Bendix BA-921 Service Manual

BENDIX® BA-921® COMPRESSOR

DESCRIPTION

The function of the air compressor is to provide and maintain air under pressure to operate devices in air brake systems. The Bendix® BA-921® compressor is a single-cylinder reciprocating compressor with a rated displacement of 15.8 cubic feet per minute at 1250 RPM.

The compressor consists of a water-cooled cylinder head assembly and an integral air-cooled crankcase assembly.

The cylinder head assembly is made up of the cylinder head, cooling plate and valve plate assembly and uses two sealing gaskets. Depending on the application, the cylinder head and cooling plate may be aluminum or cast iron. The cylinder head contains air and water ports as well as an unloader assembly. A cooling plate is located between the cylinder head and valve plate assemblies and assists in cooling.

The valve plate assembly consists of brazed steel plates which have valve openings and passages for air and engine coolant to flow into and out of the cylinder head. The compressor's discharge valves are part of the valve plate assembly. The inlet reed valve/gasket is installed between the valve plate assembly and the top of the crankcase.

Splash	Coolant Ports
Shield	(3 total)
Unloader
Cover
Unloader	Air
Piston	Intake
Discharge	Cooling
Reed	Plate
Valves (2)
Piston	Crankshaft

Piston

Rod

Crankcase

Cover

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Cylinder	Safety
	Valve
Head

Valve Plate

Assembly

Cooling

Plate

Crankcase

FIGURE 2 - BENDIX® BA-921® COMPRESSOR

The cast iron crankcase houses the piston assembly, connecting rod, crankshaft and related bearings.

All Bendix® BA-921® compressors are equipped with a safety valve to protect the compressor head in the event of, for example, a discharge line blockage downstream of the compressor. Excessive air pressure will cause the safety valve to unseat, release air pressure and give an audible alert to the operator. The safety valve is installed in the cylinder head safety valve port, directly connected to the cylinder head discharge port.

The crankcase cover located at the bottom of the crankcase is stamped with information identifying the compressor model, customer piece number, Bendix piece number and serial number. See Figures 2 and 3.

Compressor Model,

Customer Piece Number,

Bendix Piece Number and

Serial Number shown here

FIGURE 1 - BENDIX® BA-921® COMPRESSOR (CUT-AWAY)

FIGURE 3 - CRANKCASE BASE COVER

Caterpillar	Caterpillar	Detroit
C7, C9	HD Acert	Diesel S60
Engines	Engines

FIGURE 4 - TYPICAL COMPRESSOR DRIVE FLANGES	piston to move upwards, away from its seat on the valve
	plate assembly. When the unloader piston is unseated an

OPERATION

The compressor is driven by the vehicle's engine and functions continuously while the engine is in operation. Actual compression of air is controlled by the compressor unloading mechanism operating in conjunction with a governor.

AIR INTAKE (LOADED)

Just as the piston begins the down stroke, (a position known as top dead center, or TDC), the vacuum created in the cylinder bore above the piston causes the inlet reed valve to flex open. Atmospheric air (in naturally aspirated applications) or pressurized air (in turbocharged applications) flows through the open inlet valve and fills the cylinder bore above the piston. See Figure 5.

AIR COMPRESSION (LOADED)

When the piston reaches the bottom of the stroke, (a position known as bottom dead center, or BDC), the inlet reed valve closes. Air above the piston is trapped by the closed inlet reed valve and is compressed as the piston moves upwards. When air in the cylinder bore reaches a pressure greater than that of the system pressure, the discharge reed valves open and allow air to flow into the discharge line and air brake system.

At the same time air flows into the hollow center of the unloader piston through an opening in the end of the piston. Compressed air acts on the interior surfaces of the unloader piston and, along with the unloader piston spring, holds the unloader piston in the down position, against its seat on the valve plate. See Figure 7.

NON-COMPRESSION OF AIR (UNLOADED)

When air pressure in the supply reservoir reaches the cutout setting of the governor, the governor delivers system air to the compressor unloader port. Air entering the unloader port acts on the unloader piston causing the

Air	Air Inlet	Discharge
	Port	Valve	Unloader
Discharge		Closed
			Port
Port
Cooling
Plate
Valve
Plate			Unloader
			Piston
			Down &
			Seated
			Inlet
			Valve
			Open
	Piston Moving Down

FIGURE 5 - OPERATION - LOADED (INTAKE)

An Inlet Check Valve (ICV),
or alternately, an ICV plus a reservoir,
is used in the air inlet line depending on
the application
Governor	Air Dryer
Unloader Port
Governor
Reservoir
Port
Compressor
	Supply Reservoir

FIGURE 6 - BA-921® COMPRESSOR UNLOADER SYSTEM

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	Air Inlet	Discharge
Air	Port	Valve	Unloader
		Open	Port
Discharge
Port

Cooling
Plate
Valve
Plate
Inlet	Unloader
Valve	Piston
Closed	Down &
	Seated

Piston Moving Up

FIGURE 7 - OPERATION - LOADED (COMPRESSION)

air passageway is opened between the cylinder bore and the air inlet cavity in the cylinder head.

