This troubleshooting guide obsoletes and supersedes all previous publishedtroubleshooting information relative to Bendix air compressors.Advanced Troubleshooting Guidefor Air Brake Compressors*The guide consists of an introduction to air brake charging systemcomponents, a table showing recommended vehicle maintenanceschedules, and a troubleshooting symptom and remedy section with teststo diagnose most charging system problems.INDEXSymptomPage NumberAirSymptomPage NumberCoolantAir brake charging system:Slow build (9.0) . . . . . . . . . . . . . . . . . . 9 - 10Doesn’t build air (10.0) . . . . . . . . . . . . . . . 11Air dryer:Doesn’t purge (14.0) . . . . . . . . . . . . . . . . . 12Safety valve releases air (12.0) . . . . . . . . . 12Compressor:Constantly cycles (15.0) . . . . . . . . . . . . . . 13Leaks air (16.0) . . . . . . . . . . . . . . . . . . . . . 13Safety valve releases air (11.0) . . . . . . . . . 11Noisy (18.0) . . . . . . . . . . . . . . . . . . . . . . . . 13Reservoir:Safety valve releases air (13.0) . . . . . . . . . 12Compressor leaks coolant (17.0) . . . . . . . . . . 13Test ProceduresMaintenance & Usage Guidelines(1) Oil Leakage at Head Gasket . . . 14(2) System Leakage . . . . . . . . . . . . 14EngineOil consumption (6.0) . . . . . . . . . . . . . . . . . . . . 9OilOil Test Card results (1.0) . . . . . . . . . . . . . . . . . 4Oil is present:On the outside of the compressor (2.0) . . . . 5At the air dryer purge/exhaustor surrounding area (3.0) . . . . . . . . . . . . 5In the supply reservoir (4.0) . . . . . . . . . . 6 - 8At the valves (5.0) . . . . . . . . . . . . . . . . . . . . 8At air dryer cartridge (7.0) . . . . . . . . . . . . . . 9In the ping tank or compressordischarge aftercooler (8.0) . . . . . . . . . . . 9Maintenance Schedule andUsage Guidelines (Table A) . . . 3(3) Compressor Discharge andAir Dryer Inlet Temperature . . . . 14(4) Governor Malfunction . . . . . . . . 14(5) Governor Control Line . . . . . . . . 15(6) Compressor Unloader . . . . . . . . 15BASIC Test Information . . . . . . 17-20*This guide is only for vehicles that use desiccant air dryers.
Introduction to the Air Brake Charging SystemPowered by the vehicle engine, the air compressorbuilds the air pressure for the air brake system. Theair compressor is typically cooled by the engine coolantsystem and lubricated by the engine oil supply.The compressor's unloader mechanism and governor(along with a synchro valve for the Bendix DuraFlo 596 air compressor) control the brake system airpressure between a preset maximum and minimumpressure level by monitoring the pressure in the service(or “supply”) reservoir. When the air pressure becomesgreater than that of the preset “cut-out”, the governorcontrols the unloader mechanism of the compressorto stop the compressor from building air and alsocauses the air dryer to purge. As the service reservoirair pressure drops to the “cut-in” setting of the governor,the governor returns the compressor back to buildingair and the air dryer to air drying mode.As the atmospheric air is compressed, all the watervapor originally in the air is carried along into the airsystem, as well as a small amount of the lubricating oilas vapor.The duty cycle is the ratio of time the compressorspends building air to the total engine running time.Air compressors are designed to build air (run “loaded”)up to 25% of the time. Higher duty cycles causeconditions that affect air brake charging systemperformance which may require additionalmaintenance. Factors that add to the duty cycle are:air suspension, additional air accessories, use of anundersized compressor, frequent stops, excessiveleakage from fittings, connections, lines, chambers orvalves, etc.The discharge line allows the air, water-vapor andoil-vapor mixture to cool between the compressor andair dryer. The typical size of a vehicle's discharge line,(see column 2 of Table A on page 3) assumes acompressor with a normal (less than 25%) duty cycle,operating in a temperate climate. See Bendix and/orother air dryer manufacturer guidelines as needed.When the temperature of the compressed air that entersthe air dryer is within the normal range, the air dryer canremove most of the charging system oil. If thetemperature of the compressed air is above the normalrange, oil as oil-vapor is able to pass through the airdryer and into the air system. Larger diameter dischargelines and/or longer discharge