Driveline and Power Steering Fluid Service

Transmissions, differentials and power steering systems all depend on fluid, and each one needs its own type. The right fluid in the right amount keeps these parts running for years. The wrong fluid can damage a transmission in a few hundred miles. This guide covers how to choose the fluid, how to service each system, and how to spot trouble on the way.

What You Will Be Able to Do

• Find the correct fluid and service method for a transmission
• Explain when a drain-and-fill is right and when a flush is not
• Check transmission fluid level on vehicles with and without a dipstick
• Service a differential and transfer case without getting stuck
• Service hydraulic power steering and inspect CV boots

Warning: Fluids from a vehicle that has just been driven can be hot enough to burn. Wear gloves and safety glasses. Reminder: raise and support the vehicle level, and use the lift's locks. Level matters here, because a tilted vehicle gives a false fluid level.

Automatic Transmission Fluid

Automatic transmission fluid, or ATF, does several jobs. It transmits power through the torque converter, works the clutches, cools the unit and lubricates it. Modern fluids are made for specific designs, so they are not interchangeable.

Use this as a starting point, then confirm the fluid for the exact vehicle in service information. Wrong fluid can cause shudder, harsh shifts, slipping and early failure.

Reading the Fluid

Some newer fluids are a different color when new, so check the spec.

Service Methods

Some manufacturers advise against flush machines and specify drain-and-fill only. Flushing can dislodge debris and stress older clutches and seals, so a transmission that was working can start to slip. Always follow the manufacturer's recommendation.

Drain-and-Fill Procedure

1. Warm the fluid as the procedure says, and raise the vehicle level.
2. Remove the drain plug, or loosen the pan bolts to lower one edge and drain it.
3. Measure how much comes out.
4. If you took the pan off, clean it and the magnet. Fine paste on the magnet is normal. Metal chips are not.
5. Replace the filter and gasket if the service calls for it.
6. Refit the pan and torque the bolts evenly to spec.
7. Refill with the same amount you drained.
8. Check the level using the correct procedure.

Checking the Level

With a dipstick, follow the procedure printed in the manual. Most call for a warm engine, running, with the shifter moved through the gears, and the vehicle level.

Many modern transmissions have no dipstick. The level is set at a fill or check plug at a specific fluid temperature, often around 100 to 120°F. The exact range is in service information.

1. Read the fluid temperature on a scan tool.
2. Run the engine and shift through the gears.
3. With the vehicle level, remove the check plug while in the temperature window.
4. A thin stream or trickle means the level is correct. A gush means too much. No fluid means it is low.

Do not skip the temperature window. Fluid expands as it warms, so a check at the wrong temperature leaves the transmission overfilled or underfilled. Both cause problems.

Manual Transmissions

A manual transmission may use gear oil, ATF or a special fluid. Look it up. Some gear oils rated GL-5 can harm the yellow metal parts in transmissions that specify GL-4. Most use a side fill plug, with the level at the bottom of the fill hole.

Differentials and Transfer Cases

Differentials let the driven wheels turn at different speeds through a corner. They use gear oil, and the fluid spec includes a viscosity such as 75W-90 and a rating such as GL-5.

A limited-slip differential, or LSD, uses clutches that need a friction modifier. Check whether the fluid already contains it. If not, add the LSD additive. Without it, the differential chatters in turns.

Transfer cases on four-wheel and all-wheel-drive vehicles have their own requirements. Some use ATF, some gear oil, and others a dedicated fluid. Look it up each time, because a wrong guess can damage the chain and clutches.

Service Procedure

1. Drive to warm the fluid, then park level or raise the vehicle level.
2. Remove the fill plug first. If the fill plug will not come out, you cannot refill the unit, so do not drain it. Verifying this first saves you from a car that cannot be driven.
3. Remove the drain plug and catch the old fluid. Some differentials have no drain plug and need a cover removed or a suction tool.
4. Inspect the magnet. A light paste is normal. Chips or chunks suggest gear or bearing damage, so report it.
5. Reinstall the drain plug with a new washer if it has one, and torque to spec.
6. Pump in new fluid until it just begins to weep from the fill hole. That is the correct level on most units.
7. Reinstall the fill plug and torque to spec.

Power Steering Fluid

Many newer vehicles have electric power steering, which uses no fluid. It fails through software, wiring or motor faults, so there is nothing to top up.

Hydraulic systems use a pump and fluid. The fluid can be a dedicated power steering fluid or ATF, depending on the manufacturer's specification. Some European systems use a mineral-based fluid. Do not mix fluid types, since seals can swell and the pump can fail.

To check the level, read the marks on the reservoir or dipstick, which often have hot and cold ranges.

To refresh the fluid:

1. Remove the old fluid from the reservoir with a suction tool.
2. Refill with the correct fluid.
3. Start the engine and turn the wheel from lock to lock a few times. Do not hold it against the stops.
4. Repeat until the fluid stays clean.

A low level means a leak, so look for it. A whining pump that gets worse on turns can mean low fluid or air in the system. Foamy fluid suggests air.

CV Boots and Joints

While you are under the vehicle, check the drive axles. A CV, or constant velocity, joint lets the axle flex while it turns. A rubber boot keeps grease in and dirt out.

1. Turn the wheels to full lock and use a flashlight.
2. Check each boot for tears, splits and loose clamps.
3. Look for grease thrown around the wheel well.

A torn boot needs prompt attention. Once grease escapes and grit gets in, the joint wears out quickly. Caught early, a new boot can sometimes save the joint. Caught late, the axle usually needs replacing.

Common Mistakes

Check Your Understanding

1. A customer asks for a transmission flush on a car whose manufacturer specifies drain-and-fill only. What do you do?
2. You drain a rear differential and only then find the fill plug is seized. What should have happened first?
3. After a rear differential service on a truck with a limited-slip unit, the customer reports chatter in tight turns. What is the most likely cause?
4. You find metal chips on a transmission pan magnet. What do you do?

Answers

1. Explain the manufacturer's recommendation and offer the approved service. Flush machines can cause problems, and the manufacturer's method comes first.
2. The fill plug comes out first, so you know you can refill before you drain.
3. The friction modifier was missing. Check the fluid, and add the LSD additive if required.
4. Stop and report it. Chips suggest internal damage, and a fluid change will not fix that.

Key Takeaways

• Fluids are specific. Look up the exact fluid and method for each vehicle.
• Follow the manufacturer on drain-and-fill versus flush.
• Check transmission level at the correct temperature, with the vehicle level.
• Always remove the fill plug before the drain plug.
• Limited-slip differentials need the correct friction modifier.
• Inspect magnets and boots while you are there, and report what you find.