As the piston moves from bottom dead center (BDC) to top dead center (TDC) air in the cylinder bore flows past the unseated unloader piston, into the cylinder head inlet cavity and out the inlet port. To prevent the air from flowing back into the engine air induction system, an inlet check valve (ICV) is installed upstream of the air compressor inlet port. The location of the device and the way it is plumbed into the compressor air induction system is unique to the specific engine and the type of air induction (naturally aspirated or boosted air) the compressor uses. These air induction systems will be explained in further detail in the “Air Induction” section on page 4. On the piston down stroke (TDC to BDC) air flows in the reverse direction, from the inlet cavity past the unseated unloader piston and inlet reed valve, and into the cylinder bore.

LUBRICATION

The vehicle's engine provides a continuous supply of oil to the compressor. Oil is routed from the engine to the compressor's oil inlet. An oil passage in the crankshaft routes pressurized oil to the precision sleeve main bearings and to the connecting rod bearings. Spray lubrication of the cylinder bores, connecting rod wrist pin bushings, and ball type main bearings is obtained as oil is forced out around the crankshaft journals by engine oil pressure. Oil then falls to the bottom of the compressor crankcase and is returned to the engine through drain holes in the compressor mounting flange.

Lubrication for Caterpillar C11 and C13 engine applications

Bendix® BA-921® compressor - for Caterpillar C11 and C13 engine installations only - use an "oil jet" that sprays oil under the piston for purposes of cooling. This oil jet is part of a special crankcase cover that is used only on the BA-921® compressor for CAT C11 and C13 engine installations (see Figure 14).

This design slightly alters the flow of oil for lubrication. The oil supply line from the engine is directly connected to the back side of the special crankcase over. The oil flows in parallel through a passageway in the crankcase cover and through the oil jet to spray oil under pressure up onto the underneath of the piston for cooling. At the same time, oil flows out of the opposite end of the special crankcase cover, through a fitting and a metal tube and second fitting into the oil supply port of the compressor. At this point oil flows in a similar manner as in the first paragraph of this section.

COOLING

Bendix® BA-921® compressors are cooled by air flowing through the engine compartment as it passes the compressor's cast-in cooling fins and by the flow of engine coolant through the cylinder head. Coolant supplied by the engine cooling system passes through connecting lines into the cylinder head and passes through internal passages in the cylinder head, cooling plate and valve plate assembly and returns to the engine. Figure 9 illustrates the various approved coolant flow connections. Proper cooling is important in minimizing discharge air temperatures - see the tabulated technical data on page 14 of this manual for specific requirements.

Air Inlet Port	Air From
	Governor
	Unloader
	Port
Cooling
Plate
Valve	Unloader
Plate	Piston Up &
	Unseated

Air in Pistons Shuttles Back and Forth from the Piston to the Cylinder Head and Inlet Port During Unloaded Mode

FIGURE 8 - OPERATION - UNLOADED

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AIR INDUCTION GENERAL

The Bendix® BA-921® air compressor can be used both with air induction systems that are naturally aspirated (atmospheric air) and pressurized (turbocharged). The following section covers Caterpillar and Detroit Diesel engine air induction arrangements. See Figure 4, for typical flanges used.

CATERPILLAR

Caterpillar HD ACERT engines (C11, C13, C15 and C18) and MD ACERT engines (C7 and C9) are typically equipped with Bendix® BA-921® compressors. These engines provide pressurized (turbocharged) air to the compressor's inlet port. Caterpillar recommends the use of an inlet check valve in the air induction system to prevent the air from the compressor being forced back into the engine air induction system when the compressor is operating in the "unloaded" condition (not building air). Because the compressor induction system is turbocharged, an additional air volume (reservoir) is required between the compressor inlet port and the inlet check valve to prevent excessive air pressure at the compressor inlet in the unloaded mode. Figures 10 and 11 show example of the different air induction systems used by Caterpillar to perform this function.