line lengths can help reducethe temperature.The discharge line must maintain a constant slopedown from the compressor to the air dryer inlet fittingto avoid low points where ice may form and block theflow. If, instead, ice blockages occur at the air dryerinlet, insulation may be added here, or if the inlet fittingis a typical 90 degree fitting, it may be changed to astraight or 45 degree fitting. For more information onhow to help prevent discharge line freeze-ups, seeBendix Bulletins TCH-08-21 and TCH-08-22 (seepages 20-22). Shorter discharge line lengths orinsulation may be required in cold climates.The air dryer contains a filter that collects oil droplets,and a desiccant bed that removes almost all of theremaining water vapor. The compressed air is thenpassed to the air brake service (supply) reservoir. Theoil droplets and the water collected are automaticallypurged when the governor reaches its “cut-out” setting.For vehicles with accessories that are sensitive to smallamounts of oil, we recommended installation of aBendix PuraGuard system filter, designed to minimizethe amount of oil present.The Air Brake Charging System supplies theDischargeLineOptional “Ping” TankAir Dryercompressed air for the braking system as well as otherair accessories for the vehicle. The system usuallyconsists of an air compressor, governor, discharge line,air dryer, and service reservoir.Optional Bendix PuraGuard System Filter or PuraGuard QC Oil Coalescing FilterCompressorGovernor(Governor plus Synchro valvefor the Bendix DuraFlo 596 Compressor)2Service Reservoir(Supply Reservoir)Reservoir Drain
Table A: Maintenance Schedule and Usage GuidelinesRegularly scheduled maintenance is the single most important factor in maintaining the air brake charging system.Vehicle Used for:No. ofAxlesColumn 1Column 2 in.6 ft.Column 3RecommendedAir DryerCartridgeReplacement1Column 4RecommendedReservoirDrainSchedule2Column 5AcceptableReservoirOil Contents3at RegularDrain Intervale.g. Line haul single trailerw/o air suspension, air overhydraulic brakes.5orlesse.g. Line haul single trailerwith air suspension,school bus.5orlessHigh Air Usee.g. Double/triple trailer, openhighway coach/RV, (most)pick-up & delivery, yard orterminal jockey, off-highway,construction, loggers, concretemixer, dump truck, fire truck.8orlessCompressor with up to 25% duty cyclee.g. City transit bus, refuse,bulk unloaders, low boys,urban region coach, centraltire inflation.12orlessBendix Tu-Flo 750 air compressorCompressor with up to 25% duty cycleBendix BA-921 air compressorCompressor with up to 25% duty cycleFor oil carry-overcontrol4 suggestedupgrades:5/8 in.1/2 in.9 ft.9 ft.Every 3YearsFor oil carry-overcontrol4 suggestedupgrades:5/8 in.1/2 in.5/8 in.RecommendedEveryMonth Max ofevery 90days12 ft.For theBASICTest Kit:OrderBendixP/N5013711Every 2Years15 ft.EveryMonth5/8 in.12 ft.For oil carry-overcontrol4 suggestedupgrades:3/4 in.BASIC testacceptablerange:5 oil unitsper month.SeeappendixA.EveryYear15 ft.Footnotes:1 With increased air demand the air dryer cartridge needs to be replaced more often.2 Use the drain valves to slowly drain all reservoirs to zero psi.3 Allow the oil/water mixture to fully settle before measuring oil quantity.4 To counter above normal temperatures at the air dryer inlet, (and resultant oil-vapor passingupstream in the air system) replace the discharge line with one of a larger diameter and/or longer length. This helps reduce the air's temperature. If sufficient cooling occurs, theoil-vapor condenses and can be removed by the air dryer. Discharge line upgrades are notcovered under warranty. Note: To help prevent discharge line freeze-ups, shorter dischargeline lengths or insulation may be required in cold climates. (See Bendix Bulletins TCH-08-21and TCH-08-22, included in Appendix B, for more information.)5 For certain vehicles/applications, where turbo-charged inlet air is used, a smaller sizecompressor may be permissible.3BASIC testacceptablerange:3 oil unitsper month.SeeappendixA.12 ft.For oil carry-overcontrol4 suggestedupgrades:Bendix BA-922 , or DuraFlo 596 air compressorCompressor with less than 15% dutycycleBendix Tu-Flo 550 air compressorLow Air UseNote: Compressor and/or air dryerupgrades are recommended in caseswhere duty cycle is greater than thenormal range (for the examplesabove).For Bendix Tu-Flo 550 and 750compressors, unloader service isrecommended every 250,000 miles.
