Technical information

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17
Technician Guidelines for Antilock Braking Systems
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manufacturers. (TMC Recommended Practice 129, “Heavy-
Duty Vehicle Systems Wiring Checks,” is a good source of
general information on electrical diagnostic procedures.)
ii. Electronic diagnostic procedures
To diagnose an electronic system properly, specialized test
equipment approved by the electronic system manufacturer
may be required. Failure to use the correct diagnostic tool may
result in inaccurate or incomplete diagnosis or cause ECU damage.
iii. Air system diagnostics
It is beyond the scope of this document to explain air
system diagnostic procedures in great detail. However, several
TMC Recommended Practices—such as RP 619, “Air System
Inspection Procedure”—are a good source of general
information on this topic. Other references for diagnosing air
brake systems can be readily obtained from component,
vehicle, and test equipment manufacturers.
Chart 2 on page 18 is an example of a troubleshooting flow
chart for a common modulator valve problem.
Step 5: Find and isolate problem
For an active problem, the diagnosis should narrow and/or
eliminate possible causes. Find and isolate the faulty part of the
system or circuit by breaking the problem into smaller pieces.
For an intermittent problem, attempt to simulate/recreate the
conditions where the fault would exist. Monitor suspect circuits
and components to pinpoint the probable cause while the
problem is occurring.
Step 5a: Reexamine complaint
Review all information describing the complaint. When did
the problem occur? What conditions are present when the
symptom occurs (weather conditions, driving conditions, etc.)?
Contact the driver, if necessary, to gather more information or
to arrange a “show me” or test drive interview.
Step 6: Repair and verify
Once the suspect component is found, carefully disconnect
the old component and inspect its connections to the harness.
If the component connections are OK, temporarily connect a
known good component (without installing) to ensure the
problem is corrected.
Technician Tip—
If a suspect part can be
easily installed and
removed, remove and
temporarily replace it
with a known good part
to see if the problem
remains.
If the problem
disappears, reinstall the
suspect component to
see if the problem
returns. If so, replace
the suspect component.