CATERPILLAR C7/C9 ENGINES

The Bendix® BA-921® compressor in the C7/C9 air induction system (see Figure 10) receives its air from the engine's intake manifold (turbocharged). During the pumping condition (loaded mode), the air flows from the engine intake manifold through the inlet check valve and inlet line to the compressor inlet port. During the non-pumping condition (unloaded mode), the compressor cylinder pushes air back out of the inlet port to the inlet check valve. The ICV prevents the air from traveling beyond this point. Because the air is boosted (under pressure), it is important that the compressor inlet line is of sufficient length, strength and volume to minimize the build-up of air pressure in the inlet system. The air shuttles back and forth between the compressor cylinder bore and the ICV during this phase of the compressor operation.

CATERPILLAR C11, C13, C15 AND C18 ENGINES

The Bendix® BA-921® compressor in the C11, C13, C15, and C18 air induction systems (see Figure 11) receives its air from the engine's intake manifold (turbocharged). During the pumping condition (loaded mode), the air flows from the engine intake manifold through the inlet check valve, expansion tank and inlet line to the compressor inlet port. During the non-pumping condition (unloaded mode), the compressor cylinder pushes air back out of the inlet port into the expansion tank. The ICV (at the end of the

	Head	Discharge	Coolant In or Out
	Bolt (4)
		Safety Valve	(One or other not used)
	Discharge
	Port

Governor

Connection

Inlet

Unloader Port

Cover

Coolant In or Out

CYLINDER HEAD PORT IDENTIFICATION

The cylinder head connection ports are identified with cast in numerals as follows:

AIR IN	0
Compressed AIR OUT	2
Coolant IN or OUT	9
Governor Control	4

FIGURE 9 - TYPICAL BENDIX® BA-921® COMPRESSOR

CYLINDER HEAD

expansion tank) prevents the air from traveling beyond this point. Because the air is boosted (under pressure), it is important that the compressor inlet line is of sufficient length, strength and volume to minimize the build-up of air pressure in the inlet system. The air shuttles back and forth between the compressor cylinder bore and the expansion tank during this phase of the compressor operation.

Inlet

Port

Inlet

Line

			Air Supply
Inlet Check Valve		F	rom Engin	e

FIGURE 10 - EXAMPLE OF CATERPILLAR (ACERT

ENGINE) C7/C9 COMPRESSOR AIR INDUCTION SYSTEM (TURBOCHARGED)

4

Inlet

Line

Expansion Tank

Inlet

Port

Air Supply

From Engine

Inlet Check Valve

FIGURE 11 - EXAMPLE CATERPILLAR (HD ACERT

ENGINE) C11/C13/C15/C18 COMPRESSOR AIR INDUCTION SYSTEM (TURBOCHARGED)

DETROIT DIESEL

The Detroit Diesel Series 60 (EGR) engine is equipped with the Bendix® BA-921® compressor and uses naturally aspirated air induction system. Detroit Diesel recommends the use of an inlet check valve in the air induction system to prevent the air from the compressor cylinder bore from being forced back into the engine air induction system when the compressor is in the unloaded mode (non-pumping condition). A flexible high-pressure hose is installed between the inlet check valve and the compressor inlet fitting. This hose can be of various lengths to accommodate the distance between the compressor and inlet check valve. See Figure 12.

Fr
Inlet	Inlet
		Port
Check
Valve

FIGURE 12 - EXAMPLE OF DETROIT DIESEL (EGR) S60

COMPRESSOR AIR INDUCTION SYSTEM (NATURALLY ASPIRATED)

During operation, non-pressurized air from the engine's air source is routed to the compressor from a point between the engine air filter and the non-pressure side of the turbocharger. When the compressor is building air (loaded mode), the air flows from the engine intake tube, through the inlet check valve into the inlet port of the compressor. When the compressor is not building air (unloaded mode), the compressor pushes the air back out the compressor during the cylinder upstroke towards the inlet check valve. The ICV prevents the air from traveling beyond this point. The air shuttles back and forth between the compressor cylinder bore and the ICV during this phase of the compressor operation.

PREVENTATIVE MAINTENANCE

Regularly scheduled maintenance is the single most important factor in maintaining the air brake charging system. Refer to Table A in the Troubleshooting section on page 17, for a guide to various considerations that must be given to maintenance of the compressor and other related charging system components.

Important Note: Review the warranty policy before performing any intrusive maintenance procedures. An extended warranty may be voided if intrusive maintenance is performed during this period.

EVERY 6 MONTHS, 1800 OPERATING HOURS OR AFTER EACH 50,000, MILES WHICHEVER OCCURS FIRST, PERFORM THE FOLLOWING INSPECTIONS AND TESTS.

