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0A-79 General Information:
Electronic Vibration Analyzer (EVA)
Description and Operation
Special tool
J 38792-A Electronic Vibration Analyzer
The J 38792-A, electronic vibration analyzer (EVA), is a
12-volt powered hand-held device, similar to a scan tool,
which receives input from an attached vibration sensor
or accelerometer and displays the most dominate input
frequency(ies) (up to three) on its liquid crystal display.
The vibration concern frequency(ies) are obtained
through the use of the J 38792-A while following the
Vibration Analysis Diagnostic Tables. The frequency(ies)
obtained, when applied to the Vibration Analysis
Diagnostic Tables, are used as a primary input to help
determine the source of the vibration concern.
EVA Vibration Sensor
The J 38792-A vibration sensor incorporates a 6.1 m (20
ft) cord, that allows the sensor to be placed on virtually
any component of the vehicle where a vibration concern
is felt.
The J 38792-A contains 2 sensor input ports which can
be activated individually to allow for 2 individual vibration
sensor inputs. The vibration sensors can then be placed
in 2 different locations in the vehicle and their individual
inputs can be read without having to stop a test, move
the sensor and resume the test. The use of 2 vibration
sensors can help in more quickly finding and recording
an accurate frequency of the vibration concern, and in
more quickly making comparisons between 2 different
areas of a single component, or a vehicle system, during
the diagnostic process.
EVA Vibration Sensor Placement
Proper placement of the J 38792-A vibration sensor
(accelerometer) is critical to ensure that proper vibration
readings are obtained by the J 38792-A. The vibration
sensor should be placed on the specific vehicle
component identified as being the most respondent to
the vibration. If no component has been identified, install
the sensor to the steering column as a starting point.
EVA Vibration Sensor-to-Component Attachment
NOTE
The J 38792-A vibration sensor must be
attached to vehicle components in the
manner indicated in order to achieve
accurate frequency readings of the vibration
disturbance.
The vibration sensor of the J 38792-A is designed to
pickup disturbances which primarily occur in the vertical
plane, since most vibrations are felt in that same up-and-
down direction. The J 38792-A vibration sensor is
therefore directional sensitive and must be attached to
vehicle components such that the side of the sensor
marked UP is always facing upright and the sensor body
is as close to horizontal as possible. The sensor must be
installed in the exact same position each time tests are
repeated or comparisons are made to other vehicles.
The J 38792-A vibration sensor can be attached to
vehicle components in various ways. For non-ferrous
surfaces, such as the shroud of a steering column, the
sensor can be attached using putty, or hook and loop
fasteners. For ferrous surfaces, the sensor can be
attached using a magnet supplied with the sensor.
EVA Software Cartridge
The J 38792-A uses a software cartridge, the J 38792-
60, which provides various information to the J 38792-A.
The J 38792-60 provides the J 38792-A with an
additional feature which can be selected and utilized to
assist in diagnosing vibration concerns.
NOTE
The Auto-Mode function of the J 38792-A
cartridge, J 38792-60, is designed to be used
in SUPPORT of the Vibration Analysis
Diagnostic Tables ONLY.
This support-feature is available through the J 38792-A
Auto-Mode function. When selected, the J 38792-A will
prompt the user to select which one of 2 vehicle systems
(vehicle speed or engine speed), is the SUSPECTED
source of the vibration concern. Using the inputted
vehicle data parameters along with the most dominate
vibration frequency obtained, it will identify a
SUSPECTED source of the vibration concern, such as
first-order tire and wheel. This can be a useful feature
when used in conjunction with the Vibration Analysis
Diagnostic Tables, to confirm results obtained through
the diagnostic process.
EVA Smart Strobe Function
The J 38792-A can be used to identify some rotating
components/systems which exhibit imbalance IF the
component rotational speed is the dominant frequency
of the vibration concern. The J 38792-A is equipped with
a strobe light trigger wire which can be used with an
inductive pickup timing light, J 38792-25, or equivalent
included with the J 38792-KIT, or available separately.
Using the Smart Strobe function enables the user to
input the vibration frequency to which the strobe will
flash. By marking the suspected rotating component,
such as a pulley, adjusting the strobe frequency to match
the dominant vibration frequency at the engine RPM
noted during diagnosis, and then operating the engine at
that specific RPM, the mark on the object will appear to
be stationary if that object is imbalanced.
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