What Air Trac suspension is and why Peterbilt uses it

Air Trac is Peterbilt's air suspension system for the rear axles, designed to replace traditional leaf spring setups. Instead of metal springs, it uses air-filled bellows that compress and extend to absorb road shock. The system maintains a consistent ride height regardless of load, which protects cargo, reduces driver fatigue, and extends the life of the truck frame and components.

Peterbilt introduced Air Trac to compete with other air suspension options in the heavy-duty market. It's standard or optional on most Peterbilt Class 8 trucks, particularly those used in long-haul and regional delivery where ride quality and load stability matter. The system integrates with the truck's air brake system, so the same compressor that powers the brakes also pressurizes the suspension.

Key Takeaways

  • Air Trac uses pressurized air bellows instead of leaf springs to support the rear axles and automatically adjust to load changes.
  • The system shares the truck's main air compressor, so brake system pressure and suspension pressure are linked through a common supply.
  • Air springs wear out over time and typically need replacement every 5 to 10 years depending on miles, load cycles, and maintenance.
  • Common failures include leaking air springs, faulty height control valves, and compressor issues that reduce system pressure.
  • Regular inspection of air lines, fittings, and bellows can catch small leaks before they cause suspension collapse or brake problems.

How the Air Trac system components work together

The core of Air Trac is the air spring — a rubber and fabric bellows mounted between the axle and the frame. When the compressor pressurizes the system, air fills the bellows and lifts the axle. When pressure drops or the truck encounters a bump, the bellows compress. This cycle repeats thousands of times per day, which is why air springs eventually fail.

The height control valve is the brain of the system. It monitors how high the truck sits and automatically adds or releases air to keep the ride height constant. If you load cargo, the valve senses the frame dropping and opens to let more air into the springs. If you unload, it vents air to prevent the truck from riding too high. A faulty height control valve causes the truck to sit too low or too high, which affects handling and can trigger warning lights on the dashboard.

The air compressor supplies pressure to both the brakes and the suspension. It runs continuously while the engine is on and stores air in the main reservoir. If the compressor fails or cannot keep up with demand, suspension pressure drops first (because brakes take priority), and the truck sags. The air dryer removes moisture from the compressed air to prevent rust inside the lines and springs. A wet air system corrodes components from the inside out.

Signs that Air Trac components need service or replacement

A truck that sits lower on one side than the other usually has a leaking air spring on the low side. You may hear a hissing sound near the rear axle, especially when the truck is parked and the compressor cycles on and off. Some leaks are small and only noticeable after the truck sits overnight; others are large enough to drop the suspension within hours of shutdown.

If the truck rides too high or too low and does not adjust when you load or unload cargo, the height control valve is likely stuck or out of calibration. The suspension may also feel bouncy or unstable, as if the truck is not settling after bumps. A warning light on the dashboard — often labeled "Air Suspension" or "Check Suspension" — confirms a system fault and should be read with a diagnostic scanner to pinpoint the cause.

Compressor problems show up as slow pressure buildup, especially on cold mornings. The truck may take longer than usual to build enough air pressure to release the parking brake. If the compressor cycles on and off more frequently than normal, it is working harder to maintain pressure, which usually means air is leaking somewhere in the system. Moisture in the air lines can freeze in cold weather and block air flow, causing the suspension to collapse suddenly.

Maintenance intervals and inspection checklist

Air springs should be inspected visually every 50,000 miles or at least once a year. Look for cracks, bulges, or permanent wrinkles in the rubber. A small crack may not leak now but will grow under pressure and load cycles. If you see any damage, the spring should be replaced before it fails completely. Replacement typically costs between $400 and $800 per spring, depending on the truck model and whether you replace one or both sides at once.

Air lines and fittings should be checked for corrosion, loose connections, and kinks. Corroded fittings can leak slowly and are hard to spot until pressure drops. Tighten any loose fittings by hand first; if they still leak, they need to be replaced. Kinks in the air line restrict flow and can cause the suspension to respond slowly or unevenly.

The height control valve should be tested during routine service. A technician can measure the truck's ride height when empty and when loaded to confirm the valve is responding correctly. If the ride height does not change, or changes only slightly, the valve needs adjustment or replacement. The air dryer cartridge should be replaced according to the manufacturer's schedule, usually every 2 to 3 years or 200,000 miles, to prevent moisture buildup.