AIR INDUCTION

The Bendix® BA-921® compressor is designed for connection to the vacuum side of the engine’s air induction system and the pressure side (turbocharged) of the engine’s air induction system.

A supply of clean air is one of the single most important factors in compressor preventive maintenance. Since the air supply for BA-921® compressor and engine is the engine air cleaner, periodic maintenance of the engine air filter is necessary.

Inspect the compressor air induction system each time engine air cleaner maintenance is performed.

1.Inspect the intake hose adapters for physical damage. Make certain to check the adapters at both ends of the intake hose or tubing.

2.Inspect the intake hose clamps and tighten them if needed.

3.Inspect the intake hose or line for signs of drying, cracking, chafing and ruptures and replace if necessary.

4.Verify that the compressor inlet fitting is tight (check torque).

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5.Any metal tubes should also be tight (torqued properly) to the mating fitting. Inspect the metal tubes for any cracks or breaks and replace if necessary.

6.If an expansion tank is present (turbocharged air induction systems only), inspect any cracks and replace if necessary.

COMPRESSOR COOLING

Inspect the compressor discharge port, inlet cavity and discharge line for evidence of restrictions and carbon buildup. If more than 1/16" of carbon is found, thoroughly clean or replace the affected parts. In some case, carbon buildup indicates inadequate cooling. Closely inspect the compressor cooling system. Check all compressor coolant lines for kinks and restrictions to flow. Minimum coolant line size is 3/8" I.D. Check coolant lines for internal clogging from rust scale. If coolant lines appear suspicious, check the coolant flow and compare to the tabulated technical data present in the back of this manual. Carefully inspect the air induction system for restrictions.

LUBRICATION

Check the external oil supply line for kinks, bends, or restrictions to flow. Supply lines must be a minimum of 3/16” I.D. Refer to the tabulated technical data in the back of this manual for oil pressure minimum values.

Check the exterior of the compressor for the presence of oil seepage and refer to the TROUBLESHOOTING section for appropriate tests and corrective action.

Note: This is applicable only for Caterpillar engine installations. Detroit Diesel utilizes an internal oil feed design. No external oil supply line is used.

OIL PASSING

All reciprocating compressors pass a minimal amount of oil. Air dyers will remove the majority of oil before it can enter the air brake system. For particularly oil sensitive systems, the Bendix® PuraGuard® system can be use in conjunction with a Bendix® air dryer.

If compressor oil passing is suspected, refer to the TROUBLESHOOTING section and TABLE A for the symptoms and corrective action to be taken. In addition, Bendix has developed the “Bendix Air System Inspection Cup” or BASIC™ kit to help substantiate suspected excessive oil passing. The steps to be followed when using the BASIC™ kit are presented in APPENDIX A at the end of the TROUBLESHOOTING section.

COMPRESSOR DRIVE

Check for noisy compressor operation, which could indicate excessive drive component wear. Adjust and/or replace as necessary. Check all compressor mounting bolts and retighten evenly if necessary. Check for leakage and proper unloader mechanism operation. Repair or replace parts as necessary.

COMPRESSOR UNLOADER & GOVERNOR

Test and inspect the compressor and governor unloader system for proper operation and pressure setting.

1.Check for leakage at the unloader port. Replace leaking or worn o-rings.

2.Make certain the unloader system lines are connected as illustrated in Figure 6.

3.Cycle the compressor through the loaded and unloaded cycle several times. Make certain that the governor cuts-in (compressor resumes compressing air) at a minimum of 105psi (cut-out should be approximately 15 - 20psi greater than cut-in pressure). Adjust or replace the governor as required.

4.Note that the compressor cycles to the loaded and unloaded conditions promptly. If prompt action is not noted, repair or replace the governor and/or repair the compressor unloader.

IMPORTANT NOTE

Replacement air governors must have a minimum cut-in pressure of 100psi. The cut-in pressure is the lowest system pressure registered in the gauges before the compressor resumes compressing air.

Compressors with no signal line to the unloader port should have a vent cap (e.g. Bendix part number 222797) installed in the port. Under no circumstances should the port be plugged or left open.

SERVICE TESTS

GENERAL

The following compressor operating and leakage tests need not be performed on a regular basis. These tests should be performed when it is suspected that leakage is substantially affecting compressor buildup performance, or when it is suspected that the compressor is “cycling” between the loaded (pumping) and unloaded (non-pumping) modes due to unloader leakage.

