BENDIX TCH-001-045 User Manual

Technical Bulletin

Bulletin No: TCH-001-045-A Effective Date: 5/1/98 Cancels: PRO-01-32 dated 8/1/88 Page: 1 of 6

Subject:

General Comments

Some oil carryover from the air compressor into the air brake system occurs as a normal part of all air compressor
operation. Compressors require oil carry over to lubricate the compressor’s valves and rings. It is typical to find
some presence of oil at the inlet and discharge ports of the air compressor. Oil present at the exhaust of drain
valves, air dryers or aftercoolers is normal and indicates that these components are functioning properly,
removing contaminants from the system.

Vehicles equipped with a desiccant type air dryer that incorporates a filter (separator) for removing oil from the
air will normally minimize oil passed through to the reservoir. The oil collects in the air dryer with condensed
moisture and is periodically purged. Often, the routing or location of the air dryer exhaust port can amplify
concerns about oil passing, depending on where the purged oil / water mixture is expelled on the vehicle, e.g. on a
fender or onto a fuel tank. This can mistakenly be identified as “problem” oil passing, when in actuality the air
dryer and compressor are functioning properly.

Vehicles may be equipped with condensing type aftercoolers or drain valves instead of an air dryer. These devices
have little or no affect on preventing oil or water from being passed through the system and into the reservoir.

If after reviewing these general comments excessive oil passing is still suspected proceed to Step 1.

Step 1: Reservoir Draining

If excessive oil is passing from the air compressor, the air dryer filter element and desiccant may become
saturated. One of the first signs of oil saturated filter element or desiccant cartridge is moisture appearing in the
reservoirs.

Troubleshooting Compressor Oil Passing

The first step in determining if a compressor is passing an excessive amou nt of oil is to drain the reservoirs and
monitor the amount of water and/or oil removed.

Check reservoir drain valves to ensure that they are functioning properly. It is recommended that the vehicle
should be equipped with functioning automatic valves or have all reservoirs drained to zero (0) psi (0 kPa) daily
or optimally to be equipped with a desiccant type air dryer prior to the reservoir system.

If when draining the reservoirs there is an absence of water and/or oil, the compressor and all compressor support
systems are operating properly and there is no need to proceed further. If the expulsion from the dryer purge
occurs in a location such that it affects vehicle appearance, reroute the purge.

If water and/or oil are present, and the duration since the last reservoir draining is known and consistent with
Table A, go to Step 2.

If the reservoirs have not been drained according to Table A, follow the suggested frequency for two cycles. If at
the end of the second cycle, water and/or oil are drained, further investigati on is needed. Proceed to Step 2.

An absence of water and/or oil after the second draining indicates the compressor and all compressor support
systems are operating properly. Continue to perform reservoir draining on a regular basis as outlined in Table A.

Step 2: External Influences

At this point it should have been established through observations made in Step 1 that conditions exist causing
excessive oil passing. This does not indicate that immediate removal of the compressor is necessary or that the
compressor is the root cause for this issue. Removal of the compressor at this point usually leads to the
replacement of several compressors over a period of time without resolution of the issue.

The support systems, which control and contribute to the compressor operation must be examined. There are
many external influences, which can affect compressor oil passing. The influences can be grouped into
subcategorizes as follows:

• Vehicle operation

• Engine operation

• Compressor inlet air supply system

• Compressor coolant supply system

• Compressor oil supply and return system

Proceed to Step 3.

Step 3: Vehicle Operation
Vehicle operation is the driving factor for compressor performance and influences all of the systems that control

and contribute to compressor operation.
Check vehicle system leakage. Vehicle system leakage should not exceed industry standards of 1 psi (6.9 kPa)

pressure drop per minute without brakes applied and 3 psi (20.7 kPa) pressure drop per minute with brakes
applied. If leakage is excessive, check for system leaks and repair.

After determining that the system leakage meets industry standards the compressor duty cycle must be
established. Duty cycle is the percent of time the compressor is loaded during a period of operation. As duty cycle
increases, the amount of oil passed into the system increases.

Example:
If two compressors, which pass the same amount of oil when operated at the same duty cycle, are placed on two

vehicles which are identical except for duty cycle, the compressor on the vehicle with the greater duty cycle
would pass more oil into the system. The amount of effluent purged from the dryer would be greater on the
vehicle with the greater duty cycle. Thus the relative amount of oil at the purge of the dryer is not only an
indication of the level of oil passed by the compressor but also the duty cycle of the compressor.

Compressors that operate with duty cycles of 25% or less will have an expected life equal to t he normal warranty
period. As duty cycles extend beyond 25% the expected life will decrease.

It is recommended that the compressor duty cycle should not exceed 25% and the average charge period should
not exceed 90 seconds to ensure the normal life of the compressor and dryer. By keeping the charge time at or
below 90 seconds the compressor operating temperature is lower reducing oil consumption, carbon formation,
and discharge temperatures. By keeping discharge temperatures lower the dryer’s ability to remove oil and
moisture is increased.

Once the duty cycle is established review Table A for the vehicle application and make sure that the compressor
and air dryer combination installed on the vehicle match those recommended based on duty cycle. If the vehicle is
not properly equipped a change of the compressor and/or air dryer maybe necessary.

If the duty cycle exceeds the recommended 25%, two options are available:

• Reduce the amount of demand by redesigning the air system.