Air Brake Charging System TroubleshootingHow to use this guide:Find the symptom(s) that you see, then move to the right tofind the possible causes (“What it may indicate”) andremedies (“What you should do”).Review the warranty policy before performing any intrusivecompressor maintenance. Unloader or cylinder head gasketreplacement and resealing of the bottom cover plate areusually permitted under warranty. Follow all standard safetyprocedures when performing any maintenance.WARNING! Please READ and follow these instructions toavoid personal injury or death:When working on or around a vehicle, the following generalprecautions should be observed at all times.1. Park the vehicle on a level surface, apply the parkingbrakes, and always block the wheels. Always wear safetyglasses.2. Stop the engine and remove ignition key when workingunder or around the vehicle. When working in the enginecompartment, the engine should be shut off and the ignitionkey should be removed. Where circumstances require thatthe engine be in operation, EXTREME CAUTION shouldbe used to prevent personal injury resulting from contactwith moving, rotating, leaking, heated or electrically chargedcomponents.3. Do not attempt to install, remove, disassemble or assemblea component until you have read and thoroughlyunderstand the recommended procedures. Use only theproper tools and observe all precautions pertaining to useof those tools.4. If the work is being performed on the vehicle’s air brakesystem, or any auxiliary pressurized air systems, makecertain to drain the air pressure from all reservoirs beforebeginning ANY work on the vehicle. If the vehicle isequipped with an AD-IS air dryer system or a dryerreservoir module, be sure to drain the purge reservoir.Symptom:1.0 Oil Test CardResultsLook for:Normal - Charging system is working withinnormal range.Check - Charging system needs furtherinvestigation.5. Following the vehicle manufacturer’s recommendedprocedures, deactivate the electrical system in a mannerthat safely removes all electrical power from the vehicle.6. Never exceed manufacturer’s recommended pressures.7. Never connect or disconnect a hose or line containingpressure; it may whip. Never remove a component or plugunless you are certain all system pressure has beendepleted.8. Use only genuine Bendix replacement parts, componentsand kits. Replacement hardware, tubing, hose, fittings, etc.must be of equivalent size, type and strength as originalequipment and be designed specifically for suchapplications and systems.9. Components with stripped threads or damaged partsshould be replaced rather than repaired. Do not attemptrepairs requiring machining or welding unless specificallystated and approved by the vehicle and componentmanufacturer.10. Prior to returning the vehicle to service, make certain allcomponents and systems are restored to their properoperating condition.11. For vehicles with Antilock Traction Control (ATC), the ATCfunction must be disabled (ATC indicator lamp should beON) prior to performing any vehicle maintenance whereone or more wheels on a drive axle are lifted off the groundand moving.What it may indicate:What you should do:Not a valid test.Discontinue using this test.Do not use this card test to diagnosecompressor "oil passing" issues. They aresubjective and error prone. Use only theBendix Air System Inspection Cup (BASIC) testand the methods described in this guide foradvanced troubleshooting.The Bendix BASIC test should be thedefinitive method for judging excessive oilfouling/oil passing. (See Appendix A, onpage 17 for a flowchart and expandedexplanation of the checklist used whenconducting the BASIC test.)ûBendix BASIC Testü4