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IN SERVICE OPERATING TESTS

Compressor Performance: Build-up Test

This test is performed with the vehicle parked and the engine operating at maximum recommended governed speed. Fully charge the air system to governor cut out (air dryer purges). Pump the service brake pedal to lower the system air pressure below 80 psi using the dash gauges. As the air pressure builds back up, measure the time from when the dash gauge passes 85 psi to the time it passes 100 psi. The time should not exceed 40 seconds. If the vehicle exceeds 40 seconds, test for (and fix) any air leaks, and then retest the compressor performance. If the vehicle does not pass the test the second time, use the Advanced Troubleshooting Guide for Air Brake Compressors, starting on page 15 of this document to assist your investigation of the cause(s).

Note: All new vehicles are certified using the FMVSS 121 test (paragraph S5.1.1) by the vehicle manufacturer, however the above test is a useful guide for in-service vehicles.

Optional Comparative Performance Check

It may be useful to also conduct the above test with the engine running at high idle (instead of maximum governed speed), and record the time taken to raise the system pressure a selected range (for example, from 90 to 120 psi, or from 100 to 120 psi, etc.) and record it in the vehicle’s maintenance files. Subsequent build-up times throughout the vehicle’s service life can then be compared to the first one recorded. (Note: the 40 second guide in the test above does not apply to this build-up time.) If the performance degrades significantly over time, you may use the Advanced Troubleshooting Guide for Air Brake Compressors, starting on page 15 of this document, to assist investigation of the cause(s).

Note: When comparing build-up times, be sure to make an allowance for any air system modifications which would cause longer times, such as adding air components or reservoirs. Always check for air system leakage.

LEAKAGE TESTS

See the standard Air Brake System and Accessory Leakage test on Page 28 (Test 2).

Note: Leakage in the air supply system (components before the supply reservoir - such as the governor, air dryer, reservoir drain cocks, safety valve and check valves) will not be registered on the vehicle dash gauges and must be tested separately. Refer to the various maintenance manuals for individual component leakage tests and the Bendix “Test and Checklist” published in the Air Brake System Handbook (BW5057) and on the back of the Dual Circuit Brake System Troubleshooting Card (BW1396).

CYLINDER HEAD

Check for cylinder head gasket air leakage.

1.With the engine running, lower air system pressure to 60 psi and apply a soap solution around the cylinder head. Check the gasket between the cylinder head and valve plate assembly and the inlet reed valve/gasket between the valve plate assembly and crankcase for air leakage.

2.No leakage is permitted. If leakage is detected replace the compressor or repair the cylinder head using a genuine Bendix maintenance kit available from an authorized Bendix parts outlet.

INLET, DISCHARGE & UNLOADER

In order to test the inlet and discharge valves and the unloader piston, it is necessary to have shop air pressure and an assortment of fittings. A soap solution is also required.

1.With the engine shut off, drain ALL air pressure from the vehicle.

2.Disconnect the inlet and discharge lines and remove the governor or its line or adapter fitting.

3.Apply 120-130 psi shop air pressure to the unloader port and soap the inlet port. Leakage at the inlet port should not exceed 50 sccm.

4.Apply 120-130 psi shop air pressure to the discharge port and then apply and release air pressure to the inlet port. Soap the inlet port and note that leakage at the inlet port does not exceed 20 sccm.

If excessive leakage is noted in Tests 3 or 4, replace or repair the compressor using genuine Bendix replacements or maintenance kits available from any authorized Bendix parts outlet.

While it is possible to test for inlet, discharge, and unloader piston leakage, it may not be practical to do so. Inlet and discharge valve leakage can generally be detected by longer compressor build-up and recovery times. Compare current compressor build-up times with the last several recorded times. Make certain to test for air system leakage, as described under In Service Operating Tests, before making a determination that performance has been lost.

Unloader leakage is generally exhibited by excessive compressor cycling between the loaded and unloaded condition.

1.With service and supply system leakage below the maximum allowable limits and the vehicle parked, bring system pressure to governor cut-out and allow the engine to idle.

7

2.The compressor should remain unloaded for a minimum of 5-10 minutes. If compressor cycling occurs more frequently and service and supply system leakage is within tolerance replace the compressor or repair the compressor unloader system using a genuine Bendix maintenance kit available from authorized Bendix parts outlets.

COMPRESSOR REMOVAL & DISASSEMBLY

GENERAL

The following disassembly and assembly procedure is presented for reference purposes and presupposes that a rebuild or repair of the compressor is being undertaken. Several maintenance kits are available and the instructions provided with these parts and kits should be followed in lieu of the instructions presented here.

