Have you ever tried to actually read your owner's manual, especially the part about maintenance and service? As if this kind of stuff isn't confusing enough, there are always two schedules listed, one for a vehicle driven under "normal" conditions and another for "severe." But what exactly does this mean? I have yet to see an automaker that actually explains, in plain English, what these terms mean.

This article is the second in a series of four that addresses the differences between severe and normal service recommendations. We've already covered engine oil, so now we're moving on to the rest of the fluids in your vehicle: transmission fluid, differential and transfer case lubricants, engine coolant, power steering fluid and brake fluid.

Service Schedules For Fluids

Let's start by trying to understand the differences between normal and severe. The severe schedule, which always has shorter recommended intervals between fluid changes, applies if any of the following are true:

  • Most trips are less than 10 miles (16 km). This is particularly important when outside temperatures are below freezing.
  • Most trips include extensive idling (such as frequent driving in stop-and-go traffic).
  • The vehicle is frequently driven in dusty areas, like on dirt or gravel roads.
  • The vehicle is frequently used for towing a trailer or using a carrier on top, both of which place extra demands on the engine.
  • The vehicle is used for delivery service, police, taxi, or other commercial applications

If none of these conditions are applicable, you should go ahead and follow the normal schedule. Your owner's manual will tell you the specific mileage suggestions for your vehicle, thus, make sure you check it for the recommended fluid, lubricant and filter change intervals. Keep in mind that with these fluids, there's not exactly a rule of thumb like there is with engine oil. In fact, some of the items discussed in this article may not be listed in your service schedule. If this is the case, consider our recommendations suggested under each section.

Transmission Fluid

While some manufacturers suggest that transmission fluid can last for up to 100,000 miles if you're following the normal schedule, I tend to think that this is one of the cases where it's better to be cautious and follow the severe schedule if there's any question. A severe service schedule might call for transmission fluid to be flushed at 35,000-40,000 miles, and at usually less than $200 for this procedure, it's good prevention. Getting an automatic transmission rebuilt is a costly repair, easily running into the thousands of dollars, so it's one you particularly want to avoid.

The transmission fluid performs a few functions. It serves as a medium by which the hydraulic pressure, necessary to operate the transmission, is created. It absorbs heat within the unit and carries it away to the transmission oil cooler, insuring that the transmission does not overheat. This vital fluid also lubricates the moving parts inside the transmission. Finally, it keeps dirt and debris in suspension until the filter can remove it from the fluid.

Transmission fluid is an oil, and is therefore subject to viscosity breakdown and the loss of its protective, lubricating, and cooling properties. Operating the transmission on fluid that is worn out results in premature transmission wear and ultimately failure.
In determining which service schedule to use for your transmission fluid, there are a few key real-world activities that indicate you should use the severe interval. These include towing, frequent driving in stop-and-go traffic or spending long periods of time with the vehicle idling, driving your vehicle off-road, hauling heavy loads, or using a snowplow. Following this more vigorous severe maintenance schedule will help head off the damage caused by excessive friction, heat, and internal wear, which can kill your transmission.

Differential And Transfer Case Lubricants

First a note: Most modern front-wheel-drive cars do not require this sort of service, because they don't have transfer cases or serviceable differentials. Servicing the transfer case and differential is usually applicable to four-wheel-drive, all-wheel-drive and rear-wheel-drive vehicles. While not all will require frequent service in this area, it can still be necessary, especially if you're a candidate for the severe service schedule. Most manufacturers do not require the transfer case or differential to be drained unless there's a problem, but the fluid should be regularly inspected.

Differentials turn the axles that turn the wheels, while the transfer case is an auxiliary gearbox that allows a four-wheel drive vehicle to shift from low to high range and back again. Each of these gearboxes house lubricant to keep them running smoothly and keep them cool.

The best way to determine whether your transfer case or differential needs its fluid changed is by a visual inspection of the fluid. Lubricant that contains metal flakes indicates internal wear. If this is the case, the unit should be opened up and inspected. Fluid that is black has been overheated and the unit should be inspected for the cause of the overheating, alas wear is the usual cause). Fluid that is milky in color has been contaminated with water and should be changed immediately to avert premature failure. Also check for open or broken vents that allowed the water in and repair them.

Years ago, 90-weight gear oil was the only lubricant used in differential or transfer cases. However, today's carmakers use specially formulated lubricants in many of these units. These special additives can be specific to your vehicle and can make other gear oils incompatible, so make sure you check your owner's manual for the exact fluid specifications.

Engine Coolant

Engine coolant runs through your vehicle's engine, absorbing and transferring heat to the radiator where it is cooled before being re-circulated. Coolant contains specially formulated chemical packages that inhibit rust and scale buildup, lubricate water pumps, help protect against freezing, and improve heat transfer.

There are plenty of things that can cause your coolant to "wear out": Not changing it frequently enough, running the engine in an overheated condition, and even just working the engine extremely hard can result in breakdown of the coolant's chemical properties. This leaves the cooling system more susceptible to rust and scale buildup and freezing in the winter. In addition, the water pump can wear out from excessive friction and heat.

The rule-of-thumb for flushing and replacing your coolant is every two years or 24,000 miles.

Power Steering Fluid

Power steering fluid is hydraulic oil, just like transmission fluid. Your power steering system consists of a pump and fluid reservoir, lines, and a power steering gear. The pump creates hydraulic pressure from pumping the fluid, which powers the steering gear, making steering easy.

On most vehicles, power steering fluid does not show up in the maintenance schedule, so there are no severe or normal service recommendations. The only recommendation suggested here is to check it at every oil change, inspecting closely for evidence of metal flakes, indicating steering gear or pump wear, or a black or dark color, indicating overheating. Either condition calls for replacement of the fluid and inspection of the system. If the fluid appears to be overheated, the pump should be checked for internal wear. If you catch these problems early enough and replace the pump, you can usually circumvent replacing the steering gear later.

Brake Fluid

On most vehicles, brake fluid does not show up in the maintenance schedule and so there are no severe or normal service recommendations. The only recommendation suggested here is to check the brake fluid every oil change, inspecting closely for the proper level and signs that the fluid needs to be changed. Some manufacturers do suggest having the brake fluid completely changed every five to seven years, especially if you live in a state where you experience winter or a lot of rain.

Brake fluid is a hydraulic fluid used as a medium to generate the pressure needed to activate the brakes. When you press the brake pedal, the master cylinder pumps fluid through the system, which pushes the brake caliper pistons against the brake pads, which in turn make contact with the brake rotors and slow the car.

Brake fluid that is black in color has been overheated. Excess heat in the system is usually caused by a stuck brake caliper. So if the brake fluid is dark in color, the brakes should be checked for a malfunction. Rust sediment is an indication that moisture has contaminated the brake fluid. You need to keep a close eye on brake fluid because it is hydroscopic in nature, meaning it absorbs moisture, which reduces the effectiveness of the fluid.