold mark of the dipstick, the transmission is likely under filled. The check should not stop here. Go ahead to check the level when the transmission has attained normal operating temperature as described in the next points.
° Drive around for few miles to bring the transmission up to operating temperature.
° Park the car while it still idles, engage the emergency brake.
° Gently remove the dipstick and check the oil level.
This time, the oil mark should be around the hot mark of the dipstick.
If it is way below the hot mark, it signifies an under filled transmission which can cause the transmission to over heat and burn and also cause transmission performance issues.
If the transmission oil level is above the hot mark of the dipstick, it means that the transmission has been overfilled. This can lead to splashing of the transmission fluid which will cause foaming and then consequently lead to less lubrication and transmission overheat.
As temperature rises, the volume of the automatic transmission fluid also rises. That is why there is both cold and hot marks on the transmission dip stick.
Note: Honda has a slightly different procedure for checking the fluid level. They recommend turning the engine off and then immediately checking the fluid level on the dipstick.
Sealed automatic transmission (without dipstick)
Checking the fluid level on this type of transmission requires a greater level of expertise as compared to checking fluid level on transmission with dipstick. The job is better handled by an expert but it is not difficult nor impossible for any DIY inclined car user to do.
This type of transmission does not use a dipstick for checking the transmission oil level. The fluid level is rather checked mostly through the transmission refill hole.
It is important to strictly follow the procedure spelt out in the owners manual as it can slightly vary for different cars.
Checking the fluid level is best done in the morning.
Let us go straight to the procedure for checking the fluid level. Remember that it can vary for different cars. Your owner manual or repair manual is your best reference for this task.
Before you begin checking the fluid level, get an OBD2 reader that can read transmission fluid temperature handy. You can also use an infrared handheld thermometer to measure the temperature by pointing at the transmission pan.
Let the car be on a leveled surface, preferably above a workshop pit for easy access to under the car. In the absence of a pit, four strong jack stands can be used. It is important to ensure the car is stable on the jack stands for safety.
° Before you start the car, slightly losen the fill plug to check whether it will not be hard to loosen it when you need to without allowing fluid to come out. Lock it back.
° Start the car, engage the emergency brake and cycle through each gear, holding it on each gear for about 5 seconds.
° Connect the OBD2 scanner and begin to monitor the transmission fluid temperature. There is a temperature sensor inside the transmission that sends a temperature signal to the transmission control unit which could be a dedicated control unit or integrated into the engine control unit.
If you are using an infrared type thermometer, point at the transmission pan to measure the temperature. At my discretion, I normally add 1 degrees Celsius to whatever my infrared thermometer reads because of slight temperature difference between the fluid and the transmission pan.
Note before we go to the next step: there is usually a temperature window the car manufacturer gives at which you should open the fill hole. For many cars, it's in the neighborhood of 40 to 45 degrees celcius.
° Continue to monitor the transmission fluid temperature. Let's assume the temperature window given by the manufacturer is 40 to 45 degrees Celsius, once it is 39 degrees Celsius, set out to loosen the fill plug or any port the manufacturer recommends for checking the transmission fluid level.
In this temperature window, the fluid should be dripping or dropping slightly to show that the fluid is just a the level of the fill/check hole.
If the fluid is rushing out, it means that the transmission was over filled. The fluid should be allowed to drain till it just drips out slightly. The transmission fluid temperature should still be within the given range.
If the fluid does not drop at all, it means that the transmission is under filled and fluid should be added. This must be done within the given temperature window.
Beware of the fluid temperature exceeding that of the given range (in this case 40 to 45 degrees celcius). If so, the transmission will be under filled.
In some cars, the fill hole and drain hole use same port but it will have an extension that can be removed to drain fluid from the transmission.
For cars that have oil level dipstick, the level should be checked as often as possible.
Sealed transmissions are meant to require less frequent service but preventive maintenance should not be neglected on this type of transmission as there could be a fluid leak or an over fill during the last transmission service.
Both transmission under fill and over fill with oil can damage the transmission. This can be avoided by checking the fluid level, following the right procedure.