MAINTENANCE KITS & SERVICE PARTS
Cylinder Head Gasket Kit .......................................................	5008558
Unloader Kit ...........................................................................	5008557
Governor Adapter Kit .............................................................	5008561
Compressor Seal Kit (crankcase)...........................................	5008559
Components and kits for Caterpillar Engines
CAT C7/C9 Inlet Check Valve .................................................	801580
CAT C11/C13/C15/C18 Inlet Check Valve ...............................	801592
CAT C7/C9 Engine Seal Kit....................................................	5012367
CAT C11/C13/C15/C18 Engine Seal Kit.................................	5012369
CAT ST-4 Discharge Safety Valve (7/8"-14 thrd.) .....................	801116
Components and kits for DDC Engines
DDC Inlet Check Valve ............................................................	802192
DDC Series 60 Engine Seal Kit..............................................	5012371
DDC ST-4 Discharge Safety Valve (M16-1.5 thrd.)...................	800534

IMPORTANT! PLEASE READ AND FOLLOW THESE INSTRUCTIONS TO AVOID PERSONAL INJURY OR DEATH:

When working on or around a vehicle, the following general precautions should be observed at all times:

1.Park the vehicle on a level surface, apply the parking brakes, and always block the wheels. Always wear safety glasses. Where specifically directed, the parking brakes may have to be released, and/or spring brakes caged, and this will require that the vehicle be prevented from moving by other means for the duration of these tests/procedures.

2.Stop the engine and remove ignition key when working under or around the vehicle. When working in the engine compartment, the engine should be shut off and the ignition key should be removed. Where circumstances require that the engine be in operation, EXTREME CAUTION should

be used to prevent personal injury resulting from contact with moving, rotating, leaking, heated or electrically charged components.

3.Do not attempt to install, remove, disassemble or assemble a component until you have read and thoroughly understand the recommended procedures. Use only the proper tools and observe all precautions pertaining to use of those tools.

4.If the work is being performed on the vehicle’s air brake system, or any auxiliary pressurized air systems, make certain to drain the air pressure from all reservoirs before beginning ANY work on the vehicle. If the vehicle is equipped with an AD-IS® air dryer system or a dryer reservoir module, be sure to drain the purge reservoir.

5.Following the vehicle manufacturer’s recommended procedures, deactivate the electrical system in a manner that safely removes all electrical power from the vehicle.

6.Never exceed manufacturer’s recommended pressures.

7.Never connect or disconnect a hose or line containing pressure; it may whip. Never remove a component or plug unless you are certain all system pressure has been depleted.

8.Use only genuine Bendix® replacement parts, components and kits. Replacement hardware, tubing, hose, fittings, etc. must be of equivalent size, type and strength as original equipment and be designed specifically for such applications and systems.

9.Components with stripped threads or damaged parts should be replaced rather than repaired. Do not attempt repairs requiring machining or welding unless specifically stated and approved by the vehicle and component manufacturer.

10.Prior to returning the vehicle to service, make certain all components and systems are restored to their proper operating condition.

11.For vehicles with Antilock Traction Control (ATC), the ATC function must be disabled (ATC indicator lamp should be ON) prior to performing any vehicle maintenance where one or more wheels on a drive axle are lifted off the ground and moving.

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1 Head Cap Screws (4)		2 Unloader Cover Cap Screws (2)
(include washers)		3 Unloader Cover Splash Shield
4 Unloader Cover		6 Unloader Balance Piston
5 Unloader Cover Gasket		7 O-Ring
		8 Spring
12 ST-4™ Safety Valve		9 O-Ring
		10 Unloader Piston
		11 O-Ring
		14 Head Gasket
13 Cylinder Head		Alignment	(2)
		Bushings
15 Cooling		16 Valve Plate
Plate		Assembly
				30	31
				Governor
				Gasket (2)	Governor
17 Inlet Reed Valve/					Adapter
Gasket		19 Cylinder
18 Cylinder		Rod
					32 Bolt
		Crankcase			with
					Washer
20 Bearing		Alignment
					(2)
Sleeve		Pins
21 Crankcase		22 Crankshaft
27 Bottom Cover		23 Rear Bearing
				24 O-Ring
Gasket					25 Rear End
					Cover