Common Air Trac failures and what causes them

Leaking air springs are the most common failure. The rubber bellows are exposed to constant flexing, UV light, ozone, and temperature swings. Over time, the rubber hardens and cracks. Salt and road chemicals accelerate this process. Once a crack forms, air escapes and the suspension loses height. Replacing the spring stops the leak, but if the truck has high miles or sits outdoors year-round, the other spring may fail soon after.

Height control valve failure usually stems from dirt or moisture inside the valve. The valve has small passages and a sensitive diaphragm that can stick if contaminated. A stuck valve may hold air in the springs even when the truck is unloaded, or it may vent air too quickly and let the truck sag. Some valves can be cleaned and recalibrated; others must be replaced. Replacement costs range from $300 to $600 depending on the valve model.

Compressor failure is less common but more expensive to fix. The compressor can wear out internally, lose pressure, or seize. A worn compressor may still run but cannot build pressure fast enough to support the suspension and brakes. Replacement typically costs $1,500 to $2,500 plus labor. Air dryer failure causes moisture to accumulate in the system, which corrodes springs, valves, and lines from the inside. This is why replacing the dryer cartridge on schedule is cheaper than replacing corroded components later.

Differences between Air Trac and other suspension options

Leaf spring suspension, the traditional alternative, uses stacked metal leaves to support the axle. It is simpler, cheaper to maintain, and does not depend on air pressure. However, leaf springs sag under heavy loads and provide a rougher ride. They also require more frequent replacement of individual leaves and bushings. Air Trac maintains consistent ride height and comfort but requires more complex maintenance and is more expensive to repair when something fails.

Other air suspension brands, such as those made by Hendrickson or Meritor, work on the same principle as Air Trac but may have different valve designs, spring configurations, or integration with the truck's electronic systems. Peterbilt Air Trac is designed specifically for Peterbilt trucks and integrates with Peterbilt's electrical and pneumatic architecture. Parts and service are widely available at Peterbilt dealers, though independent shops familiar with air suspension can also service the system.

When to replace Air Trac springs versus repair

A single leaking air spring can be replaced without replacing the other side, though many technicians recommend replacing both springs at the same time. If one spring has reached the end of its service life, the other is likely close behind. Replacing both ensures even ride height and prevents a second failure shortly after the first repair. The labor cost is similar whether you replace one or two springs, so the total cost difference is mainly the price of the extra spring.

If the truck has over 500,000 miles or the springs are more than 10 years old, replacement is usually the right choice even if only one is leaking. Repair attempts on very old springs often fail because the rubber has degraded throughout, not just at the leak point. A new spring comes with a warranty and is more reliable than a patched old one.

Frequently Asked Questions

Can I drive the truck if one air spring is leaking?

You can drive it short distances, but the truck will sit lower on the leaking side and handle unevenly. The frame may rub on the tire or suspension components. Do not drive long distances or at highway speeds until the spring is replaced, as the uneven load can damage the axle, frame, and other components.

What does it mean if the air suspension warning light stays on?

The light indicates a fault in the system — usually a pressure drop, a faulty height control valve, or a sensor malfunction. You need to connect a diagnostic scanner to read the specific fault code. Do not ignore the light; continued driving with a suspension fault can damage the truck and make the repair more expensive.

How much does it cost to replace an air spring?

A single air spring replacement typically costs $400 to $800 in parts and labor, depending on your location and the truck model. Replacing both springs at once usually costs $700 to $1,400 total, since the labor is similar but you save on a second service visit. Prices vary by dealer and independent shop.

Why does my Air Trac system lose pressure overnight?

Small leaks in the air springs, lines, or fittings cause slow pressure loss. The compressor cannot run while the engine is off, so any leak will eventually empty the system. Inspect all visible air lines and fittings for corrosion or loose connections. If you find no obvious leak, the height control valve or an air spring may be leaking internally.

Can I switch from Air Trac to leaf springs?

Yes, but it requires significant modification to the frame, axle mounts, and brake system. The cost is usually $3,000 to $6,000 or more, depending on the truck model and the shop. Most owners keep Air Trac because the ride quality and load stability justify the maintenance cost, and switching is rarely worth the expense.