Symptom:What it may indicate:What you should do:2.0 Oil on theOutside of theCompressorEngine and/or other accessoriesleaking onto compressor.Find the source and repair. Return the vehicleto service.2.1 Oil leaking atcompressor / engineconnections:(a)Leak at the front or rear (fuelpump, etc.) mounting flange.ð Repair or replace as necessary. If themounting bolt torques are low, replace thegasket.(b)Leak at air inlet fitting.ð Replace the fitting gasket. Inspect inlethose and replace as necessary.(c)Leak at air discharge fitting.ð Replace gasket or fitting as necessary toensure good seal.(d)Loose/broken oil line fittings.ð Inspect and repair as necessary.(a)Excessive leak at head gasket.ð Go to Test 1 on page 14.(b)Leak at bottom cover plate.ð Reseal bottom cover plate using RTVsilicone sealant.(c)Leak at internal rear flange gasket.ð Replace compressor.(d)Leak through crankcase.ð Replace compressor.(e)(If unable to tell source of leak.)ð Clean compressor and check periodically.2.2 Oil leakingfrom compressor:ð(c)(a)ð(c)3.0 Oil at air dryerpurge/exhaust orsurrounding areaHeadgasketand rearflangegasketlocations.Air brake charging system functioningnormally.5ð Air dryers remove water and oil from theair brake charging system.Check that regular maintenance is beingperformed. Return the vehicle to service.An optional kit (Bendix piece number5011327 for the Bendix AD-IS or AD-IP air dryers, or 5003838 for the Bendix AD-9 air dryer) is available to redirect theair dryer exhaust.
Symptom:What it may indicate:4.0 Oil in Supply orService Reservoir(air dryer installed)(If a maintained BendixPuraGuard systemfilter or Bendix PuraGuard QC oilcoalescing filter isinstalled, call1-800-AIR-BRAKE(1-800-247-2725) andspeak to a Tech Teammember.) What you should do:Maintenance(a) If air brake charging systemmaintenance has not beenperformed.That is, reservoir(s) have not beendrained per the schedule in TableA on page 3, Column 4 and/or theair dryer maintenance has notbeen performed as in Column 3.ð Drain all air tanks and check vehicle at nextservice interval using the Bendix BASICtest. See Table A on page 3, column 3and 4, for recommended service schedule.(b) If the vehicle maintenance hasbeen performed as recommended in Table A on page 3,some oil in the reservoirs is normal.ð Drain all air tanks into Bendix BASIC testcup (Bendix Air System Inspection Cup).If less than one unit of reservoir contentsis found, the vehicle can be returned toservice. Note: If more than one oil unitof water (or a cloudy emulsion mixture)is present, change the vehicle's airdryer, check for air system leakage (Test2, on page 14), stop inspection andcheck again at the next service interval.See the BASIC test kit for full details.If less than one "oil unit" of water (or water/cloudy emulsion mixture) is present, usethe BASIC cup chart on the label of thecup to determine if the amount of oil foundis within the acceptable level.ðIf within the normal range, return thevehicle to service. For vehicles withaccessories that are sensitive to smallamounts of oil, consider a Bendix PuraGuard QC oil coalescing filter.ð If outside the normal range go toSymptom 4.0(c).Also see the Table A on page 3, column 3for recommended air dryer cartridgereplacement schedule.(a)See Table A, on page 3,for maintenanceschedule information.Drain all air tanks (reservoirs)into the Bendix BASIC testcup. (Bendix kit P/N 5013711).Duty cycle too high(c) Air brake system leakage.ð Go to Test 2 on page 14.(d) Compressor may be undersized forthe application.ð See Table A, column 1, on page 3 forrecommended compressor sizes.ð If the compressor is "too small" forthe vehicle's role (for example, where avehicle's use has changed or serviceconditions exceed the original vehicle orengine OE spec's) then upgrade thecompressor. Note: The costs incurred (e.g.installing a larger capacity compressor,etc.) are not covered under originalcompressor warranty.