Item Qty. Description

14 Head Cap Screws

22 Unloader Cover Cap Screws

3	1	Unloader Cover Splash Shield						29 Cap			26 Cap
4**	1	Unloader Cover			28 Crankcase
											Screws (4)
5**	1	Unloader Cap Gasket					Screws (4)
						Cover
6**	1	Unloader Balance Piston
7**	1	O-Ring
8**	1	Spring
										Notes:
9**	1	O-Ring
10**	1	Unloader Piston								*	included in Cylinder Head Gasket Kit
11**	1	O-Ring									(5008558)
12	1	ST-4™ Safety Valve		Item Qty.		Description				**	included in Unloader Kit (5008557)
13	1	Cylinder Head		23	1	Rear Bearing
14*	2	Head Gaskets		24	1	O-Ring				***	included in Governor Adapter Kit
15	1	Cooling Plate
				25	1	Rear End Cover					(5008561)
16	1	Valve Plate Assembly		26	4	Cap Screws
17*	1	Inlet Reed Valve/Gasket		27	1	Bottom Cover Gasket				Crankcase Compressor Seal Kit (5008559)
18	1	Cylinder		28	1	Crankcase Cover
										Unique Engine Seal Kits are available, for
19	1	Cylinder Rod		29	4	Cap Screws
										Cat C7/C9 Engine, Cat C11/C13/C15/C18
20	1	Sleeve Bearing		30***	2	Governor Gasket
										Engines, and DDC Series 60 Engines
21	1	Crankcase		31***	1	Governor Adapter
22	1	Crankshaft		32***	2	Bolt with Washer

FIGURE 13 – BA-921® COMPRESSOR EXPLODED VIEW

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REMOVAL

In many instances it may not be necessary to remove the compressor from the vehicle when installing the various maintenance kits and service parts. The maintenance technician must assess the installation and determine the correct course of action.

These instructions are general and are intended to be a guide. In some cases additional preparations and precautions are necessary. In all cases follow the instructions contained in the vehicle maintenance manual in lieu of the instructions, precautions and procedures presented in this manual.

1.Block the wheels of the vehicle and drain the air pressure from all the reservoirs in the system.

2.Drain the engine cooling system and the cylinder head of the compressor. Identify and disconnect all air, water and oil lines leading to the compressor.

3.Remove as much road dirt and grease from the exterior of the compressor as possible.

4.Remove the discharge and inlet fittings, if applicable, and note their position on the compressor to aid in reassembly.

Note: If a cylinder head maintenance kit is being installed, stop here and proceed to PREPARATION FOR DISASSEMBLY. If replacing the compressor continue.

5.Remove any supporting bracketing attached to the compressor and note their positions on the compressor to aid in reassembly.

6.Remove the front flange mounting bolts and remove the compressor from the vehicle.

7.Inspect drive gear and associated drive parts for visible wear or damage. If the compressor drive gear is worn or damaged, the compressor must be replaced. Refer the Engine Manufacturers service manual to address the associated engine drive parts.

8.If the compressor is being replaced stop here and proceed to “Installing the Compressor” at the end of the assembly procedure. (Note: Replacement compressors come with the drive gear pre-assembled on the compressor.)

PREPARATION FOR DISASSEMBLY

Remove the balance of road dirt and grease from the exterior of the compressor with a cleaning solvent. If a rear end cover or end cover adapter is used on the compressor being worked on, mark it in relation to the crankcase. It is recommended, but not specifically necessary, to mark the relationships of the cylinder head (13), cooling plate (15), valve plate assembly (16), and crankcase (21).

A convenient method to indicate the above relationships is to use a metal scribe to mark the parts with numbers or

lines. Do not use marking methods such as chalk that can be wiped off or obliterated during rebuilding.

Prior to disassembly make certain that the appropriate kits and or replacement parts are available. Refer to Figure 13 during the entire disassembly and assembly procedure.

What follows is a description of a complete disassembly, actual maintenance may only need to include portions of these instructions.

CYLINDER HEAD

1.Remove the discharge safety valve (12) from the cylinder head (13).

2.To restrain the spring force exerted by balance piston spring (8) of the unloader assembly, hold the unloader cover (4) in place while removing the two unloader cover cap screws (2) and spray shield (3). Carefully release the hold on the unloader cover until the spring force is relaxed, then remove the unloader cover.

3.Remove the unloader cover gasket (5).

4.Remove the balance piston (6), its spring (8) and the unloader piston (10) along with its o-rings (7, 9 & 11) from the cylinder head (13).

5.Remove the four hex head bolts (1) from the cylinder head.

6.Gently tap the cylinder head, cooling plate (15) and valve plate assembly (16) with a soft mallet to break the gasket seal between the valve plate assembly and the crankcase (21). Lift the cylinder head with cooling plate and valve plate assembly off the crankcase.

7.Remove the metal inlet reed valve/gasket (17).

