802767 | Detroit Diesel® Air Compressor BA-921

Cylinder Diameter Ø92 mm
Volume 360 cm³
Piston Operating Distance 54 mm
Port 0 M27x2
Port 2 M22x1.5
Port 9.1 M18x1.5
Port 9.2 M18x1.5
Crankshaft Thread M20x2.5 RH
Number of Cylinders 1
Cooling Water
ESS Feature Yes
Water Cooled Valve Plate Yes
Direct Working Yes
Maximum Operating Pressure 10.5 bar

1. Supply air pressure for safe braking

The compressor keeps the air tanks filled so the brakes can work reliably.

2. Simpler construction

With only one cylinder and piston:

• fewer moving parts,
• easier servicing,
• lower manufacturing cost.

3. Good for moderate-duty applications

Works well for:

• smaller trucks,
• buses,
• farm equipment,
• older heavy vehicles,
• industrial machines.

4. Lower maintenance costs

Single-cylinder units are generally easier to troubleshoot and rebuild than larger multi-cylinder compressors.

1. Slow air pressure build-up

One of the most common signs.

The compressor takes too long to raise tank pressure to operating range (often around 100–125 PSI).

You may notice:

• Warning buzzer/light staying on
• Long wait before driving
• Pressure gauges rising very slowly

2. Low air pressure

The system cannot maintain normal pressure.

Possible effects:

• Weak braking response
• Air brakes not releasing properly
• Repeated low-air alarms

3. Compressor runs constantly

The compressor never seems to “cut out.”

Normally, the governor unloads the compressor at maximum pressure. A failing compressor may struggle to reach cut-out pressure.

4. Excessive oil in air tanks

Worn piston rings or seals can allow engine oil into the air system.

Signs include:

• Oily residue when draining tanks
• Oil-contaminated air lines
• Damaged air brake components

5. Excessive moisture buildup

A weak compressor can overheat or fail to properly manage airflow, leading to excess condensation.

You might see:

• Water in tanks
• Frozen airlines in cold weather
• Corrosion in the system

6. Unusual noises

Bad internal components can create:

• Knocking
• Grinding
• Squealing
• Clicking

These may indicate worn bearings, valves, or piston damage.

7. Air leaks around compressor

Leaks may occur at:

• Head gasket
• Fittings
• Discharge line
• Unloader/governor connections

8. Overheating

The compressor may become excessively hot due to:

• Poor lubrication
• Restricted discharge line
• Carbon buildup
• Internal wear

9. Smoke from compressor area

Blue or gray smoke can indicate oil burning or severe internal wear.

10. Frequent governor cycling

If pressure rapidly drops and rebuilds repeatedly, the compressor may be weak or unable to keep up with demand.