Bendix Air Compressor Troubleshooting


1. Compressor passes excessive oil as evidenced by presence of oil at exhaust ports of valving or seeping inlet from air starter. Cause:

A. Restricted air intake.Check engine or compressor air cleaner and replace if necessary. Check compressor air inlet line for kinks, excessive bends and be certain inlet lines have the minimum specified inside diameter. Recommended minimum inlet line is 15.9 mm (5/8 inch) I.D. Recommended maximum air inlet restriction is 635 mm (25 in) of water.
B. Restricted oil return (to engine).Oil return to the engine should not be in any way restricted. Check for excessive bends, kinks, and restrictions in the oil return line. Minimum recommended oil return line size is 15.9 mm (5/8 inch) O.D. tubing or equivalent I. D. [12.7 mm (1/2 inch) minimum]. Return line must CONSTANTLY DESCEND from the compressor to the engine crankcase. Make certain oil drain passages in the compressor and mating engine surfaces are unobstructed and aligned. Special care must be taken when sealants are used with or instead of gaskets.
C. Poorly filtered inlet air.Check for damaged, defective or dirty air filter on engine or compressor. Check for leaking, damaged or defective compressor air intake components (i.e., induction line, fittings, gaskets, filter bodies, etc.). The compressor intake should not be connected to any part of the exhaust gas recirculation (E.G.R.) system on the engine.
D. Insufficient compressor cooling.For air-cooled portions of the compressor:

1. Remove accumulated grease, grime, or dirt from the cooling fins. Replace components found damaged.

2. Check for damaged cooling fins. Replace components found damaged.

For water-cooled portions of the compressor:

1. Check for proper coolant line sizes. Minimum recommended size is 9.5 mm (3/16inch) I.D. tubing.

2. Check the coolant flow through the compressor. Minimum allowable flow is 9.5 liter/min (2.5 U.S. gpm) at engine governed speed. If low coolant flow is detected, inspect the coolant lines and fittings for accumulated rust scale, kinks and restrictions.

3. Water temperature should not exceed 93°C (200°F).

4. Optimum cooling is achieved when engine coolant flows as shown in Figure 4 of this manual.

E. Contaminants not being regularly drained from system reservoirs.Check reservoir drain valves to insure that they are functioning properly. It is recommended that the vehicle should be equipped with functioning automatic drain valves, or have all reservoirs drained to 0 kPa (0 psi) daily optimally to be equipped with a desiccant-type air dryer prior to the reservoir system.
F. Compressor runs loaded an excessive amount of time.Vehicle system leakage should not exceed industry standards of 6.9 kPa (1 psi) pressure drop per minute without brakes applied, and 20.7 kPa (3 psi) pressure drop per minute with brakes applied. If leakage is excessive, check for system leaks and repair.
G. Excessive engine crankcase pressure.Test for excessive engine crankcase pressure and replace or repair crankcase ventilation components as necessary. (An indication of crankcase pressure is a loose or partially lifted dipstick).
H. Excessive engine oil pressure.Check the engine oil pressure with a test gauge and compare the reading to the engine specifications. Bendix does not recommend restricting the compressor oil supply line because of the possibility of plugging the restriction with oil contaminants. Minimum oil supply line size is 4.8 mm (3/16 inch I.D.) tubing.
I. Faulty compressor.Replace or repair the compressor only after making certain none of the preceding installation defects exist.


2. Noisy compressor. Cause:

A. Loose drive gear or pulley.Inspect the fit of the drive gear or pulley on the compressor crankshaft. The pulley or gear must be completely seated and the crankshaft nut must be tight. If the compressor crankshaft surface or its keyway are damaged, it is an indication of loose drive components. If damage to the compressor crankshaft is detected, replace the compressor or the crankshaft. When installing the drive gear or pulley, torque the crankshaft nut to 113 N·m (100 lb ft). DO NOT BACK OFF THE CRANKSHAFT NUT TO ALIGN THE COTTER PIN AND CASTELLATED NUT. (Some compressors do not use castellated nuts). DO NOT USE IMPACT WRENCHES.
B. Excessively worn drive couplings or gears.Inspect drive gear and couplings and engine for excessive wear. Replace as necessary. (Non-metallic gears should be replaced when the compressor is changed.)
C. Compressor cylinder head or discharge line restrictions.Inspect the compressor discharge port and discharge line for carbon build-up. If carbon is detected, check for proper cooling to the compressor. (See Cause and remedy (D) under Symptom #1.) Inspect the discharge line for kinks and restrictions. Replace discharge line as necessary.
D. Worn or burned out bearings.Check for proper oil pressure in the compressor. Minimum required oil pressure; 34.5 kPa (5 psi) engine idling, 103 kPa (15 psi) maximum governed engine rpm. Check for excessive oil temperature. Oil temperature should not exceed 115°C (240°F).
E. Faulty compressor.Replace or repair the compressor after determining none of the preceding installation defects exist.


