In this guide, we’ll discuss one of the most confusing questions related to transmission fluid: Can transmission fluid get low without a leak? The transmission fluid in an automatic transmission system is used to transfer power from the engine to the transmission. The fluid acts as a coolant and lubricant in the transmission and drivetrain of the engine and its level should be monitored on a regular basis. If the transmission fluid level is getting low in your vehicle and you’re not observing any visible signs of transmission fluid leakage, this guide is for you.
Transmission fluid cannot get low without a leak. If you don’t find a puddle of transmission fluid underneath the car and around the transmission pan, it is because the leak is too small to spot. The transmission fluid might be leaking when you drive your car and transmission fluid is circulated under pressure to exit through the small leaks. To confirm that transmission fluid is leaking, you should drive your car on a dry and smooth surface, such as a parking lot. If you find spots of transmission fluid on the driveway, it means that transmission fluid is leaking.
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How To Check The Level Of A Transmission Fluid?
To check the level of transmission fluid:
- Park a car on level ground.
- Start the car to warm up the engine to an operating temperature as the minimum and maximum marks on the transmission oil dipstick are based on the transmission temperature of around 90ºC to 100ºC. The engine should not be run for more than 90 seconds to warm it up.
- With the engine running, depress the brake pedal and place the gear lever in each gear position, holding for approximately 5 seconds in each position. Then, put the gear lever in PARK.
- Turn off the engine.
- Remove the dipstick from the transmission and wipe it with a clean cloth.
- Insert the transmission dipstick all the way into the transmission.
- Remove the dipstick and check the transmission fluid level.
- If the level of transmission fluid is below the lower mark on the dipstick, it means that the transmission oil level is low.
Note: If the car has been driven for an extended period, at high highway speeds, in city traffic, during hot weather, or while towing a trailer, the transmission fluid must cool down to 90ºC to obtain an accurate reading.
Signs of Transmission Fluid Being Low
Transmission fluid helps to shift gears smoothly by applying pressure on the clutches and bands. If the transmission fluid is low, the gear shifting will be harder, and your car will also make clunking noises due to improper lubrication in the drivetrain.
So, some of the most common symptoms of transmission fluid being low include:
1. Clunking Or Grinding Noises
Low transmission fluid makes clunking or grinding noises. In a car, transmission fluid is used to lubricate the gears of the transmission.
When transmission fluid is low, the gears grind against each other, shift harshly and make grinding sounds. This happens because the gears are not able to rotate smoothly.
Also, due to the low level of transmission fluid, the power is not being smoothly transferred from the impeller (pump) to the turbine in the torque converter, which will also cause clunking noises.
2. Clutch Slipping Or Dragging of Gears
Low transmission fluid makes a vehicle jerky while trying to accelerate. When the transmission fluid level is low, it will not be able to apply a certain pressure on clutches and bands to engage and shift gears.
As a result, the clutches will slip, and gears will not shift smoothly. Moreover, a low level of transmission fluid also wears out the high-friction clutch pads. As a result, the clutches are not able to maintain a tight grip and slip while shifting gears.
3. Burning Smell
The burning odor of smoke, plastic, or melting wires is an indication that the transmission is overheating or transmission fluid is burning in the engine. The burning smell from the transmission system is generally due to low fluid levels in the system.
In addition to lubrication of the drivetrain and keeping the clutches engaged, transmission fluid also plays the role of minimizing friction and keeping the transmission system cool. If there is a low level of transmission fluid, the transmission will overheat which will cause a burning smell.
4. Check Engine Light
The check engine light may also illuminate on the dashboard of the vehicle because the oil pressure in the automatic transmission is low. As a result of the transmission fluid being low, the engine could stall due to a lack of lubrication, which is the main reason for the check engine light to come on.
The other reason is when the engine is running, the engine control module in the computer detects a fault with the transmission and issues an advisory warning. You should immediately check trouble codes using the scan tool if the check engine light illuminates.
Causes Of Losing Transmission Fluid Without A Leak
If your engine is losing transmission fluid after driving over a few miles and you aren’t noticing any spot of transmission fluid on a driveway, here are some of the most common causes due to which your transmission is losing transmission fluid without a leak:
1. Bad Vacuum Modulator On Transmission Case
Some very old automatic transmission systems (before 1990’s models) have a vacuum modulator that could fail and cause transmission fluid to suck into the engine. Older automatic transmissions did not have electronic sensors. They could only be controlled mechanically.
The vacuum modulator is connected to the engine air intake. The vacuum modulator has a diaphragm that is computing the load in your car’s engine.
