The Fluids Time Forgot

Isn't Everything Some Kind of Oil?

Most people know that oil should be changed regularly. Most people probably know that automatic transmission fluid should be changed regularly. They might even know that coolant should be changed at some point as well. What about some of the other fluids? If you are asking yourself right now, “There are other fluids?” then chances are you have neglected your car in some way or another. If you are wondering what other fluids your car has, and when they need to be replaced, check your owner’s manual.

The owner’s manual is truly the be-all-end-all when it comes to what services your car requires. You can find out exactly what services the manufacturer recommends and at what mileage or time intervals they recommend them. A few things don’t get mentioned in the owner’s manual but it’s not a bad idea to have these things serviced as well, especially if your car has high mileage.

Some other fluids your car might have are as follows:

Manual Transmission Oil

Anything with gears in it has to have some kind lubrication. While everyone knows something about automatic transmission fluid not many people know about manual transmission fluid. The oil that is used in a manual transmissions varies somewhat. The only thing that the fluid in a manual transmission must do is lubricate the gears and the shifting mechanisms.

The many gears of a manual tranmission

Manual transmissions usually use one of the following types of fluid, 75w-90 gear oil, automatic transmission fluid (ATF), regular motor oil such as 10w-30, or some kind of proprietary synthetic oil. The service interval for the fluid will ultimately be determined by the manufacturer, and it will also depend on what kind of fluid the manufacturer designed the transmission to use. The interval may be as often as every 30k miles, while some vehicles might not list any service interval and may even indicate that the fluid doesn’t require service for the life of the vehicle (or at least for the duration of the warranty period).

No dipstick is found for this oil, but it ought to be checked on a somewhat regular basis. A good lube shop will usually check this fluid when they are servicing the engine oil. Most of the time the service of a manual transmission requires a simple drain and refill of the fluid. A drain plug will be found at the lowest point on the transmission, and somewhere half way up the side of the case will be found a fill plug. After the old fluid is drained, new fluid will be pumped into the fill hole until it starts to run out the hole. At this point the transmission is full, and the fill plug is reinstalled. This is a very simple service which means that it’s easy to perform yourself, or easy for someone else to perform. Either way this means that it doesn’t cost much but it is something that most people seem to forget.

Brake Fluid

This must be serviced more often then any of the other forgotten fluids. Most vehicles require that the brake fluid be serviced every 2 years or 30k miles. Part of the reason for this frequent service interval is the fact that brake fluid is hygroscopic. This means that the fluid soaks up moisture from anything that it is exposed to. Keeping the moisture in the brake hydraulic system in check is important. Water is corrosive to just about everything it comes in contact with, and it also freezes. Both of these things are bad for the brake system.

A brake fluid reservoir perched on top of the brake master cylinder.
Brake fluid types don’t vary much at all from one make to another. Generally most manufacturers will use either DOT 3 fluid or DOT 4 fluid. The difference is that DOT 4 has a higher boiling point than DOT 3. Brake fluid is not petroleum based but is actually glycol based. This means that brake fluid is more similar to coolant in many ways than oil. DOT 3 and DOT 4 can be mixed with the only effect being that the boiling point of DOT 4 is no longer as high as it could be once it gets mixed with DOT 3. Synthetic brake fluid that is usually referred to as DOT 3-4 or DOT 5.1 is also compatible with DOT 3 and DOT 4. DOT 5 brake fluid is silicone based and should never be mixed with anything else. This fluid is usually used in very special applications such as auto racing.

The brake fluid condition is easy to check because the fluid reservoir is under the hood in a place that easy to access. Removing the cap is usually necessary in order to look at fluid condition. If the fluid is very dark then it probably needs to be serviced. If you have had your car for a long time but you don’t know if the fluid has ever been serviced, it probably needs to be serviced. If you just purchased your car recently and you don’t know if the brake fluid was ever serviced, it probably needs to be serviced. To service the brake fluid the entire hydraulic system must be flushed or bled. This requires a special tool to force all of the old fluid out, using positive or negative pressure at one end of the system or the other. This may also be accomplished by someone pumping the brake pedal while someone else opens a bleeder screw at each brake actuator. Brake bleeding is better left to someone with a little experience because if it isn’t performed properly the brakes will not work at all.

Power Steering Fluid

This is a special hydraulic fluid that is used to transmit force as well as lubricate. The fluid is stored in a reservoir where it is gravity fed into a pump. The fluid will be pressurized so that it can act against a piston in the steering gear assembly, and help push the piston in the direction that the vehicle is being steered. Some vehicles will use power steering pressure to provide pressure to the power brake booster that helps to actuate the brakes. The fluid is doing double duty in this instance.

A p/s reservoir attached diredtly to the pump.