3. Excessive build-up and recovery time. Compressor should be capable of building air system from 586 to 690 kPa (85 to 100 psi) in 40 seconds with engine at full governed rps. Minimum compressor performance is certified to meet Federal requirements by the vehicle manufacturer. Do not downsize the original equipment compressor. Cause:

A. Dirty induction air filter.Inspect engine or compressor air filter and replace if necessary.
B. Restricted induction line.Inspect the compressor air induction line for kinks and restrictions and replace as necessary.
C. Restricted discharge line or compressor discharge cavity.Inspect the compressor discharge port and line for restrictions and carbon build-up. If a carbon build-up is found, check for proper compressor cooling. Replace faulty sections of the discharge line.
D. Slipping drive components.Check for faulty drive gears and couplings and replace as necessary. Check the condition of drive belts and replace or tighten, whichever is appropriate.
E. Excessive air system leakage.Test for excessive system leakage and repair as necessary. Use the following as a guide: Build system pressure to governor cutout and allow the pressure to stabilize for one minute. Using the dash gauge, note the system pressure and the pressure drop after two minutes.

The pressure drop for Post-1975 vehicles should not exceed:

1. 13.8 kPa (2 psi) in each reservoir for a single vehicle.
2. 41.4 kPa (6 psi) each reservoir for a tractor and trailer.
3. 55 kPa (8 psi) in each reservoir for a tractor and 2 trailers.
F. Sticking unloader pistons.Check the operation of the unloading mechanism. Check the proper operation of the compressor air governor. If the governor is operating properly, replace the unloader mechanism. Inspect for bent, linked or blocked tubing leading to or from the governor.
G. Faulty compressor.Replace or repair the compressor after determining none of the preceding installation defects exist.


4. Compressor fails to unload. Cause:

A. Faulty governor or governor operation.Test the governor for proper operation and inspect air lines to and from the governor for kinks or restrictions. replace or repair the governor or its connecting air lines.
B. Faulty or worn unloader pistons or bores.Inspect for worn, dirty or corroded unloader piston and bore. Replace as necessary.


5. Compressor leaks oil. Cause:

A. Damaged mounting gasket.Check the compressor mounting bolt torque. If the mounting bolt torque is low, replace the compressor mounting gasket before retorquing the mounting bolts.
B. Cracked crankcase or end cover.Visually inspect the compressor exterior for cracked or broken components. Cracked or broken crankcases or mounting flanges can be caused by loose mounting bolts. The end cover can be cracked by overtorquing fitting or plugs installed in the end cover. Replace or repair the compressor as necessary.
C. Loose end cover.Check the cap screw torques and tighten as necessary.
D. Loose oil supply or return line fittings.Check the torque of external oil line fittings and tighten as necessary.
E. Porous compressor casting.Replace the compressor if porosity is found.
F. Mounting flange or end cover, O-ring or gasket-missing, cut, or damaged.F. Replace as necessary.


6. Compressor constantly cycles (compressor remains unloaded for a very short time). Cause:

A. Leaking compressor unloader piston.Remove the compressor inlet air strainer or fitting. With the compressor unloaded (not compressing air), check for air leakage. Replace as necessary.
B. Faulty governor.Test the governor for proper operation and repair or replace as necessary.
C. Excessive system leakage.Test for excessive system leakage (see Symptom #3 Cause (E)). Reduce leakage wherever possible.
D. Excessive reservoir contaminants.Drain reservoirs.


7. Compressor leaks coolant. Cause:

A. Improperly installed plugs and coolant line fittings.Check torque of fittings and plugs and tighten as necessary. Over-torque fittings and plugs can crack the head casting.
B. Freeze cracks due to improper anti-freeze strength.Test anti-freeze and strengthen as necessary. Check coolant flow through compressor to assure the proper anti-freeze mixture reaches the compressor.
C. Faulty compressor (porous castings).If casting porosity is detected, replace the compressor.


8. Compressor head failure. Cause:

A. Restricted discharge line.Clear restriction or replace line.
B. Loose head bolts.Tighten evenly to a torque of 20 to 25 N·m (175 to 225 lb in).
C. Faulty compressor or head gasket.Check for rough or poorly machined head or block surfaces Replace necessary components.

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