As you increase the throttle, the vacuum decreases and the vacuum modulator calculates the engine load. This allows the transmission to shift appropriately. As more load is introduced, the engine produces less vacuum and the transmission knows it needs to enter a lower gear.
If the diaphragm of the vacuum modulator is leaking, the transmission fluid will leak through it and burn into the engine’s cylinder with the fuel. This will cause white smoke from car exhaust and will also lower the level of transmission fluid without any leak.
Remove the vacuum line going to the transmission case and see if it is wet inside or if any fluid is leaking out of the vacuum modulator.
2. Leakage Through Transmission Cooler
The transmission cooler is located inside the same radiator as the engine coolant. Just like a coolant is continuously circulated through the radiator to maintain its temperature, transmission fluid is also circulated through the transmission cooler so that its temperature can be maintained and it could prevent overheating in transmission.
If there is an internal wall rupture in the transmission cooler, the transmission oil will mix with the coolant due to its high pressure.
If you remove a radiator cap and notice a milky texture on the coolant, the chances are that transmission fluid is leaking into the radiator. This will cause transmission fluid to get low without a leak.
Moreover, you should also check if the transmission fluid is leaking through the hoses going in and out of the transmission cooler.
If there is any wetness on the hoses going in and out of the transmission cooler, it means that the hoses are decayed. Moreover, if there are crimps on the hoses of the transmission cooler, they could block the flow of transmission fluid and stop it from circulating in the transmission system.
3. Bad Seal Of Transfer Case
The transfer case is present in four-wheel drives. If the transfer case is overfilled with the transmission fluid, the chances are that transmission fluid is being pushed from the transmission into the transfer case through a transfer case input seal, due to which the level of transmission fluid is lowered in the transmission pan. If you remove the fill plug of the transfer case and the transmission fluid comes running out, it means that the transfer case is overfilled.
4. Leaking Of Transmission Fluid Into Bell Housing
If the front seal of the transmission is leaking, the transmission fluid will settle in the bottom of the bell housing. Bell housing is the part of the transmission system which encloses the torque converter and flywheel.
The transmission fluid pooling in the bottom of the bell housing gets flung out or evaporated upon driving. Due to this reason, transmission fluid gets low without any noticeable leak.
A transmission fluid leaking through the front pump seal of the torque converter drips off the bottom of the bell housing, and directly onto the exhaust pipe. As a result, you might notice the cloud of fog following you.
Can Transmission Fluid Evaporate?
No, the transmission fluid does not evaporate as it is filled in the air-tight casing of the transmission. But if the transmission fluid gets leaked out, the transmission fluid evaporates into the air. A leak in the transmission system is caused by a number of factors including the damaged drain plug, bad seal of the transfer case, and torque converter.
If your transmission is frequently low on the transmission fluid and you can’t spot the signs leak, it isn’t because the transmission fluid is evaporating. It is just that the transmission fluid is leaking internally, such as through the radiator, transfer case, or bell housing.
Can Transmission Fluid Get Low Over Time?
Yes, transmission fluid gets low over time due to wear and tear or internal leaks.
The transmission fluid is very important because it is used to transfer torque from the engine to the gears, thereby preventing damage to the gears.
Transmission fluid helps lubricate the gears and other parts of your engine so they work smoothly. Transmission fluid can get low over time due to many reasons. The most common cause is that transmission fluid becomes dirty or thick due to the buildup of metal particles.
When transmission fluid lowers, the car will not be able to shift gears smoothly and may stall if you drive over rough surfaces as the insufficient transmission fluid will hinder the transmission process due to a lack of appropriate hydraulic pressure/
It is recommended that you change the transmission oil every 30,000 miles to prevent this.
If you are not changing the transmission fluid every 30-40K miles, there will be some buildup inside the transmission. While changing the transmission fluid, you may end up knocking loose some of these deposits, which in turn could block critical valve body passages.
Why Is My Transmission Fluid Disappearing?
The most common reason for transmission fluid disappearing is its leakage through the bell housing, drain plug of the transmission pan, input shaft seal of the transfer case, hoses of transmission cooler, and transaxle.
If your transmission fluid is disappearing, then you must check those areas for leaks. The transmission fluid is used to lubricate the various moving parts of the transmission and cools the engine.
The transmission fluid is stored in the pan and is pumped through the system by a transmission pump. It is important to replace this fluid periodically to ensure proper operation of the transmission.
In order to keep the transmission fluid flowing properly, it is very important to maintain its level. When you change the transmission fluid, you must remove the old fluid and add a new one.