8.Gently tap the cylinder head, cooling plate and valve plate assembly with a soft mallet to break the gasket seals. Then separate the cylinder head from the cooling plate (15) and valve plate assembly and remove the two gaskets (14) between them.

CRANKCASE COVER

1.Remove the four crankcase cover cap screws (29) securing the crankcase cover (28) to the crankcase (21). Using a soft mallet, gently tap the crankcase cover to break the gasket seal. Remove the crankcase cover gasket (27).

2.In the case of the Caterpillar C11 and C13 engine application, the BA-921® compressor utilizes an “oil jet” that sprays oil under the piston for purposes of cooling. This oil jet is part of a special crankcase cover that is used strictly on the BA-921® compressor for the C11 and C13 engine installation (Figure 11). Refer to section OPERATION – Lubrication for description of the system. To disassemble, follow the instructions below.

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a.Remove the oil supply line from the engine at the inlet to the special crankcase cover.

b.Remove the metal oil supply tube at the compressor oil supply port and at the outlet fitting of the special crankcase cover.

NOTE: Mark position of the special crankcase cover. It must be re-installed with the same orientation to assure proper operation of the oil jet.

c.Remove the four crankcase cover cap screws securing the special crankcase cover to the crankcase. Using a soft mallet, gently tap the crankcase cover to break the gasket seal. Remove the crankcase cover gasket (27).

REAR END COVER

1.Remove the four end cover cap screws (26) that secure the rear end cover to the crankcase.

2.Remove the rear end cover from the crankcase. Remove the o-ring seal (24) from the end cover.

CLEANING OF PARTS GENERAL

All parts should be cleaned in a good commercial grade of solvent and dried prior to inspection.

CYLINDER HEAD ASSEMBLY

1.Carefully remove all gasket material adhering to the cylinder head (13), cooling plate (15), valve plate assembly (16) and cast iron crankcase (21). Make certain not to scratch or mar the gasket surfaces. Pay particular attention to the gasket surfaces of the head.

2.Remove carbon deposits from the discharge and inlet cavities of the cylinder head, cooling plate and valve plate assembly. They must be open and clear in both assemblies. Make certain not to damage the head.

3.Remove rust and scale from the cooling cavities and passages in the cylinder head, cooling plate and valve plate assembly and use shop air to clear debris from the passages.

4.Check the threads in all cylinder head ports for galling (e.g. abrasion, chafing). Minor thread chasing (damage) is permitted.

5.Make certain the unloader vent passage under the unloader cover (4) in the head is open and free of debris.

INSPECTION OF PARTS

CYLINDER HEAD, COOLING PLATE, VALVE PLATE ASSEMBLY AND UNLOADER MECHANISM

1.Carefully inspect the head gasket surfaces on the cylinder head (13) for deep gouges and nicks. Also, inspect the cylinder head for any cracks or port thread damage. If detected, the compressor must be replaced. If large amounts of carbon build-up are present in the discharge cavity such that it restricts the air flow through the cylinder head, the compressor should be replaced.

2.Carefully inspect both sides of the head gasket surfaces on the cooling plate (15) for deep gouges and nicks. Also, inspect the cooling plate for any cracks or other damage. If found, the compressor must be replaced.

3.Carefully inspect the valve plate assembly (16) gasket surfaces (both sides) for deep gouges and nicks. Pay particular attention to the gasket surface. An inlet reed valve/gasket (17) is used between the valve plate assembly and crankcase. This gasket surface must be smooth and free of all but the most minor scratches. If excessive marring or gouging is detected, the compressor must be replaced. If large amounts of carbon build-up are present on the two main surfaces, in the two discharge valve holes or between the discharge valve and the discharge seat, the compressor should be replaced.

4.If the unloader assembly has been removed from the cylinder head, the unloader assembly must be serviced using an unloader kit (5008557).

5.If large amounts of carbon build-up are present on the unloader piston (10) seat or orifice or if the return spring exhibits compression set, the unloader components must be replaced with an unloader kit.

REAR END COVER

Check for cracks and external damage. Check the crankshaft rear bearing diameter in the rear end cover (25) for excessive wear, flat spots or galling. Check the rear support bracket threaded holes and hydraulic pump attachment threaded holes (if present) for thread damage. Minor thread chasing is permitted but do not re-cut the threads. If any of these conditions are found, replace the compressor.

CRANKCASE

Check the cylinder head gasket surface on the deck (top) of the crankcase (21) for nicks, gouges, and marring. A metal gasket is used to seal the cylinder head to the crankcase. This surface must be smooth and free of all but the most minor scratching. If excessive marring or gouging is detected, the compressor must be replaced.

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