Power steering fluid will break down over time and loose its ability to lubricate. Oxidation is also a problem just as it is with most other fluids in the vehicle’s various systems. Replacement intervals for power steering fluid are all over the map. Some experts say that it really doesn’t need to be serviced, and some would say to service it every 60,000 to 90,000 miles. All fluids break down over time and the properties that allow them to do their job are diminished, so every fluid must be serviced at some point.

The good news regarding power steering fluid service is that new technology is eliminating hydraulic power steering. Many new cars have electronic power steering that uses an electric motor to provide the assist. This is good because there is no pump and no fluid, which means no leaks, and more reliability.

P/S reservoir mounted away from the pump.
The type of power steering fluid that a car requires varies a little from one make to another but most of them require one of two different fluids. Standard power steering fluid is used by many vehicles. This fluid is just a special hydraulic fluid used to lubricate and transmit force. If the car doesn’t use standard power steering fluid, then it will probably use automatic transmission fluid. ATF is also a special hydraulic fluid that can transmit force and lubricate. Honda cars and trucks require a special “Honda Type” power steering fluid that is not compatible with regular fluid; many European cars might require some kind of specialized mineral oil. None of these fluids should ever be mixed.

Most power steering fluid reservoirs have either a small dipstick attached to the cap, or the reservoir is translucent so the level can be seen through the side of it. Servicing the power steering fluid is similar to servicing the brake fluid in that the system must be bled. This is not a process of ‘drain and fill’ like most fluids on the vehicle.  The best way to perform this service is to remove the return line from the reservoir, and direct it to a container to catch the old fluid as it cycles through. Start the engine and quickly pour new fluid into the reservoir to replace the old fluid as it flows out the return line into your catch pan. Some machines are available to help with this flushing process but they are not at all necessary.

Transfer Case Fluid

Any vehicle that has four wheel drive has an extra gear box on the back of the transmission that is responsible for splitting transmission output between the front wheels and the rear wheels. This device also contains gears that will multiply the torque output from the transmission. This device is the transfer case. Because it is full of gears and shafts and other rotating assemblies it too must be lubricated at all times.
A typical transfer case. The drain plug is the round thing at the bottom,
and the fill plug can be seen in the middle of the housing.
The fluid that is usually used in the transfer case can be 75w-90 gear oil, ATF, or any proprietary lubricating fluid that the manufacturer might come up with (ATF is amazing stuff, that’s why it gets used in all sorts of different things besides the automatic transmission). The average service interval for the transfer case is about 60,000 miles, but some manufacturers will claim it’s a lifetime fill and doesn’t require service. Service of the fluid is just a drain and fill similar to the manual transmission service.

Differential Fluids

The differential is the set of gears that make up the final drive and gear reduction in the power train. If you look under the back end of a rear wheel drive vehicle, this is the round box of gears that can be seen in the middle of the axle where the drive shaft connects to the axle. These gears split the power between the drive wheels, multiply torque one last time, and allow one wheel to turn faster than the other when the vehicle is turning.

Every drive axle must have a differential so any vehicle that has all wheel drive, or four wheel drive, will have two differentials, one for each axle. On a car with front wheel drive the differential is in the same housing as the transmission and most of the time they share the same fluid. This is why the transmission on a front wheel drive car is often referred to as a transaxle. A few front wheel drive cars have the differential and the transmission in the same housing but the fluid is not shared, nor is it the same type of fluid. Both fluids must be serviced separately.
Rear differential with the pan removed
Differential fluid must be serviced anywhere between every 15k to 90k miles, and of course some manufacturers say that their vehicle’s never require differential fluid service. Most differentials can be serviced through a simple drain and fill. Some of them do not have a drain plug however, and some kind of pan or cover must be removed so the fluid can flow out. Filling the differential requires pumping the new fluid in through the fill hole somewhere on the differential housing until it reaches the level of the fill hole.

The fluid is usually a gear oil such as 75w-90, but with all of the specialty differentials that are found on today’s SUVs and crossovers, some require a proprietary fluid that must be obtained from the dealer. Differentials that are known as “limited slip” require a special additive to go with the gear lube. This additive keeps the clutch packs located in this special type of differential conditioned in such a way that they will slip when needed and grip when needed.

Never Ending Fluids

Getting behind on fluid services is easy to do and therefore catching up again and getting things back on course is very difficult. The service schedules found in the owner’s manual will never let you down, and if you combine these recommendations with the recommendations that a trusted mechanic can make, you will never fall behind. None of these fluid services are difficult for a well trained service technician so none of them are very expensive all by themselves, but together it could be costly. Nevertheless fluid service is important and all of these things must be remembered. If they are forgotten, it will only lead to failure of the systems where they are found.

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