ð If the compressor is correct for thevehicle, go to Symptom 4.0 (e).The duty cycle is the ratio of time the compressor spendsbuilding air to total engine running time. Air compressorsare designed to build air (to "run loaded") up to 25% of thetime. Higher duty cycles cause conditions that affect airbrake charging system performance which may requireadditional maintenance. Factors that add to the duty cycleare: air suspension, additional air accessories, use of anundersized compressor, frequent stops, excessive leakagefrom fittings, connections, lines, chambers or valves, etc.6
Symptom:What it may indicate:4.0 Oil in Supplyor ServiceReservoir*(air dryer installed)(continued)What you should do:Temperature(e) Air compressor discharge and/orair dryer inlet temperature too high.ð Check temperature as outlined in Test 3on page 14. If temperatures are normalgo to 4.0(h).(f) Insufficient coolant flow.ð Inspect coolant line. Replace as necessary(I.D. is 1/2").ð Inspect the coolant lines for kinks andrestrictions and fittings for restrictions.Replace as necessary.(f)(e)ð Verify coolant lines go from engine blockto compressor and back to the water pump.Repair as necessary.(g)Testing the temperatureat the discharge fitting.Inspecting the coolant hoses.(g) Restricted discharge line.(g)ð If discharge line is restricted or more than1/16" carbon build up is found, replace thedischarge line. See Table A, column 2, onpage 3 for recommended size. Replace asnecessary.ð The discharge line must maintain aconstant slope down from the compressorto the air dryer inlet fitting to avoid lowpoints where ice may form and block theflow. If, instead, ice blockages occur at theair dryer inlet, insulation may be addedhere, or if the inlet fitting is a typical 90degree fitting, it may be changed to astraight or 45 degree fitting. For moreinformation on how to help prevent dischargeline freeze-ups, see Bendix BulletinsTCH-08-21 and TCH-08-22 (Appendix B).Shorter discharge line lengths or insulationmay be required in cold climates.Kinked discharge line shown.Other(h) Restricted air inlet (not enough airto compressor).(h)ð Check compressor air inlet line forrestrictions, brittleness, soft or sagginghose conditions etc. Repair as necessary.Inlet line size is 3/4 ID. Maximumrestriction requirement for compressors is25 inches of water.ð Check the engine air filter and service ifnecessary (if possible, check the air filterusage indicator).Partlycollapsedinlet lineshown.*If a maintained Bendix PuraGuard system filter or Bendix PuraGuard QC oil coalescingfilter is installed, call 1-800-AIR-BRAKE (1-800-247-2725) and speak to a Tech Teammember.7
Symptom:4.0 Oil in Supplyor ServiceReservoir*(air dryer installed)(continued)What it may indicate:What you should do:Other (cont.)(i) Poorly filtered inlet air (poor airquality to compressor).Inspect theengine aircleaner.ð Check for leaking, damaged or defectivecompressor air inlet components (e.g.induction line, fittings, gaskets, filter bodies,etc.). Repair inlet components as needed.Note: Dirt ingestion will damagecompressor and is not covered underwarranty.(j) Governor malfunction or setting.ð Go to Test 4 on page 15.(k) Compressor malfunction.ð If you found excessive oil present in theservice reservoir in step 4.0 (b) above andyou did not find any issues in steps 4.0 (c)through 4.0 (j) above, the compressor maybe passing oil.Replace
air dryer, and service reservoir. 3 Compressor with up to 25% duty cycle Footnotes: 1 With increased air demand the air dryer cartridge needs to be replaced more often. 2 Use the drain valves to slowly drain all reservoirs to zero psi. 3 Allow the oil/water mixture to fully settle before measuring oil quantity.